Back fires at high rpms

[sebas54]

Hi, I have a 1987 gm 4.3l merc. I/O. Two barrel carb. that I just rebuilt.

Today I took the boat for a test drive and it ran good except at high rpms it would backfire or make a noise and the rpm gauge would tick up or down 1 rpm.

Could the adjustment screw cause this on the carb? I just sort of screwed it in.

Also when going over wave or wakes it sounded like it was losing power. this only happened sometime as well with the backfire noise.

This only happenes when the boat was under load; in neutral it runs fine.

I have recently replaced the fuel pump, fuel filter, water separator and other things. I tested it for blockage in the fuel system and there was non. i also tested for a vacum leak and there was non.


Thanks.

 

[mkast]

Re: Back fires at high rpms

Backfire through the carburetor is usually a lean fuel condition.

 

[Bt Doctur]

Re: Back fires at high rpms

And this happens at what RPM?
intake popping can be a valve, valve adjustment, a lean fuel condition.
The idle mix screw should have no effect past idle speed.

 

[mkast]

Re: Back fires at high rpms

Today I took the boat for a test drive and it ran good except at high rpms it would backfire or make a noise

High RPM

 

[sebas54]

Re: Back fires at high rpms

3500-3900 rpms. Can I diagnose this myself. This didn't start happening till I rebuilt the carb.

Thanks

 

[mkast]

Re: Back fires at high rpms

How accurate is the float set.
Low float condition will lead to a lean condition at high RPM.

 

[sebas54]

Re: Back fires at high rpms

I got it very close. It was off by 1mm. Is that enough to make it run lean?

Thanks again!!

 

[sebas54]

Re: Back fires at high rpms

When I removed the carb the float level had a solid needle. I installed a spring needle. Can this be the reason. I also adjusted the levels according to the spring needle.

 

[sebas54]

Re: Back fires at high rpms

Any ideas???
After rebuilding my carb. I have experienced lean conditions. I installed a spring needle and set the adjustments on the float for the spring needle. When i remove the old needle it was solid needle. Is this my problem??

 

[danond]

Re: Back fires at high rpms

You should stick with one thread