Re: Chasing Rough idle...compression?

Posted by Tim Cole On 2011/5/21 7:48:31

Dear Cortcomp:

Let's see. Is the exhaust smooth or does it feel like random misfires? Random misfires are probably due to vacuum or idle circuit problems.

The compression figures you show are not catastrophic but they aren't great either. I've looked at Packards with uneven compression and poor idle quality that are acceptable on the road because the motor doesn't have enough power to shake the car. If you are lucky your motor might benefit from running decarbonizers through the fuel system.

Disconnect and plug all vacuum lines. Another culprit is the choke pull off.

Another test you can do is called a running compression. In this test you test each cylinder with the motor running.
Such a test will detect broken and stuck rings that cause problems at idle.

The ignition should be showing a good hot steady 20-25KV using a KD tester from NAPA.

After that I don't think it is reasonable to expect proper idle unless the OEM fuel system is being used.

I was looking at an 18th series car with a good Stromberg on it and, after servicing, it ran perfectly. Just like the book and as if it had computer control. I don't run into carburetors that good very often, but when they were new they were really good.

Good luck.