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  1. #1
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    Cool RB Concepts TM523 Carb The Real Info you need to know

    I just recived my RB Concepts TM523 carb. You have to cut off this plastic sleeve around the base of it. Becareful not to cut your self it's very tuff to get off. You need to be carefull not cuting into the carb when trying to remove the plastic cover. It could possible create air leak and make it hard to slide into engine case. Once you do this the carb aclty fits nice and snug as it should be. Then you need to grind a little bit off the front of the carb to alow for it to sit flush on the base of the engine case and also to clear the fly wheel. It just fits with the stock fly wheel after you do a little grinding. Theres no way a 40mm fly wheel will fit. Now You have to remove the stock throttle lever thats on the carb and replace it with the stock traxxas unit. Next you need to move the throttle horn thats on the engine motor mount and move it out to the next hole. Next to get wide open throttle you need to remove the throttle and brake horn off the servo and turn it one spline towards the front of the truck and re-adjust your set screw coller. With a little tinkering you will get it to go wide open.

    I added a Motor Save Air filter Part# RT80

    Running Traxxas 33% fuel

    Traxxas Glow plug

    Engine temp 245

    Stock gear ratio 15/36

    Wide ratio trans

    Stock Pipe

    Stock tires

    My opinion there needs to be a heaver gauge wire for the throttle, it's to thin and flexible. I watch it flex when it's going wide open.

    The Test: it's too fat out of the box but that's a good thing. I leaned it out a little bit and all I can say is WOW Mission control we are ready for take off. It has silly power now. That little hesitaion bog or bobble off the line is gone. When I got it dialed in and cracked the throttle it would just rev to the moon, I thought I broke something or the clutch was bad or even the slipper was bad. None of the above It was just spinning the tires out of control. I never laughed so hard in my life. It was like electric truck or like hitting nitrous button.

    Caution: slide the carb down as far as it goes and see were it needs to be clearanced, it just needs to be grinded a tiny bit. If you grind to much you will cut into the base of it were the O-ring goes and it might not seal and creat a air leak and ruinon the carb.

    I have more testing to do but it was alot of fun and nite and day difference in throttle response and performance.
    Last edited by slayer17; 01-14-2012 at 11:41 AM.

  2. #2
    RC Turnbuckle Jr. CarGuy7a's Avatar
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    I've always said that the tuning difficulties the 3.3 engine has stems from the carburetor. Those problems went away after I started using a Force .28 or the Force .26 carb on my 3.3. The 11K carb off an OS 21TM will work in the 3.3 also. The biggest difference I noticed was like you said the throttle response. Also at optimum tune with the Force .28 carb is that the motor ran about 10 to 20F cooler.

    Look into the Tekno Revo ball bearing throttle linkage setup. It comes with a different style bell crank that bolts onto the motor mount and it has a linkage that routes on the outside of the roll bar. It works very well and is really smooth.

  3. #3
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    What carb worked the best for you? I tried a few force carbs and they didn't fit there base was 13.96mm were as the 3.3 is 13.01mm. Traxxas needs a new carb 110% BUT they spent alot of money in tooling for this carb wich was for the 2.5. The new carb for the 3.3 should be 7.5mm and the low side needle needs to be exstend so it give you more low to mid range control over the fuel mixture.
    Quote Originally Posted by CarGuy7a View Post
    I've always said that the tuning difficulties the 3.3 engine has stems from the carburetor. Those problems went away after I started using a Force .28 or the Force .26 carb on my 3.3. The 11K carb off an OS 21TM will work in the 3.3 also. The biggest difference I noticed was like you said the throttle response. Also at optimum tune with the Force .28 carb is that the motor ran about 10 to 20F cooler.

    Look into the Tekno Revo ball bearing throttle linkage setup. It comes with a different style bell crank that bolts onto the motor mount and it has a linkage that routes on the outside of the roll bar. It works very well and is really smooth.
    Last edited by slayer17; 01-14-2012 at 03:14 PM.

