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Pinch Test

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MatthewJ

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I'm trying to tune my tmaxx 2.5 but it's not going well. When I do the pinch test the engine turns off after 2 seconds, but it does not speed at all. What do I have set wrong ?

-Matt
 
That probably means it's a bit lean on the LSN.

http://www.rchobbies.org/cars_eng-tuning.htm

rchobbies.org said:
LOW-SPEED NEEDLE
The high-speed needle is dialed in, so now let’s properly set the low-speed needle. It’s important to set it last because it simply regulates the fuel that flows from the main needle at low throttle settings. Lean out the main needle, and you automatically lean the low-speed needle as well. For this reason, it’s wise to finish with the low-speed setting.

Again, it’s imperative for the engine to be at full operating temperature. There are many methods of testing the low-speed needle setting; one is to pinch the fuel line. When you pinch the fuel line, the engine rpm increase slightly. Keep pinching it, and the engine will eventually stall. If the engine rpm increase dramatically, it indicates that the low-speed setting may be too rich. Or, if the low-speed setting is already too lean, the engine rpm may not increase much at all, and the engine will stall rather quickly. It’s a somewhat crude method and doesn’t tell you what to expect from the engine on the track, but it will get you into the ballpark.
Click to expand...

But you may want to do as it suggests, get your HSN closer to correct prior to tuning your LSN, unless you already know it is.
 
I think my problem is my hsn is not set correctly. Here's what's happening.

My engine is broke in.
Today it was 75 F (25 C) outside. My altitude is 0.
I reverted to stock tuning as a base for my engine tuning.
The truck idles fine.
Should there be blue smoke while the truck is idling ?

I've been making small adjustments to my hsn leaning it as per tmaxx manual.
I still can't get it to hit 2nd gear.

My engine hit 320F during my testing, I'm sure the engine is already pretty rich, so where is this heat coming from.

Finally, all of a sudden, my truck won't start. After about 20 min of running it dies and won't stay running. IF I can get it started, it dies almost instantly. Any Ideas ?

Thanks.
 
If your engine is hitting those kind of temps despite being rich (which you said you are sure it is) then you have one hell of a serious air leak somewhere. It is leaning you out so bad that a full rich condition will not help it. It is also why the pinch test results were like that.

You really might have cooked that mill. Don't get it that hot again till you figure out where your leak is!!!

As for the engine not running now: One of two things--a soaked plug from you masking the lean condition with the excessively rich tune, or a dead plug.
 
Last edited:
I'm in the process of removing the engine to check for a leak, but I can't find any documentation on how to remove the engine.
 
It's been well documented that the 4 screws directly under each corner of the engine hold it on.;) Seal the back plate and the carb neck and make sure the head bolts are tight. There's a ton of info on this site explaining engine sealing.
 
MatthewJ said:
I'm in the process of removing the engine to check for a leak, but I can't find any documentation on how to remove the engine.
Click to expand...

Engine removal is pretty straight forward. I find that it is usually easier to remove the engine mount from the chassis than the engine from the mount. Once you remove the bolts, disconnect the pipe, fuel lines and throttle linkage, engine comes out.

For sealing an engine, go to post six in this thread:
https://www.rcnitrotalk.com/forum/showthread.php?t=60832
 
So i could not find any air leaks. I still we ahead and sealed the engine as per the video and thread above.

I reset both needles to factory but still having issues. Once i got it running, i drove back and forth the driveway a few times and the engine temp was up too 300 again.

I'm at a total loss as how to get this working.

-Matt
 
welcome to my world..

bump for any ideas..been having the same issues too. if i can get it running it just runs hotter and hotter no matter what i do..getting ready to erp it..
 
I had the same problem. Ended up being the head bolts. I "forced them" a tad tighter. About a 1/4 turn. It solved my problem. But crank them at your own risk. I was ready to let my maxx sit until I could erp. I pretty much got lucky. All I know is the more times that motor gets that hot, the less likely you are to ever get it running right. I'd get a new head gasket and re-torque the bolts. I can't think of any thing else that might help. Make sure there isn't any debris in the hsn.
 

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