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rough running after long idle or long low rpm driv


Jedi Knight
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I am starting this thread as a result of something captainde said in his overheating TR3 thread.

"after fifteen minutes the engine idle was very rough, to the point where I had to keep goosing the throttle to keep it running, "

I suspect this has nothing to do with his heating problem. BUT I have that same rough idle issue with my TR4A. The engine in my TR4A is all rebuilt as are the carbs. It runs great, starts great and all that. If I let it idle for a long time (say 10 minutes then it runs real rough. The same thing happens if I am driving through a nice area looking at houses and trying not to make noise with my Stebro exhaust so I drive at 1500 RPM tops, after ten minutes or so I get that rough running. If I then run at high RPM for a bit it clears out and all is well again.

I assume its carbon fouling and the high RPM clears it.

Is there something that can be done about this other then high RPM clearing. Like different temp plugs or timing. I think my timimg is right, with just a very slight ping on heavy load .

Not a big deal but I thought there might be some interest and advice on the subject.


Great Pumpkin - R.I.P
Rumor has it that adding a heat shield to the carbs helps a great deal. Apparently modern fuel percolates easier than the old stuff did, so anything that keeps exhaust heat away from the carbs/jets should help. Increased air flow (electric fan) should help too.

Some have even reported noticing increased fuel mileage from adding a heat shield, presumably due to reduced fuel evaporation.


Personally, I just avoid driving slow
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