engine misfire. Very tricky

[buffblazer]

ok so i have talked about this before but nobody really posted on it.

I have an 86 k5 with a 350, bored .30 flat top pistons, stock heads edlebrock performer intake. holley truck advenger 650. headers, no emissions at all.

now the truck runs great, if you ease into the throttle in any gear it will run as smooth as a new truck. but if you put it to the floor it will misfire and pop through the exhaust around 2500-3200 rpm and then clear itself out.


Im running an SM465. it will not do this in first gear, just second and third. and the weird thing is, it will not do it on the dyno,


so what im trying to say is. on a dyno sitting still, it runs like a champ. but on the open road it misfires.


never did this before the shop put the holley on it so i was assuming that was the root of the problem. on the dyno it runs a perfect 14.7 air/fuel.

now the misfire is the type that you would get when the engine was cold. i have a 205 thermostat in it and yes it is colorado but the other day it was 50 out and it still did it.

the shop is dumbstuck and im out of ideas. what says CK5?

 

[djtejas]

have you checked the mechanical timing in your distributor?

i had a similar problem and it was all my total timing coming in too soon...

i used Crane cams adjustable vacuum advance and mechanical advance springs...i set the adjustable vacuum advance to 8 degrees instead of the stock 20 degrees and i tried different springs in the mechanical advance until it ran like i wanted it to...
it seems that the heavier vehicle liked the total timing coming in later than lighter vehicles i have had in the past

 

[4X4HIGH]

There are several issues with Holley carbs and once they are fixed they are an excellent carb. The easiest to fix first the the accelerator pump adjustment and NOBODY i've talked with yet understands how this is done correctly (well i know how). You might need a 50cc accelrator pump though as it sounds like you have an off idle BOG which means not enough fuel in the transistion between the idle fuel circuit and the main fuel circuit.

 

[Thunder]

The acelerator pump was my first thought also. Sounds like your engine is going lean under high loads untill the power valve opens fully and the main fuel circut gets up to speed.
Speaking of the Power valve. Yours may be sized worng for your engine. causing the fuel enrichment to come in late. That will cause mid rpm problems on hard aceleration too.
Rule of thumb is: Power valve should open at 1/2 of the idle vaccum. EX: If your vaccum at idle is 16 HG (Engine warm in nuteral manual trans, Auto trans should be in gear) you should be running a power valve marked 8 or 7.5.

 

[buffblazer]

have you checked the mechanical timing in your distributor?

i had a similar problem and it was all my total timing coming in too soon...

i used Crane cams adjustable vacuum advance and mechanical advance springs...i set the adjustable vacuum advance to 8 degrees instead of the stock 20 degrees and i tried different springs in the mechanical advance until it ran like i wanted it to...
it seems that the heavier vehicle liked the total timing coming in later than lighter vehicles i have had in the past

the shop said they adjusted the timing correctly when they were trying to figure out the problem.

 

[buffblazer]

There are several issues with Holley carbs and once they are fixed they are an excellent carb. The easiest to fix first the the accelerator pump adjustment and NOBODY i've talked with yet understands how this is done correctly (well i know how). You might need a 50cc accelrator pump though as it sounds like you have an off idle BOG which means not enough fuel in the transistion between the idle fuel circuit and the main fuel circuit.

im not much of a holley guy, but is the accelerator pump hard to change? what makes it so hard to understand how to adjust it?

 

[buffblazer]

The acelerator pump was my first thought also. Sounds like your engine is going lean under high loads untill the power valve opens fully and the main fuel circut gets up to speed.
Speaking of the Power valve. Yours may be sized worng for your engine. causing the fuel enrichment to come in late. That will cause mid rpm problems on hard aceleration too.
Rule of thumb is: Power valve should open at 1/2 of the idle vaccum. EX: If your vaccum at idle is 16 HG (Engine warm in nuteral manual trans, Auto trans should be in gear) you should be running a power valve marked 8 or 7.5.

i know the shop put in different jets but i can ask them if they touched the power valve.

 

[mrdrinksalil]

I agree with the ACC pump. I have a 670 truck avenger and EXACTLY the same engine setup as you. it would do the same thing. Getting a lean condition and the backfire under load when you stab the throttle. In the driveway it's fine but you get against a rock and get into the pedal is would stall or backfire sometimes. The accelerator pump was fine, diaphram was good ect. BUT holley makes different accelerator pump cams that control how much fuel the acc. pump pushes. Mine came with the orange one and I had to get a larger one to correct my lean condition under load. Just like someone said previously the ACC Pump controls fuel between the idle circuit and the main jet circuit. If you are missing off idle you wanna look at acc pump on a holley.

You could also have a power valve issue. Which was a lot more common on older holleys because they didn't have a check valve to protect the power valve during a backfire but your carb should have one (after 92)

http://www.summitracing.com/parts/HLY-20-12/

This should also help.

http://www.holley.com/data/TechService/Technical/Carburetor%20Tech%20Info.pdf

And if that doesn't work, do what I'm doing...

http://www.binderplanet.com/forums/showthread.php?t=45292