1979 L48 4 speed, mild cam, recurved distributor, edlebrock 1406, daily driven, fuel regulator set to 5.5 psi

Had the car dyno tuned about 5 months ago when I installed a dual exhaust. Drove great for about 4 months, then noticed some surging at low rpm loads and some popping in the exhaust. Clutch went out, so it stood in jackstands for about 2 weeks. After I got everything back together, surging is still there, if not even worse. I read seemingly every forum on here, and did some troubleshooting

Most people said surging is from being lean or too advanced. I dont know how I could suddenly be lean in just one section of the powerband, idle and WOT performed the same. So I checked ignition, I pulled the vac advance off and drove around, surging gone, popping in exhaust gone. So i pulled out my timing light, and everything was the same!? 36* total at 2800. I had to detune all the way down to 22* total to get the car to drive without surging, with vac advance hooked up. Thats not right. Was absolutely dead on hard acceleration, sounded really sick too.

so now i’m pretty lost. My next effort was going to put larger primary jets in, and see what that does. I checked vacuum hoses, they seem ok, i’m thinking about buying a vac hose kit as my plan C.

any thoughts appreciated

 

Do you have a MityVac? I use an adjustable vac can that limits vac advance to 12 degrees at full vac. There are other ways to accomplish this, and Lars has them in his papers. How much advance do you get just from vacuum?

 

If it ran well after dyno tuning, believe me, jet sizes will not suddenly change. Popping as you describe it is caused primarily by a lean condition. you have something plugged up or not working in your carburetor.

 

You have developed a vac adv problem. It's not a carb issue. Larger jets and it will run rich and poorer fuel economy.

 

Put it all back the way it was. It sounds like you already checked the advance curve and it is working properly. I believe you have a vacuum leak. Start with a thorough visual inspection looking for any disconnected vacuum hose or cap. If all checks out, disconnect all vacuum lines from the engine, except brake booster and distributor. Then plug the ports on the engine they connected to and go for a drive. If it is fixed, you know where the problem is, if not use a vacuum tool to check your distributor and brake booster.

 

Is your popping at idle, or also at part-throttle load?

If it is at idle, add the search term "afterfire". I cured mine with proper timing (hooking up the vac advance to manifold vacuum), and idle mixture adjustment (I used a vacuum gauge for this part).

As for the part-throttle surge/stumble, perhaps you have a vacuum leak (or timing change) on a ported vacuum line? EGR? Following @Factoid's suggestion should identify it. A vacuum leak can also be in the carb base, or even in the intake manifold.

 

With his vac adv disconnected, the car ran fine. He may be getting too much vacuum advance when connected. Hook up timing light and check timing with vac adv connected. If its +50* at 3000 rpm, that might be the problem.

 

Maybe a spring on the mechanical advance broke or came off and it's over advancing at idle. Or the mechanical advance is seizing at an advanced position.

 

It pops under part throttle load. Idle is fine and never changed since dyno tune. I bought some carb cleaner, i’m gonna take it apart, shoot some cleaner through all of the passages, reassemble, then shoot some around the assembled carb to check for vacuum leaks

 

I believe it was set to 52* total +vacuum. It has the stock vac advance. Ran fine that way for the last year though

 

If it's timing related how could the vacuum advance suddenly be adding in timing? That is limited physically by the slot on the can.
What is the timing at idle without vacuum? and again at 1200-1500 RPM? If it's advancing prematurely then look at the mechanical advance. That can change.

If that checks out then look into junk in the main jets or float level change, float inlet needle obstruction, etc. Something that's going to make the mixture leaner somehow.

 

Re-check your timing curve. Odd your car runs fine with vac disconnected. Don't touch the carb until you confirm you have a proper functioning ignition system.

 

Well guys... sometimes its the easy stuff that gets ya. I tore down the carb this morning, cleaned all of the passages. Put back together, car ran the same. Looked over vacuum lines again, carefully. Didnt notice anything. Went to the auto store and grabbed a vacuum plug kit, plugged all of the ports on the carb. Then look ked at my ported vacuum port, which is plugged. Thought, ‘might as well take it off and put a new one on, and when I did, the plug disintegrated and ripped wide open in my hand. Put a new plug on, reconnected everything else, and she ran perfect. If the plug showed any signs of wear, I missed it. Thank you everyone for your help.

 

It’s always the little details. OCD tendencies can be your best friend with old cars.

 

Sounds like Factoid got it right. Thanks for reporting back. It was lean but not for the reason you thought is all.