Thread: 350 Chevy-carb Backfire

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Hello all,
Need some expert advice. The 350 chevy engine with a quadrajet carb in my boat is experiencing a strange problem. It only happens above 1900 RPMS.

It backfires through the carb. It will idle all day long up to about 1800-1900 RPMs but then when you put a load on it the problem starts. We took it out last weekend with no issues, but then it did it again yesterday. All standard parts are new (plugs, wires, cap, rotor) rebuilt carb. It never fully shuts the engine off, it just starts cutting out until I bring it down to idle and then it is fine there

One side note. the crossover passages on the intake manifold that feeds warm air to the carb base have turned white on both sides of the carb.

Any advice would be much appreciated! thank you, Don

did you just put this carb on with the plugs, wires????

It is super lean but the fix/cause depends on what type carb---you didn't put a Holley on an quad jet manifold???

Thank you for the reply.

It is a quadrajet cast iron GM manifold. (no adapters)

I did rebuild the quadrajet along with putting the plugs, wires, etc. on at the same time.

Thanks again for the assistance! Don Jr.

I hope I'm wrong but if you hold the throttle at a steady rpm of about 2000 to 2300 and it pops through the carb that is a classic symptom of a flat cam like I said hope I'm wrong and its just a tune up problem good luck.....ted

Hey Ted,
Thank you. Yes I hope it is not a cam lobe to be sure. I do not think it is as the problem has been intermitent.

A flat cam lobe would create a problem at all rpms and it runs like a champ and idles perfect.

Are the metering rods stuck shut, or partially shut?

Going to check all of the basics this weekend. My schedule (and the marinas hours. she is in dry storage inside) prevents me from getting to her to check the basics. Will look into all aspects of the carbs operation as well as plugs condition etc. I just dont feel like this is a major engine issue as the boat runs great most of the time. I feel i have a lean condition perhaps. Thoughts?

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.

Rained???? Snowed up here

Now I don't feel so bad.......I will take rain over snow any day of the week. (and it was in the low 80's too!)

Next trip out will be Easter Sunday (weather permitting) We shall see if we cured the problem!

Don Jr.