Rough Idle (Missing) Under Load

[glrestomod]

**UPDATE**

So sorry for the long delay! Wanted to give everyone an update and thank everyone again for their help!

Following the advice I received here we did the following:

1. Checked the tuning on the newly rebuilt carburetor and adjusted it appropriately.

2. Pulled plug wires one at a time while engine was running and found one plug wire (despite being new) was bad. Ordered a new set that I’ll put on this weekend that are specific for my application instead of universal fit like the current ones.

3. The chrome hasn’t worked since we put the rebuilt carburetor back on. Found the choke adjustment (has electric choke on a 600CFM single pumper Holley 4160) was way far to the right past the marking range on the carburetor. Also, found that the linkage to the choke that were supposed to be underneath the red fast idle cam was above it and wasn’t operable. We fixed that and tried adjusting the fast idle cam screw but can’t get the RPM’s to increase. We’ll look at that more this weekend too.

4. Car backfired (at the intake) while revving it up after putting an old plug wire on in place of the bad one. My friend said that it’s got a timing issue.

So this weekend we will check those things and add the spark plug wires.

Any other advice or suggestions would be much appreciated, as you have all been helpful so far!

 

[Don Frelier]

If pulling a plug wire doesn't change the engine speed or sound it doesn't necessarily indicate the wire is bad.
It just tells you that that cylinder isn't producing combustion.
Since you put on another old wire and it didn't change (start firing) it's likely something else.
You could try the supposedly bad wire on another cylinder as a test but I'm guessing that cylinder will fire just fine.

It could be one of several other things:

1. Spark plug - sometimes they are bad right out of the box
2. Bad cap on that terminal tower
3. Bent/broken push rod
4. Stuck/burnt valve
5. Bad cam lobe
6. Some other reason internally for no compression on that cylinder

All this assumes it's a problem on just the one cylinder you've mentioned and the other 7 are working OK.

 

[glrestomod]

If pulling a plug wire doesn't change the engine speed or sound it doesn't necessarily indicate the wire is bad.
It just tells you that that cylinder isn't producing combustion.
Since you put on another old wire and it didn't change (start firing) it's likely something else.
You could try the supposedly bad wire on another cylinder as a test but I'm guessing that cylinder will fire just fine.

It could be one of several other things:

1. Spark plug - sometimes they are bad right out of the box
2. Bad cap on that terminal tower
3. Bent/broken push rod
4. Stuck/burnt valve
5. Bad cam lobe
6. Some other reason internally for no compression on that cylinder

All this assumes it's a problem on just the one cylinder you've mentioned and the other 7 are working OK.

Thanks for the quick reply! My mechanic friend checked it for me and said it is “leaking electricity,” as it shocked him as he pulled it off and put it back on (he had a rag over his hand when he did it though).

They were universal for plug wires and didn’t really fit snug in the distributor anyway. Just got the new ones installed and will do more testing. Thanks again for the reply, will definitely look into all of the possibilities you mentioned above!

 

[glrestomod]

**UPDATE**

I replaced the plug wires, we adjusted the timing and I had a carburetor expert check out the carburetor. He took the carburetor off and tore it down again (despite it having just been rebuilt by a friend). He found the choke wasn’t setup properly, did some more cleaning and it he mentioned that it wasn’t level around the edges of where the floats are. He straightened them out and then we spent a couple of sessions adjusting the floats and it runs great now!

Thanks to everyone who commented and made suggestions! There is still a slight miss here and there so we’re unsure what it might be, although we suspect it’s because I have a bad ground somewhere (it’s obvious I do when the headlights are on, as the drivers side hood mounted turn signal stays on while driving and burns like a light).

Thanks again to everyone!