  4. #4
    RC Turnbuckle Jr. CarGuy7a's Avatar
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    The Force .28 ran the best due to the bigger inside diameter (I believe it is 8mm) and was easy to tune. It did require modding to fit into the 3.3 block and after a while of running it didn't want to stay in the motor. The force .26 carb was a direct fit into the block but the tuning was a little more finicky and the LSN being towards the inside of the truck made it a pain to get to. I've heard from others that the 11k and the TM523 carbs are hands down the best ones to run on the 3.3.

  5. #5
    RC Turnbuckle Jr. humayrayakongkinaon's Avatar
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    I thought it was just me, but swapping carb from different engine really open up the potencial of my 3.3. I have put a BB carb on mine from an unkwon engine and just happen to fit in nicely, idle nicely, revs higher and I dont have that high revving sound after you let off the throttle. the inside diameter is alot bigger than the 3.3 carb.
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  6. #6
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    Two reason for this. It's a bigger carb your letting more air in and do you see your low side needle inside the carb that sticks farther out into your main well. It controls the fuel better at low to mid range. On the Stock 3.3 carb there realy isn't much of a needle that realy controls your low to mid it just mostly works for idle and off idle.This is why there is this little bobble or hesitation when you crack it wide open. From idle you should be able to crack this deal wide open and have it take off and hit realy hard in the power band, provide your geared correclty and not for some inssane top end mph gearing. The charistics of nitro methane is the more you can load the motor the more power it makes, the more power it makes the more fuel its going to need. It's funny when I parken lot bash first thing out of everybody mouth is OH YOUR TO FAT WHY DONT YOU LEAN IT OUT. About the 3rd lap it's cleared out warmed up and it's gone lapping people silly. There like OH mother of God WOW that thing is insane fast and it has incredible throttle response.

    Quote Originally Posted by humayrayakongkinaon View Post
    I thought it was just me, but swapping carb from different engine really open up the potencial of my 3.3. I have put a BB carb on mine from an unkwon engine and just happen to fit in nicely, idle nicely, revs higher and I dont have that high revving sound after you let off the throttle. the inside diameter is alot bigger than the 3.3 carb.
    Last edited by slayer17; 01-15-2012 at 12:07 AM.

  7. #7
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    Very interesting on the carb. I'm a newbie at Nitro, but not engines. (I'm a 50 year old engineer.) I purchsed 2 T-maxx's and 1 Revo for me and my boys to play with this past Christmas. Great fun. I know exactly the trouble you are talking about with the initial bog/delay from stand still, even if I tune lean. I had little trouble tuning the T-Maxx's. One of the T-Maxx's tunes better than the other two cars and doesn't have the bog. (Luck of the draw I suppose on getting a decent carb on that one.) But the Revo... I can't get it off a very high idle that engages the clutch. It is also very twitcy to get it to tune at all. Very annoying. And believe me, I've tried everything with the carb except taking it off and seeing if there is an internal problem, like a peice of dirt or machining anomoly. I was about to blame it on an air leak, but now, you've got me thinking about the carb.

    I will probably swap carbs with one of the other cars soon and see if the problem follows the carb. I suspect it might.

    The TM523 carb looks intriguing, but it is a huge portion of the cost of the TRX 3.3, at $123 plus shipping. Seems I might as well just buy a Picco red dot and a pipe and call it a day. That should well outperform the TRX 3.3, or so I read. But I'm told buy some engine modders it is not a long lived engine and starts leaking at the bearing rapidly. I don't know if that is true, but that is something for another thread.

    But everything is a trade off. I welcome your comments.

