Curburettor backfires in 350 Pontiac motor

PONTIACNUT

MEMBER
1935 PONTIAC LEMANS

I have a 1974 Pontiac Lemans coupe with original 350 Pontiac V8 motor, 350 gearbox and 2 barrel rochester carburettor. The only mods are oversized valves for a little bit more oomph, K&N air filter, straight through exhaust with just resonators and recently put in new hydraulic lifters on the passenger side bank as these were worn.
My problem is that although the car has been garaged for 6 months I managed to get her running in no time, but she has a backfire through the carburettor and when I accelerate hard from standstill she like wants to die on me and then suddenly comes to life. Her stock specs are between 28-32 degrees for the dwell angle and timing must be 12 degrees BTDC. I had to adjust the timing to 16 degrees to try and compensate for the backfiring which has helped a bit but not much. Even with the old lifters clucking away she did not have this backfiring problem and there is even a little bit of smoke coming out the exhaust when I rev her up in park. Please help, I am getting gray hairs!

1 Reply

BACKYARDMECHANIC

MECHANIC
337 POSTS

Hi
My car doesn't give me gray hairs, its my children that do LOL. It sounds like you keep the car well tuned and you know what you are talking about, so I will skip the stuff about plugs, wires, cap, rotor and timing. You may want to try and do a compression check on the motor. Since it is the original motor I 'am assumming it has high mileage. A compression test can give you a good indication of what condition the internal parts are in (rings and valves). Also hook a vacuum gauge up to her and see what kind of reading you get. If you don't have one and buy one save the instruction sheet that comes with it since it gives you good information. A healthy motor should have a vacuum reading of 17- 22 with a steady needle. ( Neelde movement is more important then the actual reading) With out making any test and from what you told me I would have to say there is a problem with the cam or valves. Hope this helps Backyardmechanic :)