78 Carb pop on start

[Ervin S.]

My friend has a 78 L48 Automatic that has idle problems and pops through the carb on start-up. It starts, however it almost always pops back through the carb on firing. This is not a loud backfire, rather like a small pop one time on initial firing and start.It also has a rough idle and will die easily when idling.

Any suggestions on potential causes would be appreciated

[Tom H.]

Re: 78 Carb pop on start

Have you checked the timing ?? Also make sure carb is sealed securely to the intake.

[Stuart F.]

Re: 78 Carb pop on start

I agree with Tom about timing first. Also look for a vacuum leak causing a lean condition. There are a lot of vacuum lines on the 78's that could be broken/loose, etc. You didn't say how well or not it drives (besides idle).

Stu Fox

[Ervin S.]

Re: 78 Carb pop on start

Have you checked the timing ?? Also make sure carb is sealed securely to the intake.

According to him the timing was recently set, however I have not checked it myself and will do that. I have found serveral vacuum leaks and he has repaired them. The next step is to spray carb cleaner around the base of the carb to set if it speeds up the idle.

Any suggestions on methods to find vacuum leaks more easily?

It sounds like the thoughts are that a lean condition is causing the pop on startup. I would have thought that a vacuum leak would result in the rough idle and a rich condition, not lean.

What about a possible exhaust valve not seating and causing the pop on startup?

Thank you both for your thoughts and help.

[Tom H.]

Re: 78 Carb pop on start

A valve issue would be my next area to explore if timing and vacuum leaks check out OK. How does the car run on the road ?? Good power ? Any drivability issues there ??

[Ervin S.]

Re: 78 Carb pop on start

The drivability report is that performance is down, all though an L-48 in 78 is not high performance with the limited HP. It was driven back from the IL Chapter meet this weekend.

[Tom H.]

Re: 78 Carb pop on start

Has any recent work been done or did this condition just show up for him ?? Just thinking out loud trying to help pinpoint the problem. Is this a high or low mile car ?

[David S.]

Re: 78 Carb pop on start

Does the car have an A.I.R. Smog system? We had a 69 Z28 that had a "popping" issue that was actually the smog pump. Once it was rebuilt the popping went away. Then again, you would here the popping on more occasions than just the start, and you wouldn't lose tremendous power.

-Dave

[Ervin S.]

Re: 78 Carb pop on start

The car does not have an AIR system.

The car has been having progressively worse issues. He did have a local mechanic look at the car within the last few months. This mechanic missed at least three vacuum leaks. I have no idea of exactly what efforts he did, except that the issue persists.