  8. #8
    RC Turnbuckle Jr. humayrayakongkinaon's Avatar
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    TM523 carb takes the hassle of tuning.
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  9. #9
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    Theres other carbs out there that will bolt up that are cheaper. I saw one for the offna carbs brand new some were online around 75. bucks. Check Ebay. When I bought the Slayer Pro and ran it I was like Man they came along way from the .15 and it's nite and day difference from the 2.5. But it still had that little delay in the throttle response and it was the same carb they used on the 2.5 wich there is a fix for it and I can't remember ecactly. LOL The mind is the first to go when you get old. It was ripping around good. Then I bolted up the TM523 and adjusted it and all I did was laugh, it was a deep belly laugh, I was like OMG now we are talking. It woke the engine rite up. Yes exspensive for the engine, BUT you need to look at it as a bolt accessory. You will always have that carb for any 2.5 to 3.3 traxxas engine. The rebuild for the engine is cheap compared to all others. I have had every engine just about made since 1988 and this 3.3 for the money you cant beat it. Mine now is keeping up .26 and .28 with no troubles. In my opinion and 25 years of racing and playing in the long from a financial stand point, your better off with the 3.3. I will try to remember that fix for the carb I know it had to do with drilling out the main well because the needle wasn't fitting inside properly.
    Quote Originally Posted by Smyers View Post
    Very interesting on the carb. I'm a newbie at Nitro, but not engines. (I'm a 50 year old engineer.) I purchsed 2 T-maxx's and 1 Revo for me and my boys to play with this past Christmas. Great fun. I know exactly the trouble you are talking about with the initial bog/delay from stand still, even if I tune lean. I had little trouble tuning the T-Maxx's. One of the T-Maxx's tunes better than the other two cars and doesn't have the bog. (Luck of the draw I suppose on getting a decent carb on that one.) But the Revo... I can't get it off a very high idle that engages the clutch. It is also very twitcy to get it to tune at all. Very annoying. And believe me, I've tried everything with the carb except taking it off and seeing if there is an internal problem, like a peice of dirt or machining anomoly. I was about to blame it on an air leak, but now, you've got me thinking about the carb.

    I will probably swap carbs with one of the other cars soon and see if the problem follows the carb. I suspect it might.

    The TM523 carb looks intriguing, but it is a huge portion of the cost of the TRX 3.3, at $123 plus shipping. Seems I might as well just buy a Picco red dot and a pipe and call it a day. That should well outperform the TRX 3.3, or so I read. But I'm told buy some engine modders it is not a long lived engine and starts leaking at the bearing rapidly. I don't know if that is true, but that is something for another thread.

    But everything is a trade off. I welcome your comments.

  10. #10
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    You make a great argument for spending the $130+ for the carb. Future rebuild costs are always a consideration, especially when I'm footing the bill for myself and my two boys.

    Thanks.

  11. #11
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    Some good nitro fuel and a good glow plug is the key to a good tune,and a new Traxxas crab is only $45.00

  12. #12
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    carburetors compared technically on paper / found stock carb issue

    This is a lengthy post on this carb subject, but I've turned up some interesting comparisons.

    Comparing the three carburetors that we've mentioned:
    I measured the throat diameter of the standard TRX 3.3 carb and it is .227" (about 5.75mm) ($45). The OS 11KR carburator ($85) has a throat diamter of .35" (9mm). The TM523 has a throat diameter of 8mm ($123). But what is important is square area of the throat. Doing area calculations in mm:

    TRX 3.3 throat area = 26.95 sq mm
    OS 11KR throat area = 63.59 sq mm (2.36 times larger flow area than stock)
    TM523 throat area = 50.24 sq mm (1.86 times larger flow area than stock)

    Slayer17 made a keen observation, from an engineering persective. Consider that the 63 sq mm carb is what is designed to be on an OS .21 displacement engine.; the 50 sq mm carb is designed to be on a .23 engine, while the 27 sq mm carb supposedly designed to be on a .20 displacement engine. Hmmm. My engineering sirens are going off. How can this be? The .23 and the .21 carbs are relatively close in size, yet the .20 engine carb is tiny in comparison. How can engines that move about the same amount of air per stroke use carburetors that are so grossly different? Physics are physics. It would appear that the TRX carb is an afterthought for the TRX 3.3 engine and from an older and smaller engine design, as pointed out by Slayer17.

