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Issue with TRX2.5 in a T-Maxx Classic

Issue with TRX2.5 in a T-Maxx Classic

Old 05-19-2016, 01:20 AM
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Default Issue with TRX2.5 in a T-Maxx Classic

Yes, I know the engine is undersized for a T-Maxx, but this is the engine it came with. I figure it's a good learning experience to try to wring as much as I can out of this engine before replacing it with something more suitable.

So, here's the problem I'm having. When the engine is at full operating temperature, if I run it hard for ~20 seconds before letting off the throttle again, the engine will idle super-high for a good 15 seconds before settling down again. If I blip the throttle it will settle down faster. This would seem to suggest the fuel mixture at idle is too lean, but if I enrich it any more than I already have, it will repeatedly choke to death at idle as the engine is warming up. If I turn up the idle to compensate for a richer LSN, it goes back to the original problem where it idles super-high after a hard pull.

It seems odd that this would be happening at all, because the engine has a composite carburetor that should be keeping the fuel from overheating in the fuel feed. Is there a tuning trick I haven't thought of yet, or am I getting the best performance I can expect from this engine and I'll just have to upgrade to a bigger engine to get it to run better?

For the record, I've geared-down from the stock 20t clutch bell to the 18t clutch bell, to make the engine's life a little easier. The truck doesn't go super fast, but I have other vehicles for that.
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Old 05-19-2016, 09:18 PM
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it doesn't sound like you have a problem. that little engine is workin its [email protected]# off, it just wont sound good doing it.
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Old 05-19-2016, 11:07 PM
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You know how the T-Maxx transmission has a set of centrifugal ratchets that are used to engage forward and reverse gears? After a hard pull, the TRX2.5 will idle so high it engages those ratchets, causing the truck to shudder repeatedly as the ratchets engage, slow the engine down, disengage, and repeat. So for several seconds after a hard pull, it sounds like "zzzzzzZZZZZZ*tick*zzzzzzZZZZZZ*tick*zzzzzzZZZZZZ* tick*..." before it calms down again. I'd rather it didn't do that, because it can't be good for the transmission. I have a BuKu clutch that's tuned so the clutch shoes are only lightly dragging at the RPM when the ratchets engage inside the transmission, so it's not like there's a ton of torque being transferred when that happens, but I'd still rather it not do that.

If you have any tuning suggestions to make the engine calm down after a hard pull without choking while it's still warming up, I'd like to hear them. Replacing the engine is of course an option, but I'd like to learn more about tuning a small engine at the limit of what it can handle.
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Old 05-20-2016, 03:25 PM
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I think you need to richen the hsn and lean the lsn. What fuel and plug are you using?
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Old 05-20-2016, 07:57 PM
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30% nitro (with a re-shimmed head as appropriate) and an O'Donnell medium glowplug. Those glowplugs are tanks, I've only ever burned out one.

Today I didn't seem to have much trouble with it. Something the owner of my LHS suggested was that idling high after a hard run indicates the HSN is too lean, and sure enough enriching the HSN *very slightly* did improve the issue without making it choke to death while idling. It *did* wanted to choke to death when it was cold, though; I had to hold the EZ-Start button while revving the engine until I got the crankcase flushed out, which was odd because I hadn't run it in a month and there shouldn't have been any fuel in the crankcase to start with. But after that it behaved well enough; a little sluggish on takeoff until it got to the second half of the tank, but overall it ran pretty well. It actually pulled a rolling wheelie at one point, which surprised me since it has such a small engine; I'll occasionally see a standing wheelie, but I've never seen a rolling wheelie before.

And then I clipped a telephone pole, so now my main concern is replacing the busted suspension parts. *sigh*
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Old 05-20-2016, 09:52 PM
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There could be many reasons for this.

The low should be flush with the inside ring. That is about 2 out. The idle gap should be .75 to 1mm and the high should be around 4.

The engine should drop down to idle much faster then what you are describing. If the needles are set around what I stated maybe it is your linkage that is binding up and keeps the carb slightly open after a wot pull before going back to 0 right away.

How many gallons has it gone thrue? The crank bearings usually need to be replaced after a gallon or so. I use the cheapo avid's with great success.

Another good upgrade that everyone talks about is changing the carb to a losi 3.4 as its a direct drop and in supposed to eliminate all tuning issues.

Although i've been running 2.5's for 13 years and never have had any issues with the stocker.

Hope this helps...
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Old 05-21-2016, 04:23 AM
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How often do you change the plug? I've had idle issues from a worn plug before. I know the od plugs are good but something else to check.
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Old 05-22-2016, 03:27 AM
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Can't remember the last time I've had to change a glowplug, honestly. I have some in-use that are more than a year old and still shiny or light-grey. The plug in this particular engine has had maybe 25 runs on it at the most.
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