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Light acceleration misfire?
My 440 runs quite well except for two issues: horrifying mileage and a misfire under light acceleration. They may be intertwined.
The 440 is a KRE 85cc headed 400 with 4" stroke and 0.065 overbore. Mild roller cam with Edelbrock ProFlow 4 fuel injection. I suspect electrical gremlins, perhaps coil? Engine idles rock steady - though timing varies 2-3 degrees, not sure if this is due to a mediocre timing light or other issue. |
Light acceleration misfire? First thing that occurs to me is excess spark advance.
Of course lean fuel mix including a vacuum leak into one or more cylinders could be a factor. |
Agree. I would guess that the misfire is caused by too much vacuum advance under light throttle. Would suspect that slowly bringing the engine revs up in neutral probably will cause the same misfire.
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I am working on fine tuning a pro flo 4 as well. What are your settings for cruise and accel AFR targets? What are your timing settings for idle spark, advance start, total spark and vacuum advance? What is your IAC % at idle when fully warmed up?
So far, I am very impressed at the drivability of the system and have found that fine tuning my spark tables has paid off the most in gettting a very smooth and effortless accelleration at mild throttle. My only issue is a surge at 65-70mph that equates to 2700-3000rpm in my car. Pretty confident with some more seat time I will get that ironed out too. |
I had similar where under light load I was getting misfire chased it for weeks and turned out to be plug wires.
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If your hearing the misfire out the exh then it’s most likely a tuning issue or you could even have the firing order wrong.
If the misfire is causing popping back thru the Carb then you may have a intake valve set too tight. If all was fine and this issue just started after doing something to the motor, then it’s related to that last thing you did more then not! |
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Tom V. |
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I did examine the firing order - it is correct. Quote:
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For me in the past it was ignition related. i have had it with fouled plugs, poor wires.
When a plug goes to fire, the coil output builds up to the point when the mix ionizes, then discharges thru the spark. As the mixture density increases, (more throttle) , the voltage has to build higher (to the limits of the coil) to the point where the mixture ionizes and the spark discharges. At the increased voltage, poor plugs and wiring start to show up. George |
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I started this morning with the task - clear eyed and open minded. Pulled off the driver's side wires and then went to pull off the passenger side when I noticed this:
https://youtu.be/H8Qx8-C_ZAI WTF? Is the cap loose? I removed the remaining wires then the cap. Distributor still moved. Mnnh. I noted the middle bowl moved while the lower housing remained secure. Pulling the dizzy revealed all 4 upper bolts (red arrow) were loose enough to allow slippage. No lock washers nor locking fluid evident. Green arrow holds upper bowl. http://forums.maxperformanceinc.com/...1&d=1630873435 Thanks Edelbrock :mad: Pulled the shaft and removed the screws one at a time, adding a drop of Locktite and resecured. Reassembled the distributor, reinstalled, set timing and road testing shows that this was most likely the issue. 62posbonny: tune map I am using is #2019; pretty mild. Will examine advance curves and fuel more in the future as I am sure this can run much smoother. I wonder if there is a group that does this with Pontiac V8's only? |
Glad you found the problem! Makes me glad again that I went with a crank mounted trigger wheel.
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