    It would seem to be apparent as to why the 11KR and TM523 carburetors let a TRX 3.3 engine have so much more power and low end snap. Lower flow velocity & less pressure drop = greater possible air volume into engine. Lower flow velocity also mean less change in flow velocity occurs from one RPM to the next, so throttle response should feel quicker. There is a "sweet spot" with a carb's throat area where velocity and pressure drop is optimal for still good atomization with the lowest drop and enough fuel. (Not enough fuel is not normally an issue with Nitro engines however, since nitro is its own oxidizer.) A carb can be too big. Is the 11KR past that point and the TM523 what is optimal? That could be argued to the end of time, as there are too many variables to be able to answer this. But clearly based upon several's experiences (including Slayer17's here) and comparison with OS and TM carbs, I suspect the stock TRX 3.3 carb is well undersized.

    I may have to buy the 11KR carb and give it a try. It's price point seems fair. I hope it doesn't take too many mods to make it fit.

    I've torn apart the TRX carb on my Revo and made sure everything was OK. Found a couple suspect leaky point and a rough edge on the sleeve's seat that seals the o-ring against the engine. Carefully cleaned that up and made sure the carb was well seated with the o-ring. Put some fuel tubing to it and held it underwater with the ends capped and blew hard into the tubing enough to expand the bellows. No air leaks in the carb. Reinstalled and the engine now idles well. But frankly, the base of the carb where the o-ring seats and the seat area on the engine are not very good. The chamfer in the engine where the o-ring seats isn't concentric with the bore. Some RTV would have been good as well. So I had some bad tolerancing on the engine's carb seat and faulty base of the carb at the sleeve's o-ring area causing air leaks. Not very impressive QC. But any company can make errors and this carb is just an injection molded plastic part, so I can excuse that.

    Smyers
    Last edited by Smyers; 01-24-2012 at 11:55 AM.

  13. #13
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    Follow up. I added a large motor saver air cleaner Big Difference. I had to re-adjust the hi and and low end needle when I installed it. Defenetly makes the engine breath better. I also drilled out the stinger on the resinator pipe to .2200 what a huged difference now. I'm pulling power wheel stands every were at will. It's running realy hard with insane throttle response. I'm working my way towards drilling the stinger out to 15/64. Alot of people said works realy well on the forums here. 15/64 is .2344. This is still with the easy start system on it. I want to remove that and see if there is any gain in performance.

  14. #14
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    Show us some vids. It would be nice if the carb was still available but I haven't seen them around for a while.
    The Super Derecho

  15. #15
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    There still for sale online I found a few places that had them. Just type in TM523 and it pops up. I will work on video have to see if my camera still works and see how to get it online. I have old stuff from 2000
    Quote Originally Posted by Double G View Post
    Show us some vids. It would be nice if the carb was still available but I haven't seen them around for a while.
    Last edited by slayer17; 01-26-2012 at 09:03 PM.

  16. #16
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    A-Main hobbies has plenty of the TM523 in stock. I just ordered another one.
    Quote Originally Posted by Double G View Post
    Show us some vids. It would be nice if the carb was still available but I haven't seen them around for a while.

  17. #17
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    Update on the testing. I changed from the wide ratio to the standard trans ratio. I drilled stinger out to .2275 now

    Running Traxxas 33% fuel

    Traxxas Glow plug

    Engine temp 235

    Stock gear ratio 15/36

    Standard trans ratio

    Stock resanator Pipe Drilled out stinger to .2275

    Carb: Tm523

    Easy start system

    Air cleaner: Larger Motor saver

    Results: Improved throttle response. Pulls power wheelies from idle when it shifts into 2nd gear it does a violent power wheel stands and blows over backwards. You can wheelie when you want. Realy crisp clean throttle response. It's reving higher. Defently has a faster top end

    Next test I will drill the stinger out to .2335. I allso want to test Traxxas bigger exshaust header part#5340x
    Last edited by slayer17; 01-28-2012 at 10:17 PM.

