Engine “miss” or stumble - ‘68 442

[Jrbrks]

I have a 1968 442 with factory A/C.

The 400 ci engine is completely stock, except for an electronic ignition module in the distributor.

With the A/C on, it feels as if there is a miss or stumble in the motor.  It is especially noticeable at reasonably steady speeds.   It does not seem to be missing or stumbling on hard acceleration.  I am not getting any “pinging” or other noises, when it is missing or stumbling.

As soon as I turn off the A/C, the miss or stumble stop occurring (or, it becomes unnoticeable).

I have had the car about nine years but driven it less than 4,000 miles.  I have not changed the plugs, plug wires or distributor cap, since I have purchased the car.

 

Thoughts on first things to troubleshoot or the correct approach to identify the cause?

This issue has only just started within the last 30 days.

 

Thanks

Jim

[joe_padavano]

More likely, the miss becomes less noticeable with the A/C off due to the reduced load on the engine. How long has the electronic points conversion been in place? Is it Pertronix or a different brand? What coil do you have? I would also check for a vacuum leak.

[edinmass]

Use an ignition scope and a five gas exhaust analyzer. A slight miss could be a bunch of different things on this car. Don’t go changing parts or adjusting anything. It will make it ten times harder for a technician to find. 

[Jrbrks]

Joe,

The electronic points conversion has been in place since April, 2013 and it is a Pertronix (1181LS).  I believe the coil is the stock Deco-Remy coil that has been in the car, since my purchase 9+ years ago.  I do mot believe it has ever been replaced.

 

What might be the best method to check for a vacuum leak?

 

Additionally, I recently had the heater control valve replaced and the A/C switch wired to turn the compressor off without stopping the engine.  I noticed the issue after these repairs, but I do not know if they are related.

Thanks

Jim

[joe_padavano]

3 hours ago, Jrbrks said:

Joe,

The electronic points conversion has been in place since April, 2013 and it is a Pertronix (1181LS).  I believe the coil is the stock Deco-Remy coil that has been in the car, since my purchase 9+ years ago.  I do mot believe it has ever been replaced.

 

What might be the best method to check for a vacuum leak?

 

Additionally, I recently had the heater control valve replaced and the A/C switch wired to turn the compressor off without stopping the engine.  I noticed the issue after these repairs, but I do not know if they are related.

Thanks

Jim

 

The first thing to do would be to put a vacuum line on it and see if vacuum is normal or low. If the vacuum lines were messed up when the heater control valve was replaced, that would explain why you only have a vacuum leak when the A/C is on. One option would be to disconnect the vacuum line to the A/C (and any other options except power brakes and the AT) and cap it on the engine. Drive the car and see if the misfire is still there. If not, you know where to look.

[Jrbrks]

Thanks Joe.

Specifically, where would I connect the vacuum hose and gauge to look for a leak?

 

I looked at the vacuum lines under the hood and they appear correct.

 

I did not look under the dash, but I do not believe those were disturbed at the last repair.

 

My 442 is a 4-speed, but it does have power brakes.

 

Thanks

Jim

[joe_padavano]

Connect a vacuum gauge to any port that provides full time manifold vacuum - basically anything that's screwed directly into an intake runner.

[Jrbrks]

Thank you, Joe.

I will try this.