Engine Missfire in my 65. - Impala Tech
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post #1 of 9 (permalink) Old 09-07-2014, 08:54 PM Thread Starter
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Engine Missfire in my 65.

Hey guys, I'm experiencing what seems to be an random misfire. It occurs during part throttle acceleration and while holding the throttle at certain RPM's.

Example: holding it in first gear part throttle at 3500 rpm's the engine suddenly bucks or jerks. I've noticed it's generally during high engine vacuum situations i.e. 13-15 hg.

Engine = 283 with Powerglide, Pertronix kit and a rebuilt Carter 4 bbl.

What I've done = Replaced the fuel pump and the ignition coil to no avail.

The most vacuum I can get at idle is 13 hg.

I have not checked the cap, plugs or wires.

The car starts, runs, shifts fine and under full throttle zero vacuum it doesn't miss a beat.

Is it possible that this may be a lean misfire, or too much timing? I do have the vacuum advance hooked to manifold not the "timed" port.

1965 Impala ss 283
Ermine White / Red interior


1970 Chevelle ss L78
Black Cherry / White Stripes

Last edited by sam65; 09-07-2014 at 09:22 PM.
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post #2 of 9 (permalink) Old 09-07-2014, 10:14 PM
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It could be ignition or carburetor issues. Hard to diagnose over the internet.

I can say that the only difference between the ported and full vacuum manifold for the distributor is in exhaust emissions. With less advance (ported) the motor has less power off idle but it could pass a sniff test if you are borderline passing in those few states that still bother to check.

It sounds more like a vacuum leak initially, but you state it runs best at WOT and has a smooth idle which is the exact opposite of the symptoms of a vacuum leak. Your motor should have 18-21 inches of vacuum at idle. The vacuum you have is usually found in cars with a long duration cam.

Only guessing but I will wager you will discover you need a valve job. I would run a compression test and a leak-down test (or have it done for you at a garage).

Big Dave
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post #3 of 9 (permalink) Old 09-07-2014, 10:27 PM Thread Starter
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I'm going to try and plug the vacuum advance and see if that helps, then check my cap, plugs and wires just to cover it all. If all else fails I will take it to the next step and get it professionally checked.

I've owned the car for a year and haven't had this issue until today. It doesn't pop or sputter like it's loosing spark (i.e. weak coil) it's literally a "buck" as if I'm nailing the brakes!

Just curious Big Dave, what in the valve train would cause this, weak springs?

1965 Impala ss 283
Ermine White / Red interior


1970 Chevelle ss L78
Black Cherry / White Stripes
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post #4 of 9 (permalink) Old 09-08-2014, 07:18 AM
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Wear accounts for a lot and the lack of hardened valve seats.

Big Dave
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post #5 of 9 (permalink) Old 10-21-2014, 07:10 PM Thread Starter
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I inspected my cap and rotor and they look good. I have a Pertronix kit in place of points, but I noticed there's a condenser still hooked up?

Could this be causing some interference with the ignition pulse causing a intermittent miss and hesitation?

1965 Impala ss 283
Ermine White / Red interior


1970 Chevelle ss L78
Black Cherry / White Stripes
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post #6 of 9 (permalink) Old 10-22-2014, 09:54 AM
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The condenser is to maximize voltage in the coil. It does this by reversing the polarity in the coil by dumping the contents of the condenser into the coil to completely collapse the magnetic field. This allows the greatest concentration of magnetic flux to build for the next discharge.

I am not familiar with how Petronix points conversion systems work but a factory HEI module doesn't use a condenser.

Have you examined all vacuum lines from one end to the other to verify no loose lines or holes in a cracked hose? If you have I suggest taking the cap off the distributor and grabbing the weight platform on top and wiggling it to see if it moves around as result of a worn brass bushing that holds it in place normally. If you have access to another distributor I would install it temporarily to see if the problem goes away. Same for the carb.

Big Dave
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post #7 of 9 (permalink) Old 10-24-2014, 07:55 AM
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Any play in the distributor shaft will affect how the car runs. I have switched both my 65's to HEI units. The difference is remarkable.
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post #8 of 9 (permalink) Old 11-29-2014, 11:18 PM
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If it didnt miss before and is now after the Pertronix swap....I would get a Used stock HEI and just drop it in.Or a cheap summit one for $75. I'm willing to bet its something electric. The only time I've had an issue with a bad/erratic spark was my power wire to my distributor not being a full 12V and the ignition module going bad. Again not sure whats the pertronix kit involves....but something may be amiss. Especially if its sporadic and at part throttle, off idle, etc.
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post #9 of 9 (permalink) Old 11-30-2014, 12:31 PM
 
 
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Sounds like the distibutor to me , i agree id find a hei and drop it in, atleast to try it out for elimintaton process...around what rpm does it cut?
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