  18. #18
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    That's pretty amazing. Sounds like you need to make the shift point just a wee bit later! LOL

    Keep us posted.

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    LOL I never thought about that, good point. I'm just so focused on making more power and getting better throttle response. I need to get a video up online so people can realy see whats going on. Anybody in central NJ that has a good video deal that can take some vids and do some good quailty R/Cing.

    Quote Originally Posted by Smyers View Post
    That's pretty amazing. Sounds like you need to make the shift point just a wee bit later! LOL

    Keep us posted.

  20. #20
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    Update on the testing. I drilled stinger out to .2330

    Running Traxxas 33% fuel

    Traxxas Glow plug

    Engine temp 223- 245

    Stock gear ratio 15/36 14/36

    Standard trans ratio

    Stock resanator Pipe Drilled out stinger to .2330

    Carb: Tm523

    Easy start system

    Air cleaner: Larger Motor saver

    Took it out and it seemed little lazy on the bottom end but it had even more mid and top end. Crashed it early in the run and had a idle issue (Bent the throttle linkage a little bit) cracked it one more time and blew the spur gear apart. Replaced the spur gear and adjusted the low side just a tiny bit and adjusted the throttle linkage. I had my crazy bottom end back and super throttle response back. This deal is just crazy fast and out of control. So yes the 15/64 drilling out the stinger works. I then replaced the 15 clutch bell with the 14. All I can say is wow it's like a electric truck you crack the throttle and its gone. It has so much low and mid range it's insane. I raced my brothers Stock jato 3.3 and I beat him the whole time until the very top end were his 60+ mph kicks in. But off the line and mid range I was ahead of him until I just ran out of gear and engine rpm.
    I thought when I first drilled it to the .2330 and tried it I was like man I lost that perfect balance between low and mid range throttle response. But it wasnt the case. Little carb adjustment and I was back to were I was.

  21. #21
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    It would be interesting to use a Spektrum radio with telemetry with the stock pipe and carb and then swap over to the RB carb and drilled pipe. This could tell us a number of things objectively, including how muc hthe top end engine RPM incrased. I will throw my Spektrum on our Revo or T-Maxx in the next week or so and see what it it really does and get back with you, if you are interested. But perhaps you have a telemerty radio yourself.

  22. #22
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    That would be great we can compare notes. You can never get enough test data
    Quote Originally Posted by Smyers View Post
    It would be interesting to use a Spektrum radio with telemetry with the stock pipe and carb and then swap over to the RB carb and drilled pipe. This could tell us a number of things objectively, including how muc hthe top end engine RPM incrased. I will throw my Spektrum on our Revo or T-Maxx in the next week or so and see what it it really does and get back with you, if you are interested. But perhaps you have a telemerty radio yourself.

  23. #23
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    I picked up a Force .28 carb on E-bay for $14.00. I had to turn down the base to get it to fit in the engine and build a new (longer, offset to the right) throttle arm, and i had to cut a notch out of the roll bar to be able to adjust the LSN, but other than that, it was a simple conversion. Right now i am geared 17/36 and it is crazy fast. The engine sounds like it turning much faster on the top end, Throttle response is crisp and the low to midrange transition is smooth. What impressed me the most, was how easy it was to tune. 1/4 turns were resulting in noticeable and forgiving changes not the 1/8 to 1/16th adjustments that i was having to make on the stock carb. i hope to get the GPS on it soon to check the top speed. Its barely able to lift the wheels from a stand still, but i am only running 20% fuel at this time. i plan on switching to 33% and lowering the gearing some.

