Backfiring through carb when over 4K rpm’s - 3.0l

[itsa svx]

2004 Stingray 190ls with a 3.0l MerCruiser. Anytime I get above 4K rpm’s or like 34mph I start to get popping or what seems like backfiring through the carb. Motor has had a full tune up and carb cleaned but the issue is still present. Also had valves adjusted and a fuel/water separator installed but no change. I can floor it from a stop and the popping doesn’t start until it’s over 4K. Runs great under 4K rpm’s with no evidence of any issues at all. Looking for suggestions of what I should check. Thanks in advance...

 

[Bondo]

Motor has had a full tune up and carb cleaned but the issue is still present. Also had valves adjusted and a fuel/water separator installed but no change.

Ayuh,..... Welcome Aboard,..... I guess that leaves the fuel pump, or the fuel tank,.... No doubt a fuel delivery issue,.....

Hook up a can of fresh clean gasoline, right to the fuel filter to by-pass the boat's tank,.....

Check the pump's output,.... should be 4 to 6 psi at the carb,.....

 

[Rick Stephens]

While it could be fuel delivery, I'd still point at the carburetor as most likely culprit. Main jet is running lean.

 

[Bt Doctur]

popping back thru an intake valve is too tight of a valve adjustment allowing the lifter to pump up and slightly keep the valve open or a broken valve spring allowing the valve to start floating at high rpm

 

[itsa svx]

Thanks for the input - this issue started last year and the troubleshooting began with the first step doing the full tune-up since I wasn't sure when it had been done last. I replaced the Fuel line as well at that time because it crimped when I changed out the fuel filter and strainer. Next was the fuel/water separator. Then the carb cleaned after that. Each change requires some time on the water to see if it fixes the issue since it only seems to happen under load. That was as far as I made it last season. First time trying to get it out this year the cylinder head cracked while on the hose so now it has a new head with new valves, springs, pushrods, and all that as well and the issue is still present. I haven't had a change to check fuel pressure as Bondo suggested above so that will be my next step I guess and maybe another carb cleaning. Any other suggestions are very welcome.

 

[Rick Stephens]

Re--run the valve adjustment.