Engine Revs Up With Throttle Closed

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nova72

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Hi, I have a Traxxax T-Max 3.3 that I cannot get tuned. On the bench after starting, the motor will rev up with the throttle closed. I have to pinch the fuel line to shut it off. When I can tune it to idle somewhat, the motor will run for a minute or so then die. The carb is set to factor settings. I removed the carb, cleaned it and replaced all the o-rings. I did find some debris in the main jet. None of the carb work changed anything. Suspecting an air leak, I removed the motor and tested for air leaks. I found none on the motor. I did notice a small amount of air leaking from the bellows on the throttle slide on the carb. Compression feels good when turning the motor over by hand. Any suggestions on what to look at next? Thanks in advance.
 
Hi, I have a Traxxax T-Max 3.3 that I cannot get tuned. On the bench after starting, the motor will rev up with the throttle closed. I have to pinch the fuel line to shut it off. When I can tune it to idle somewhat, the motor will run for a minute or so then die. The carb is set to factor settings. I removed the carb, cleaned it and replaced all the o-rings. I did find some debris in the main jet. None of the carb work changed anything. Suspecting an air leak, I removed the motor and tested for air leaks. I found none on the motor. I did notice a small amount of air leaking from the bellows on the throttle slide on the carb. Compression feels good when turning the motor over by hand. Any suggestions on what to look at next? Thanks in advance.
I agree that it sounds like an air leak. If its not then I dont know either.

What do you think Sensei? @cbaker65
 
Is it a new or used 3.3. What factory setting did you set it to?Did you check the exhust o-ring. Is there a head gasket under the head? Almost sounds like the idle screw is out to far.. the carb boots/throttle cover is good? You can cheek the idol gap. Pull off airfillter and see how open is you slide gap. I set mine with a paper clip.. unfolded slide clip in the gap and it should be somewhat tight fit..

what fuel are you using.
 
Sensei ,says its a traxxas!..:hehe:
Well that too. But if tuned and set up rite shouldn't have a problem. I mean I don't have one on either of mine 2.5 or 3.3. But the TRX is a bi*ch to get just rite. Alot of patience and runs to get it just rite.. and not to use top fuel except for the break in....
It also might sound like your control trims are off. Have you tried to set throttle to 0 and re-enter servo?
 
You may want to check the crankcase gasket. also. The crankcase is part of the intake on a 2 cycle engine. The 3.3 uses an O ring. Air can be sucked around the bearings also. Just something to be aware of.
 
Hi, sorry in the delay to respond. It's a used motor. Carb settings are 4 turns out for the high speed needle, low speed needle is set even with the inner rind on the throttle slide. Idle gap is set to 1 mm. If I try to close the idle gap, the motor will run for a few moments then die. Carb boot is new. I'm running Traxxas 20% Top Fuel. Unknown about the head gasket, but I know the history of the motor and it's never been apart. Throttle trim is set so the servo is not activating the throttle.
 
Hi, sorry in the delay to respond. It's a used motor. Carb settings are 4 turns out for the high speed needle, low speed needle is set even with the inner rind on the throttle slide. Idle gap is set to 1 mm. If I try to close the idle gap, the motor will run for a few moments then die. Carb boot is new. I'm running Traxxas 20% Top Fuel. Unknown about the head gasket, but I know the history of the motor and it's never been apart. Throttle trim is set so the servo is not activating the throttle.

Just like JerBear said ,the engine is getting its air from somewhere else besides the carb. ,could
be coming in from head ,carb throat o-ring ,backplate ,front bearing or all places ,leaks can &
will wreak havoc on the tune!..:thumbs-up:
 
Hi, sorry in the delay to respond. It's a used motor. Carb settings are 4 turns out for the high speed needle, low speed needle is set even with the inner rind on the throttle slide. Idle gap is set to 1 mm. If I try to close the idle gap, the motor will run for a few moments then die. Carb boot is new. I'm running Traxxas 20% Top Fuel. Unknown about the head gasket, but I know the history of the motor and it's never been apart. Throttle trim is set so the servo is not activating the throttle.
That 1mm gap is way to big
.5 -.7mm is the most you want. And what plug are you running? The tops fuel really isn't helping either.. need a better fuel with less oil content. Then the traxxas top fuel.
Look for fuel with a 8 - 10% no higher then 11. 12% lubrication.. bones brew or odonnnells or VP fuel are good fuels. You also should be using a medium plug or a hot. Depending where you live..
 
Just like JerBear said ,the engine is getting its air from somewhere else besides the carb. ,could
be coming in from head ,carb throat o-ring ,backplate ,front bearing or all places ,leaks can &
will wreak havoc on the tune!..:thumbs-up:
My initial leak test turned up nothing except maybe on the throttle slide bellows. I'll check it again. Would the muffler or fuel tank problems cause it to also rev up with the throttle closed?
 
