Bad misfire

truckman · Jan 5, 2009

Little history on the motor its a remain w/ 27,000ish on it has a comp cams stage 2 cam, shorty headers, offy intake and holley 390cfm, dura sparked w/ the GM 4 pin unit.

Now for the problem i put a new dist. from napa in after i trans plated the motor in to my new truck. got a new modual, new cap rotor and plugs and wires.

Go to fire it up runs ok at idel set base timming at 10deg. go to take it for a drive it runs ok at low rpm like just a little throttle. Put it under any hard load it will kick and buck take off and sounds like its missing on 1 maybe 2 cylinders let it go back to idel it runs fine and free revs fine a lil pop out the carb every now and again.

I've played with the timming for 2 days and had now luck i've never had this problem before with this.

I'm looking for suggestions to fix the miss fire.

Thanks,
Zack

Original_Ranger84 · Jan 7, 2009

I did that GM 4 point module too and am also having troubles with misfiring... or was only when its cold out now. I don't have any performance mods though... I think I ended up with 6-8 degrees right now but I'll have to double check, I should note that while timing my truck, the mark seemed to move when I revved it up even with the vacuum advance off and I am puzzled, does yours do that?

Jorley · Jan 7, 2009

truckman said:
Little history on the motor its a remain w/ 27,000ish on it has a comp cams stage 2 cam, shorty headers, offy intake and holley 390cfm, dura sparked w/ the GM 4 pin unit.

Now for the problem i put a new dist. from napa in after i trans plated the motor in to my new truck. got a new modual, new cap rotor and plugs and wires.

Go to fire it up runs ok at idel set base timming at 10deg. go to take it for a drive it runs ok at low rpm like just a little throttle. Put it under any hard load it will kick and buck take off and sounds like its missing on 1 maybe 2 cylinders let it go back to idel it runs fine and free revs fine a lil pop out the carb every now and again.

I've played with the timming for 2 days and had now luck i've never had this problem before with this.

I'm looking for suggestions to fix the miss fire.

Thanks,
Zack

When I did my duraspark, I didn't have a timing light, so just set it by ear. With the mods you have the normal base timing may not work. Try letting the engine warm up and turn the dizzy until it runs smoothly. Another thing, the Holley 390 cfm is way more than it needs, the stock feedback carb is only 287 cfm. Do a search in the forums, as there are other threads about the Holley carb.

Original_Ranger84 said:
I did that GM 4 point module too and am also having troubles with misfiring... or was only when its cold out now. I don't have any performance mods though... I think I ended up with 6-8 degrees right now but I'll have to double check, I should note that while timing my truck, the mark seemed to move when I revved it up even with the vacuum advance off and I am puzzled, does yours do that?

The duraspark dizzy has an internal centrifugal advance in addition to the vacuum advance. So even with the vacuum advance not hooked up, the timing will still change.

Original_Ranger84 · Jan 7, 2009

Jorley said:
The duraspark dizzy has an internal centrifugal advance in addition to the vacuum advance. So even with the vacuum advance not hooked up, the timing will still change.

That would explain it... Thanks. :icon_thumby:

Bad misfire

truckman

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Original_Ranger84

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Jorley

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Original_Ranger84

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