Good afternoon, I have a 1970 Buick Skylark, 350 W/ 2bbl Rochester 2GV. The engine has been refreshed,with approx 5,000miles on the engine. Before the refresh, I had a slight popping(thup-thup) out of the tailpipe( duals w/iron manifolds) it is timed correctly, with no issues on vacuum. Valves and compression where it should be.Have any of you had this issue? It goes away at 950-1000 RPM, if idle speed is raised. Slight cam profile, but otherwise stock. It still does this,before and after. Anyone else have this issue? Thank you!
When refreshed , what was done? Was this just a ring and bearing replacement?New pistons? Actual machine hone? Actual valve job? Carb rebuild? Distributer rebuild? did you replace rocker shafts on refresh? change plugs, wires, cap, points and condenser?
New carb,distributor and rockers. Rings,and bores cleaned,with very minor honing. Complete valve job with hardened valve seats. All ignition is new w/wires on a pertronix distributor conversion.
No sure what "popping" is. Is it whn you back off the gas? Idling? Could be something as simple as an ignition missfire. Could be a bad exhaust valve. Could be a lean miss from the carb Here's a quick and dirty method to check the exhaust valves. Take a piece of paper and put it against the tailpipe. If its a bad exhaust valve the paper will get sucked to the tailpipe when it 'pops"
That was my initial thought as well. Luckily, the paper trick yielded no bad results. It is only when idling, and sporadic. I have checked for the dreaded bleed over and wire cross, everything seems to be in order.
Then I would check the mixture on the carb. You can try the old school method and enrich the mixture with propane. See if it goes away with some propane enrichment
Interesting idea, I will give it a try! I have fiddled with it so much to try and get it right. Tried other carbs,jets etc.
You should probably record it . I had dual exhaust that had an odd noise, only once it had dual exhaust added. It was an echo in pipe.
I'm not sure what popping is? From your initial post, I thought the engine had been gone through. I therefore thought that it might be something loose in a muffler, or other exhaust resonance. Maybe you can describe it better or post a video?
Alright, here we are! I did not have the best quality on my phone,so the laptop had to do! Hopefully it can show the odd/even fire noises Thank you everyone for your help in this matter!
V8 Camaros and Firebirds from the early 70's were notorious for popping thru the exhaust. They had a cross flow muffler that would push and pull making a popping sound. Could be a muffler baffle or other issue.... Just guessing.
So, its overlap? Shes been this was since I remember, always had a bit of a pop and slight shake at idle/rumble. Maybe i'm used to the calm nature of modern engines. Means I need to drive more! Rosaline and I appreciate it!
Could be. A bone stock motor will make at least 18-20" of vacuum at idle, fully warm, in Park. A longer duration but mild cam can make 13-15" depending on how big it is.
