Help! Ok. . .misfire at idle. . .with leakdown test passing. . .what could it be?

[Bolisk]

Hello all!

I'm still trying to root cause my misfire at idle from my passenger side bank. Here is a video of what it sounds like:
https://www.youtube.com/watch?v=d_pNIEegRt0

The driver side runs nice and smooth. . .and the puffs at the exhaust pipe are all even and nice.

The passenger side spits and puts. . .you can tell when the camera shifts to the passenger side.

I performed a leak down test on both banks of the motor ( when cold ) and all the cylinders checked out at about between 10 and 15% loss. During the compression stroke. . .the only air noise was from the oil fill cap (e.g. past the rings). There does not appear to be any leaking exhaust or intake valves as far as I can tell.

The vacuum at idle is nice and steady around 20. However every couple of seconds or so. . .when it seems like the engine misfires. . .the vacuum will drop by no more than one point.

I did have at one point the spark plug gaps too tight. . .and once I fixed that gaps. . .it ran a ton better. . .but it still does it. . .just much less frequently than in the video provided.

I have not performed cranking pressure test yet.

I would have bet money that this was some kind of weak spring or bad exhaust valve. . .but now I have no evidence to suggest any of those.

This only appears to be an issue at idle. . .however, the car will backfire (out the right exhaust) when the car is downshifted and the rpms go up while not applying any gass (engine breaking).

Any ideas?

Plug wires?
distributor? (I have brakerles SE electronic ignition in it)
Coil? (would expect a problem on both banks then)
fuel dilevery?
Exhaust poor back pressure? (I do have headers with poor fitting pipes just down stream of the collector.)

Any help would be appreciated.

-Jon

 

[mmesa005]

Hello all!

I'm still trying to root cause my misfire at idle from my passenger side bank. Here is a video of what it sounds like:
https://www.youtube.com/watch?v=d_pNIEegRt0

The driver side runs nice and smooth. . .and the puffs at the exhaust pipe are all even and nice.

The passenger side spits and puts. . .you can tell when the camera shifts to the passenger side.

I performed a leak down test on both banks of the motor ( when cold ) and all the cylinders checked out at about between 10 and 15% loss. During the compression stroke. . .the only air noise was from the oil fill cap (e.g. past the rings). There does not appear to be any leaking exhaust or intake valves as far as I can tell.

The vacuum at idle is nice and steady around 20. However every couple of seconds or so. . .when it seems like the engine misfires. . .the vacuum will drop by no more than one point.

I did have at one point the spark plug gaps too tight. . .and once I fixed that gaps. . .it ran a ton better. . .but it still does it. . .just much less frequently than in the video provided.

I have not performed cranking pressure test yet.

I would have bet money that this was some kind of weak spring or bad exhaust valve. . .but now I have no evidence to suggest any of those.

This only appears to be an issue at idle. . .however, the car will backfire (out the right exhaust) when the car is downshifted and the rpms go up while not applying any gass (engine breaking).

Any ideas?

Plug wires?
distributor? (I have brakerles SE electronic ignition in it)
Coil? (would expect a problem on both banks then)
fuel dilevery?
Exhaust poor back pressure? (I do have headers with poor fitting pipes just down stream of the collector.)

Any help would be appreciated.

-Jon

check for crossed plug wires. ignition timing incorrect. or cracked distributor cap or spark plugs.

 

[Bolisk]

I actually figured this out about 20 minutes ago.

First I got my hands on a known good quadrajet. . .thanks Hib!!!

After installing it and firing the car up, I found that the car still ran poorly. Still I decided that I should adjust the carbs idle mixture and idle speed setting and see if it got any better. So I hooked up my vaccume gauge and service tach to set everything up. Normally I walk around the front of the car to adjust the other idle screw. However this time I went behind the car and found that the car was running perfectly!!!!! So I adjusted the idle mixtures, then unhooked everything and prepared for a test drive. When I walked back arround the back of the car, I was horrified to find that the car was running like crap again. Damn it!!!!

So I hooked everything back up to readjust and found once again. . .that the car was running great!!! Ah!! Must be something with my tools. So I went and disconnected my vacuum gauge, and reconnected the line back to the car and ah ha!!!! The car ran like crap again. I have a vacuum leak in the car somewhere!!!

I put the old car back on for giggles...but mostly to see how I missed this obvious condition for the last year. Turns out it was a combination of bad carb and vacuum leak!!!

Anyway, the car runs great and I don't need to redo my heads. . .well not anytime soon anyway!

On to debugging my vacuum system!!

Regards,
-Jon