backfiring out of carb and revs not going up

[Shea_G]

I recently swapped my 4.3 lx with a 95 chevy 350 vortec, edelbrock performer intake, black marine carb with number 9776S 0483 stamped on it, i believe for a 502 originally. I swapped out the ignition with a delco est voyager. The carb sat for a while and i never saw the engine run before but was told it was fine. So we set the timing, it fired right up. Runs great at idle but after about half throttle it starts back firing like crazy out the carb and the rpms dont go up. Any idea where to start? Carb issue or timing or valve need adjustment? And idea what carb it is so i can try to track down a rebuild kit it thats the issue and should i rejet for the 350? It doesn't have to run perfect, just decent is fine by me.Right now boat (21' fastrac hull) tops out at 25-30ish on plane with three people and 13 with 7 people. But im only giving it half throttle like i said.

 

[Fun Times]

Yes that carb was off of an 8.2l MIE engine model.
http://www.marinemechanic.com/merc/16B5R2-webber.pdf

MIE Carburetor Adjustment Specifications
Engine Model 7.4L 8.2L Carburetor Manufacturer Number 9772, 9780S 9774, 9776, 9776S, 9776SB Float Level 1-9/32 in. (33 mm) Float Drop 2 in. (51 mm) Pump Rod Hole Location Third Hole From End Accelerator Pump 7/16 in. (11 mm) 1 Choke Pull-Off 15/64 in. (6 mm) Choke Coil Rod Top Of Rod Even With Bottom Of Lever Hole 2 Preliminary Mixture Idle Screw Setting 1-1/4 Turns
1 Measured from top of carburetor to bottom of S-link 2 Remove choke rod from lever hole. Choke held closed and choke rod pushed down with top edge of rod even with bottom edge of hole.

Between the two service literatures from Marine Mechanic, you should be able find helpful info with what jetting you may need with each engine model and carb you have vs need. But I believe there may be a difference in CFM's between the two carb models so be carful with that one.

http://www.marinemechanic.com/merc/d...ive/webber.pdf

You should be able to find some similar carb numbers in the link below for the new engine size you have now,
http://marinecarburetor.com/Marine-B...r-OMC.htm#merc

 

[72GT160]

Anytime you have a backfiring issue in an engine you first eliminate ignition issues, then fuel. So, make sure everything you have is working properly spark wise. From control box, coil, dist. + cap to the plugs making sure one isn't bad.

I'm going to say its a safe bet that the carb is jetted too fat. If not cleaned, clean! You never know what's in there. Some use that Seafoam and in just one season will dry out the inside and leave a white powdery mess clogging passageways.

"Just descent is fine by me". Are you ok If running a lean condition and burning up a piston because it runs fine?

I am have a similar issue and it's due to lack of fuel pressure. Installed in-line gauge and it showed 0, yes 0 psi. But, it runs fine, to 4000rpm then backfires through carb which makes it a lean condition. And it's a new pump!