Engine Backfiring

[mikealston2428]

Hi all,
I got a 350 Chevrolet inboard.
When engine running at different revs on the trailer there is no backfiring.
Put it in the water no backfiring at low or high revs but it backfires at middle rev range.
I know there are a few things that can cures backfiring but with the sinareo I'm having what should I be looking at first ?
Thanks,
Mike

 

[mikealston2428]

It as a quadrajet that has just been rebuilt.

 

[gonzo]

It can be running lean or the advance is not working on the distributor.

 

[mikealston2428]

Hi Gonzo,
Am I correct in assuming that turning the x2 fuel mixture screws in 1/4 turn will close the air and let more fuel in meaning making the mixture richer

 

[whitepointer23]

Backfiring Is A Result Of A Lean Condition. It Can Be Caused By Vacuum Leaks Or Incorrect Carb Settings. Turning Idle Mixture Screws In Will Close Off Fuel And Make Idle Leaner. Midrange And Up Are Not Controlled By Idle Screws. If There Are No Air Leaks Your Quadrajet Is Probably Setup Wrong. A Worn Exhaust Lobe Can Also Cause Midrange Backfire.

 

[PAR]

Did you have distributor issues previously on this engine? This would be the first place I'd look. Take a look at the base plate and weights and see if you have good, free movement. Are the springs in good shape, etc. Most backfires are a too lean issue, but timing and distributor problems can also cause them.

There are a number of places to look on the quadrajet. The bowel plug is a known leak source, but I'd look at the accelerator pump first. It's a "pay attention thing" with these puppies. For example, if it recovers quickly after it pops and falls over (backfires and stumbles), this would tell me to check the pump. On the other hand, if it bogs (after the backfire), but power eventually comes on, as RPM increases, I'd look at the secondary butterfly tension adjustment. There's other stuff, like the power piston sticking or a bent hanger, etc., but check the obvious stuff first, like timing and base settings.

 

[mikealston2428]

Hi Par,
The distributor is a brand new marine DUI HEI distributor.
The advance is preset for you in the factory based on engine details I provided them.
The carburetor has been rebuilt and just fitted.
How do I look for vacuum leak ?

Revs below and above the backfiring the engine run smoothly.
Backfiring is around 1300rpm

 

[gonzo]

The needle weights and springs are supposed to match the engine. Unfortunately, you can't look at them without taking the carb apart. The needles have the letter/numbers stamped and the springs are color coded. To find out if the engine is running lean, check the sparkplugs after running for maybe ten minutes at the RPM it is missing. Stop the engine without letting go down to idle. The sparkplugs will be very white and clean if the engine runs lean.

 

[whitepointer23]

Hi all,
I got a 350 Chevrolet inboard.
When engine running at different revs on the trailer there is no backfiring.
Put it in the water no backfiring at low or high revs but it backfires at middle rev range.
I know there are a few things that can cures backfiring but with the sinareo I'm having what should I be looking at first ?
Thanks,
Mike

I GIVE UP TRYING TO HELP PEOPLE THAT DON'T EVEN HAVE THE MANNERS TO SAY THANKS. ITS NOT HARD. I AM a MECHANIC AND I DO RUN A 350 IN A BOAT MYSELF AND HAVE JUST FIXED THE SAME ISSUE WITH MY MINE. BUT THERE IS NO ROOM ON THIS FORUM FOR ME ANYMORE. I GET OVERIDDEN BY ONE LONG TERM MEMBER OR IGNORED ALTOGETHER. SO GOOD LUCK WITH IT AND GO **** **********.

 

[mikealston2428]

Hi Brendan,
My most sincere apology to you.
In no way did I mean to insult you by not thanking you for your input that by the way was very helpful.
I have found this forum and members to be fantastic help for me during the rebuild of my engine.
I don't in anyway want to upset any members.
Thanks all,
Mike.

 

[PAR]

I certainly didn't "override" you, in fact just reconfirmed what you'd already stated. In any case, sorry you have to leave and good luck with your next adventures.

 

[gonzo]

What the hell? I see no reason for whitepointer's fit of rage. We are marine mechanics too.

 

[mikealston2428]

Hi guys,
Back on topic thanks for all the input I'll go over the engine as per suggestions and let you know how I go.
Cheers,
Mike