Thread: 350 Chevy-carb Backfire

Tweet

were you trying to fix an old problem or did you create this problem with your tune up ?

This was a freshening up of the engine that was in there already. After the tune up we had 6-7 no problem trips. This problem surfaced after she had been running fine for a while and even then it does not occur every time. In fact the engine ran perfect all the way to our destination that day (about an hour run) and then started the problem on the way back (about 3/4 of the way back) so strange. Thanks for any input. Don Jr

Ok,
Finally got to work on that backfire issue this weekend.

Heres what I found.

Timing was way off. my marine manual for that boat recommends 4 degrees BTDC. My timing was set at 6 degrees ATDC (way off) I had a very hard time getting the engine timed as the harmonic balancer is sitting almost directly against the boats front bulk head (in other words I was bent like a pretzel trying to get to the timing marks with the timing light)

I adjusted the air/ fuel mixture on the quadrajet also (bottomed them out gently and backed out 3 turns each)

Accelerator pump arm has two settings. This enging was on the farthest out hole, the good running engine was on the inside hole. Changed the location.

My thoughts of this engine running lean were not true. All of the plugs showed that the engine was running rich,not lean. Cleaned them, regapped and installed.

Checked all spark plug wires to be safe that they were not crossing eachother for a chance of a crossfire between cylinders.

Test ran the engine on land (on the garden hose) ran great. Went to go out Sunday after working on it all day Sunday..........AND IT RAINED!! So I will have to wait to see if it cured my issues under load. We shall see. Any thoughts from my diagnosis? Thanks to all. don Jr.