345 surging

[grnbinder]

Hi everyone. Jim Adams here. haven't posted in a while, however I have a problem that is driving me nuts. Truck is a 1971 1110 4x4,345. The issue I have is, I think, a carb issue. This might be one for carb guru mike mayben. OK, here it is. The carb is, I believe, the good holley. Don.t exactly remember the number, 2210? Anyhoo, start it up cold, choke works, runs great, after it warms up and doesn't have to be fully warm it will not Idle. At idle with no load it surges badly and usually dies. In gear it will surge and definately die. I can keep a little throttle on and it will stay running and not surge. I have been into the carb several times, it used to fix it, not any more. I can use a full can of carb cleaner and get it clean enough to eat out of and it makes no difference. I thought I had it fixed when I disconnected the line to the purge valve and plugged the vacuum port. However after a short time the problem returned. I have checked as well as I can for vac. leaks and cannot find one. Can spray carb cleaner everywhere and not get bit of change in the idle. It is difficult to spray the bottom area of the intake, however. Maybe? Compression runs at 150 last time I checked,all within 5lbs of each other. Dwell is 30, Timing a little advanced, maybe 5 degrees or so past specs. Manifold vac is at 17-18 when its idling correctly. I don't know if it matters, the first time I went through the carb I did not boil it out in carb cleaner, just used the spray cleaner which I thought would be fine as is wasn't really grungy. Thoughts? I also clamped the hose shut that goes to the brake booster, no difference. So there you have it. It is driving me crazy!!! Hope someone out there might have some answers. Thanks, Jim. If anyone would like to call me and ask me something I haven't thought of, my phone number is 503-206-7493. Thanks again.

[Allan E.]

Does the surging continue if you let it warm up and drive it with the vac advance line from the carb plugged? You are describing the symptoms of the vacuum advance having a crack in the diaphram. This would only show up after the thermal switch kicked in, when the engine was partly warmed up. The surging would be caused by the advance, release, advance, release of the vacuum advance, and would cease as the vac advance no longer worked.

If this is the case, you can advance the timing at idle by the amount the vac advance normally provides and disconnect (and plug) the vac feed from the carb. It may be a little harder to start with the timing at 12 degrees, but probably not much. If you just left the timing where it is, the engine might run a little warmer on the highway, but this time of year it doesn't matter that much.

The next thing I would look at would be other sources, such as a worn distributor, that could affect the stability of the timing or vacuum. A choke not properly opening might do the trick, like the "cold" and "warm enough to run" adjustment isn't done properly. If you are running an automatic choke, it should be adjusted open all the way just as the engine is fully warmed up, but just barely. Kind of a pain to get it right.

[grnbinder]

Thanks for the reply Allan. I realized after I had posted that the vac. advance was the one thing I had not even thought of. I will be checking that soon and post the results. Weathers real nasty right now and I have no garage so it will be a few days. Jim