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    Sounds good mrwaug. How about starting up a new thread (so this one isn't highjacked) showing us photos and details of the modifications? It is certainly a lot more complex than the TM523 conversion, but a whole lot cheaper. Turning down the carb base isn't something most can do, as lathe access is required. Sounds promising, but complex, considering the lathe and throttle arm work. Having a plastic body rather than a metal carb body would be better than the TM523 on a hot day, to avoid vapor lock.

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    I guess I sold a few carbs for A-MAin there out of them now. They have more coming in they said

  26. #26
    RC Turnbuckle Jr. Nitronaught's Avatar
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    Am I missing something? What, the size of the venturi has nothing to do with these carbs? Just the throat size? How about the metering of the venturi?

    Does anyone have a flowbench? It's about the only real way you can determine these issues.......
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    Good point. I should explain what we mean by throat size and recap how we have come to this conclusion.

    The throat size is the minimus of the venturi size. We're not talking about the opening sizes, but rather down in the carb at the slide. The entire thing from the opening to past the slide valve at the smallest point is a venturi. Venturis follow a mathematical curve to reduce turbulence and pressure drop. If I recall, a hyperbolic curve is the ideal, but I've been out of school too long to be sure. On such a small venturi with such a short stack length, the exact curve is probably not that important.

    Your right in that getting exact flow figures to hit the ideal pressure drop for best atomization with least restriction while does require a flow bench and a lot of expensive equipment. And since engines run at various RPM's, a venturi size has to be a trade off on lost atomization at the low RPM's or lost flow at the top end. You have to make some trade-offs in engine RPM VS venturi size, especially on engines that have a very large RPM range. (This is just one reason why fuel inection runs circles around carbs.)

    However, we don't have to do any heavy math or even bench test to make some observations. Comparison among other existing engines and carburetors on the market is a tell-tale if things are properly sized. Carbs have always been sized by total displacement of the pistons of an engine at the designed optimum operating RPM (for max HP, fuel economy, etc.) for that engine size (which dictates air required through the venturi at atmospheirc pressure at some average hegiht above sea level) on a normally aspirated engine. There are some variances based on head design, exhaust design, torque/RPM requirements, etc., but it is normally in the same ballpark for a particlar displacement. This was all worked out through engineering and testing many years ago. So if you have several .20 cu in engines of similar design, designed for the same purpose and similar RPM ranges, they will all end up with very similar carburetors sizes. This seems to point to the stock TRX 3.3 carb being too small a venturi when compared to other engine manufacturers, without crunching any math at all. So do the engineers that desgined the 3.3 engine and carb combo know somethign all the other engine manufacturer's dont? I don't think so.

    I never looked at this until I read Slayer17's carburetor review and started looking at what others have done with carbs on a TRX 3.3. I think he has hit it spot on.

  28. #28
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    Its simple like Smeyers said. You look at all the .21s and .26s even marine nitro engines runing 30K plus engine rpm and there all 7.5mm to 9mm venturys in the carb. I run a 6mm carb on my .12s and .15s. The 6mm carb is way to small for the 3.3. They used the same carb on the 2.5

  29. #29
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    One thing i remember from tech school was, the two factors that limit engine RPM's is friction and breathing. After replacing the stock carb with the force 28, and having any way to prove it, it sounded like the motor was running much faster at full throttle. It makes me wonder if Traxxas is trying to limit the maximum power output to increase engine durability and life by restricting the amount of air going into the engine.

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    I feel it just a money issue for them. They design the carb for the 2.5 and they have alot of money invested in the injection molds, they ran it on the 3.3 and they got 50% more power out of the 3.3 over the 2.5 so it was a win win situtation for them. They know it could use a bigger carb but then it's more money to re-design and make new injection molds. And who knows that might be a Ace up there sleeve down the road. The all new Pro series 3.3 and only thing they will change is the carb and guess what with the carb change that's going to make 50% more power then what they have now with the 6mm carb and everybody will be WOW I'm getting the new pro series 3.3.
    Quote Originally Posted by mrwaug View Post
    One thing i remember from tech school was, the two factors that limit engine RPM's is friction and breathing. After replacing the stock carb with the force 28, and having any way to prove it, it sounded like the motor was running much faster at full throttle. It makes me wonder if Traxxas is trying to limit the maximum power output to increase engine durability and life by restricting the amount of air going into the engine.