That 1mm gap is way to big
.5 -.7mm is the most you want. And what plug are you running? The tops fuel really isn't helping either.. need a better fuel with less oil content. Then the traxxas top fuel.
Look for fuel with a 8 - 10% no higher then 11. 12% lubrication.. bones brew or odonnnells or VP fuel are good fuels. You also should be using a medium plug or a hot. Depending where you live..
Using a new Traxxas 3232X glow plug.
 
My initial leak test turned up nothing except maybe on the throttle slide bellows. I'll check it again. Would the muffler or fuel tank problems cause it to also rev up with the throttle closed?

Loss of tank pressure will cause the idle to raise ,it will cause lean issue!
 
Using a new Traxxas 3232X glow plug.
That's a long plug.. that is one of the problems. You should be running a OS LC3 hot or P3 Ultra hot plugs.. to hell with the traxxas plugs they are crap. And the fuel. Only good for break-in. That's it... and the 1mm slide gap is to much should be .
.5 - .7 no more.. its probably running messed up because the fuel your running. And the plug... along with big gap. You will never get a good tune running the top fuel
And along plug. The fuel is to high in oil. Which makes the engine run lean. You might not have a air leak. Just not the rite fuel and plug. Does your linkage snap the slide closed 100%. With no movement at all?
Thats first off the slightest ope. Slide will mess it up. You don't need to touch or adjust the LSN. Only HSN. And the idle screw..
Here's some reads about the top fuel. I haven't used top fuel since 2000.. and. Run. LC3 and OS P3 ultra hot. All the time..And OS A4 A5 plugs cold when it's super hot out side 80° and above. In my TRX 3.3 and 2.5. Engines And in my other nitro engines as well.. if I even go out to run them. In that heat not to good to run in super hot weather IMO...

https://www.rcuniverse.com/forum/traxxas-monster-truck-forum-251/8972220-traxxas-top-fuel.html

https://www.rcuniverse.com/forum/tr...291673-safe-nitro-oil-content-3-3-motors.html
16340085822131768807029762199765.jpg
 
@StonerRC365 Thanks for the input. I did check the fuel tank and lines. No leaks or issues found. I assume the muffler/resonator is working correctly to pressurize the tank. I'll check the linkage when I get the motor back in the truck. I will switch the glow plug and fuel as you suggested and see where that takes me. Will also recheck for leaks in case I missed something.
 
@StonerRC365 Thanks for the input. I did check the fuel tank and lines. No leaks or issues found. I assume the muffler/resonator is working correctly to pressurize the tank. I'll check the linkage when I get the motor back in the truck. I will switch the glow plug and fuel as you suggested and see where that takes me. Will also recheck for leaks in case I missed something.
At this point I don't think it's a air leak. I honestly think it's the fuel and plug. And the linkage not set properly. Trust me it took me a while when I first got my Tmaxx 2.5 yrs ago... just got to have patience when it come to nitro. It get very frustrating I know. But when you get it tuned and set. There's nothing like it...👍👍👍
 
Update on this issue. New LC3 glow plug, new Traxxas muffler/resonator, running VP 20% fuel. Same issue. If there is an air leak (which I still suspect) I haven't been able to locate it. Any suggestions on how to find it would be appreciated - otherwise it's going to get trashed.
 
This is a good video on how to seal a nitro engine against air leaks. I do the stuff in this video plus I will seal the HSN & LSN using some Associated Green Slime (or any silicone grease will work) around the orings just for added measure.

 
Update on this issue. New LC3 glow plug, new Traxxas muffler/resonator, running VP 20% fuel. Same issue. If there is an air leak (which I still suspect) I haven't been able to locate it. Any suggestions on how to find it would be appreciated - otherwise it's going to get trashed.
Quick and easy got a air compressor? With a air gun attachment?
 
Hi, I have a Traxxax T-Max 3.3 that I cannot get tuned. On the bench after starting, the motor will rev up with the throttle closed. I have to pinch the fuel line to shut it off. When I can tune it to idle somewhat, the motor will run for a minute or so then die. The carb is set to factor settings. I removed the carb, cleaned it and replaced all the o-rings. I did find some debris in the main jet. None of the carb work changed anything. Suspecting an air leak, I removed the motor and tested for air leaks. I found none on the motor. I did notice a small amount of air leaking from the bellows on the throttle slide on the carb. Compression feels good when turning the motor over by hand. Any suggestions on what to look at next? Thanks in advance.
You said the throttle was closed, did you check barrel spacing with the air filter removed?
 
Update on this. I ended up having to repair the centrifugal clutch on the back (front?) of the motor. The spring was broken and the wheels kept turning with the truck on the stand. I was fighting this trying to tune it to keep the wheels from spinning. Once I repaired the clutch I could tune it on the ground so it would run somewhat decent.
 
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