67 250 runs rough, backfires when shut down.

[67CHedges]

My 67 C-10 250 three on the tree is not running as smooth as it used to.

The symptoms are:

1). It seems like it is running on 5.5 or five cylinders. You can really tell when you are trying to pull a hill.

2). When I shut it down after it operates under normal operating temperature, it has a loud backfire.

We have recently converted the distributor to a HEI pointless ignition and replaced the carb a few months before that. The carb has never worked as well as the old one.

I have tried to troubleshoot and have done the following.

1). reset the timing several times.
2). verified the wiring, distributor and coil.
3). I checked the compression to see if I had a valve problem. The book I have says the compression is 130 psi. From front to back, starting with the first cylinder, I range from 125-150. Cylinders 2-6 are within 5-10 pounds. The first cylinder is about 20 off from the others. Don't know if that is contributing to the problem or not, but wanted to check it anyway.

I have not changed the plug wires, they are new, but I know that doesn't necessarily mean they are good. I checked the gap on the plugs when I checked the compression, they were all close, if not right on.

I am frustrated and wondered if anyone has some tips before I hand it over to a garage. I checked some of the archieves and didn't see anything that addressed my issue.

I am wondering if we have the distributor off a tooth? Or if the new carb is the culprit.

Thanks in advance.