  31. #31
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    Good point about the financial issue, makes since.
    It would cost them quite a bit to have to design and produce a new carb, so they just went with a much bigger cylinder.
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    Im working on finding out what OS carbs will just bolt in. I know the 11k fits but its only 6.5 mm and We need 8mm

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    OK I found a nice cheater deal that works. I tested it and it realy wakes up the 3.3, its a 100% bolt in with no modfications. It looks 100% like the traxxas carb but it is not. It's 6.9mm vers the stock traxxas 5.9mm. It's the carb from the Losi 3.4 engine and its cheap your looking at 38 to 45 dollars. You could bolt this deal on and have the best look at it and they would never know it wasnt the traxxas carb unless they took it off and messured it. LOL and just for you smarty pants people Yes I know the high side needle deosnt look like the traxxas. You can take the high side needle from the stock 3.3 and bolted it up and worked great if you had to do that.
    Last edited by slayer17; 03-20-2012 at 11:18 PM.

  34. #34
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    can one of you guys send me a pm of where you go this carb from im very interested in getting one for my 3.3 engine

  35. #35
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    Hi there.
    New here on the forum. Looking for advice. What kind of carbour is the best?
    It is very important.
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  36. #36
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    The losi off the 3.4 engine they have is a direct bolt in and it works great and is also bigger then the stock traxxas unit. If you can find a TM523 carb they work great also.
    Quote Originally Posted by RMitchell View Post
    Hi there.
    New here on the forum. Looking for advice. What kind of carbour is the best?
    It is very important.
    ___________
    http://ytubedownloader.org/
    download youtube videos

  37. #37
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    I bought a couple carbs a while back after spending some time talking to Slayer17. I bought both the RB and the OFNA. Over this past weekend, I finally got to install one of the carbs.

    I put the new RB carb (8mm throat) on my Revo 3.3. I only had time to ran a couple tanks of fuel. The first thing I noted was how much easier it tuned and held a good idle. That in itself was really nice. It had no low end hesitation once I got the idle mixture screw right. The only sluggishness on take-off is due to the relatively slow Traxxas servo, but it is NOT a hesitation. It had A LOT more low end torque. I had to tweak on the clutch (I use a Buku), but once I had that right, the acceleration was great. Much better than stock for sure. (I have a center diff installed, so no wheelies.) I didn’t try the stock Traxxas pipe yet, as I need to drill the stinger, so I was running my THS pipe. (My workshop is broken down and most of my tools are packed up for a move in a couple weeks, so that will have to wait.)

    The potential negative I did note is that if you get the mix too lean, it will let you know right away, as it will vapor lock. This is due to the all metal carb of course. Some people will not like that. But it is really a good thing, as it keeps you from running too lean, which protects the engine. So unless you are a hard core racer and will sacrifice an engine on purpose in a final heat, the metal carb can help save your bacon.

    I was able to use the stock carb lever that came with the RB. I’m not sure why Slayer17 couldn’t use it. (Perhaps he can explain.) The extra length of the RB lever didn’t interfere with anything on my car. The only interference I had was that the slide end at the low idle mix screw would hit the lower electrical connector on the EZ-Start motor, stopping it a full 2mm from full open. The ultimate solution is to get rid of the EZ-Start and go to bump start. But for now, some creative bending of the electrical connection allowed near full opening of the carb. It’s probably less than 1/2mm from full open, which is good enough for now. I’ll tweak that more later.

