I fired up my 63 Lark 259 canadian V8 after replacing the electric fuel pump this AM. Engine has a AFB 4BBL from a GM application, source unknown. This has not yet been on the road. It starts, idles and runs well, but it had an intermittent exhaust back fire from both banks but primarily and most frequently from the drivers side. It runs and revs fine with no load. When in gear the back fire is more frequent but not constant. At 1500 to 2000 rpm with no load it's either gone or no longer noticeable. the fuel pump is an inline 35 gph at 7 psi max. Turning the dist to change the timing does lese it some but doesn't clear it up. Plugs are new, points cond and rotor are what was i there when I got it points and rotor look good, cap looks good with no visible carbon tracks. It has those yellow "performance" plug wire also. Coil operated at normal temps. Ran it for about 35 minute got it up to 190 degrees. Any ideas appreciated.
Backfiring as almost always a timing problem, misrouted plug wires, ect, unless it's something odd like a sticky valve.
If it has been sitting up for a while(2+years?) not running, it's highly likely it's sticking valves. Intermittent back firing is very often sticking valves. Slowly drizzling about a cup of automatic transmission fluid down the carb, while running at a fast idle, will often cut the deposits on the valve stems and eliminate the problem. Be careful not to dump a big "glug" down the carb at once. It could cause a hydraulic lock and bend a connecting rod. Be aware that it will smoke like crazy until the ATF all burns up. It won't hurt anything. I have used this trick many times over the last 40 years.
Wouldn't it be likely for sticky valves to show up on a compression test??? They were all 120 to 130 dry. Maybe need to slap the vacuum gauge on it tomorrow. See if anythnig shows up when it burps. I have some MM oil, I will try misting that in and see what happens. It's been sitting for at least 5 years under my ownership and several more years with the PO.
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