Quote:
Originally Posted by Texasdoc
Chasing a high RPM/high throttle miss. Looking for thoughts - possible things I am missing. It idles fine at 13inHg. I can accelerate at normal speeds just fine. Cruise on the highway is fine too. However, if I start to get into it, it initially feels down on power. If I continue to apply more throttle, it starts to break up, missing pretty badly. I haven't gone WOT yet since I don't want to damage anything in case it is detonation instead of just a miss. Doesn't *seem* like an ignition problem but lots of forums say 95% of misses are ignition related, not carb related.
408W, AFR195 Renegade heads
Performer RPM Intake - 3/4 inch OPEN wooden spacer under carb.
New QuickFuel 750 carb (not sure exactly which one)
New MSD street fire wires
New Autolite 3924 plugs
Cam 230/236 - 112 lobe separation
MSD distributor - verified rotor phasing
Mallory Hi Fire 6AL
Recently reinstalled the intake to confirm no intake gasket leak. All cylinders appear to be firing at idle - headers are all hot.
Verified timing at 36 degrees, all in at 3000. Initial advance at 18. Black MSD bushing, one light silver one light blue spring. Timing appears stable while holding the throttle at 3000 rpm - no timing bounce.
No vacuum advance currently (make sure not over advancing)
No PCV currently - removed and the miss is a little better but not gone.
Verified TDC with piston stop.
Took the carb apart to see what was inside. It is one of their race carbs - no choke. 4.5 Power Valve. Main jets 74, secondary jets 82, secondary plate with PV plug.
Seems to me that the power valve is a little low. MSD says to use half the idle vacuum but I've read that this is not the case - just a possible starting place. What I think is happening is as I get deeper into the throttle, and move out of the transition circuit it goes lean (since the PV hasn't opened yet) causing a lean miss. Removing the PCV system helped since it is making it a touch richer (less unmetered air). Since I can cruise on the highway at 2500 RPMs without problems, seems the jets are fine. Idle circuit is fine. Just goes lean on heavy acceleration.
I have ordered a new PV 6.5 but haven't gotten to install it yet. Any other carb gurus have thoughts on what may be going on?
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This doesn't seem to be just an adjustment, as has been suggested by others.
How long has the Quick Fuel carb been on there?
Have you measured the plug lead resistances, including the coil?
Have the spark plugs been out for inspection/comparison?
Gary