    I started to mount the modified OFNA carb (7.5mm throat) on my T-Maxx 3.3, but I found I need to change the linkage to the carb, as it is a bit too short. (I have not tried it on the Revo yet.) This is because the carb’s slide is about 15mm longer than a stock TRX carb from the carb’s centerline when fully open. The other thing to note about the RB carb is that the sleeve that slides into the engine is about 1mm shorter than the stock Traxxas carb, while the OFNA sleeve is exactly the same length. Another thing about that OFNA carb is that it has this raised rib on the square part of the plastic body with metal stickers. I pulled the sticker on one side and filed that ridge off. This allowed for good clearance of the larger T-Maxx flywheel. This may allow for another 2mm of flywheel clearance on the Revo, although I have not actually tried it yet. I suspect I will need to drill the stinger on the T-Maxx as well so it can breathe.

    Keep in mind that the OFNA carb it is NOT bolt on like the RB or Losi 3.4 carb and requires machining of the sleeve. It does have a metal sleeve under the plastic once machined down to the right OD to fit the TRX 3.3. The TRX requires a .510” sleeve and the OFNA carb comes at .550”, so it requires lathe work.

    Regardless of OFNA, RB or Losi 3.4 carb; there’s no going back to the Traxxas carb for me. It’s poorly made in comparison with the RB or OFNA and just not the right carb for that engine, based upon jsut two tanks of fuel I ran and the tremendous amount of work that Slayer17 has done.

    I have no intent on trying the Losi carb at this point, as I want the larger throat of the RB or OFNA. Assuming it has a decent low speed needle arrangement, the Losi carb will be better than the stock TRX carb as well. Besides the small throat of the TRX carb, the low speed needle arrangement on the stock TRX carb is a design problem as well and probably a BIG part of the bog from idle problem. If the Losi uses the same plastic raised boss around that low speed needle as the TRX carb, I'd steer clear. A good carb will use the tube/needle arrangement to keep that fit precision and repeatable in manufacturing. Both the RB and OFNA use this arrangement.

  38. #38
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    I'm seriously considering on going this route sometime. Couple questions: I haven't looked into the barrel itself for a while, but is it possible to bore out the Traxxas carb to say, 7 or 8mm? I may head to the LHS and buy one to try.
    Also, Traxxas carbs have a factory settings of 4 HSN, 1.75 LSN and 0.7-1.0mm idle gap. What baseline settings are you using to start with?
    The Super Derecho

  39. #39
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    No, you can't bore out a TRX carb. The slide is only slightly larger in diamter than the bore of the carb. If you bore out the carb's throat, air will leak around the slide rendering the carb useless. You have to have a carb with the approprate slide diameter. There is no way to mod a stock carb unless you repalce the slide wit ha larger diameter, which is not proctical, if even possbile. Cheaper and easier to buy a new one with a larger throat diamter.

    I can't recall at turn on the screws. I just took a swag at about 4 turns HSN and 1.5 turns LSN. I adujsted from there for optimum smoke and performance with the idle dropping down quickly from full on. I started with the idle screw at about 0.7mm gap, but I had to go smaller and probably have less than 0.5mm now.

  40. #40
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    Yea what he said
    Quote Originally Posted by Smyers View Post
    No, you can't bore out a TRX carb. The slide is only slightly larger in diamter than the bore of the carb. If you bore out the carb's throat, air will leak around the slide rendering the carb useless. You have to have a carb with the approprate slide diameter. There is no way to mod a stock carb unless you repalce the slide wit ha larger diameter, which is not proctical, if even possbile. Cheaper and easier to buy a new one with a larger throat diamter.

    I can't recall at turn on the screws. I just took a swag at about 4 turns HSN and 1.5 turns LSN. I adujsted from there for optimum smoke and performance with the idle dropping down quickly from full on. I started with the idle screw at about 0.7mm gap, but I had to go smaller and probably have less than 0.5mm now.

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