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The engine is just rebuilt with everything,all testing proves out OK but the engine runs perfect up to about 2500 rpm and then what they call revolsion,I call it blowback through the carb but does not backfire,it just falls off and blows fuel up through the carb.I'm out of ideas ,anybody got any ideas cause I could sure use them
Is this in neutral or underload? Need to know more about the engine, cam, valvetrain, etc. Could have the timing chain set wrong, valves too tight? Anyway, welcome to CHR! Lots of knowledgeable folks on here and all will be glad to help you sort this thing out!
First thing that comes to mind is that the valve lash is too tight. Next in line, in my mind anyway, would be weak valve springs.
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that sounds like a flat cam to me ...ted
Hi Ted Tks for the reply.I didn't think that it was the new cam because the leakdown test showed 170# in each cyl and didn't lose any so I ruled it out and the engine runs perfectly up until the problem starts.I've had 2 different intakes on it ,2different ignitions on it ,mallory Hy Fire and a new oem ignition,5 different carbs and loosened the rocker arm shafts 1/4 turn at a time up to one full turn with no change,regsapped the trigger two different times,disconnected one carb on the duel carb setup with no change.I've not been able to make it run better or worse.Nothing seems to make a difference..John(shadyj)
I had a clogged exhaust system do this (muffler). And, if equipped with an EGR valve, the valve sticking can cause this also.
Times TWO! I've had plugged cat.converters do this. Put a vacuum gauge on it and see if the readings fall to zero. It could easily be the engine can't "breathe". Also double wall pipe separation is a potential problem if oem pipe was used. Again, a vacuum gauge can help diagnose.