5.7 Mercruiser backfiring under load

[burkey41]

[SUB][/SUB]Hi Guys-
I've used the forum for awhile, but this is my first post so I apologize for the length of the post and any info that may be missing. The engine in question is a 5.7 Mercruiser in my 93 Larson w/ a alpha 1 outdrive with thunderbolt ignition, engine serial #0f05074 and what I believe to be a Weber 4bbl carb (only # I can find on the carb is 8867). A couple of weekends ago I took the boat out and on my return trip it began to backfire under load and eventually killed even at idle speed. I changed the fuel filter, plugs, and distributor cap. After those items the engine will run at an idle, but still was backfiring under load. Next I filled the fuel filter full with seafoam and while the boat was trailered pulled that thru under load and the backfiring quit. So I un-trailered and run it about 1.5mi at WOT and it ran perfectly, the second I pulled it back to 3000 rpm it began to backfire again. Does this sound like an internal issue with the carb that I should get a kit and rebuild? If so can anyone direct me to the proper kit for this carb? Or am I way off and need to look into another issue. I appreciate any feedback and if there is any more info that I can provide please let me know.

Thanks
Matt

 

[Reel Poor]

Re: 5.7 Mercruiser backfiring under load

The Weber/Edelbrock 8867 is not a carburetor designed for marine application. I would recommend replacing with an Edelbrock marine carburetor part #1409.

 

[burkey41]

Re: 5.7 Mercruiser backfiring under load

As far I know the carb on it is original marine and is identical to the Weber 4bbl in my service manual for this engine. When I look up the Weber Carb with the 8867 it does not look like the one I have, so I may be not finding the correct #, but I cannot find any other carb number on the carb itself or the tag on the engine block. Anyone have any ideas on this problem. I have owned the boat for four years and never had an issue with the carb that is on it currently.

 

[Bondo]

Re: 5.7 Mercruiser backfiring under load

As far I know the carb on it is original marine and is identical to the Weber 4bbl in my service manual for this engine. When I look up the Weber Carb with the 8867 it does not look like the one I have, so I may be not finding the correct #, but I cannot find any other carb number on the carb itself or the tag on the engine block. Anyone have any ideas on this problem. I have owned the boat for four years and never had an issue with the carb that is on it currently.

Ayuh,... Yer carb should have a 9600# stamped in a corner of the base...

Yes, yer fightin' a Bad gas/ Water in the Fuel issue...

 

[burkey41]

Re: 5.7 Mercruiser backfiring under load

Ok, so I finally found the half wore off # on the carb. It is 9661S 0263. Can anyone help me with a part # for a rebuild kit for this carb? Thanks

 

[Reel Poor]

Re: 5.7 Mercruiser backfiring under load

Sierra #18-7748
Mercruiser #809065

 

[burkey41]

Re: 5.7 Mercruiser backfiring under load

In need of some more help, got the kit and cleaned and replaced parts from the kit. However the threading in the end of the choke shaft is worn and does not want to hold the bolt onto it tightly. I would just replace the choke shaft but can not find the part anywhere for sale. The only number on the choke shaft and lever is 8529. Any help tracking this part down would be great. Thanks

 

[NHGuy]

Re: 5.7 Mercruiser backfiring under load

I looked in Mercruiser, they don't show that part separately. Try looking up the carb online and seeing if the parts are available separately that way. Or, can you go to a bigger bolt, can it be epoxied in, or can you somehow repair the shaft bore? Otherwise get the piece off a bad carb from a marine salvage place if you have one nearby. And if that does not work start looking for carb shops, you'd be surprised what a motivated person can pull off.
Last option is get a whole carburetor. Yow, $.

 

[grewvin1]

Re: 5.7 Mercruiser backfiring under load

My 5.7L had backfire through carb under load, bad coil.

 

[burkey41]

Re: 5.7 Mercruiser backfiring under load

Still fighting with this backfiring issue and got looking at the coil yesteday and it was leaking oil out the top, not sure if this is the only problem, but it is obviously shot. I am hoping that the coil has been the issue the entire time and was just breaking down as it warmed up causing the backfire/timing issue. Appears that it was the original coil from '93 with part # 392-805570. I can only find coils with 392-805570A2, are these two part #'s interchangable?

 

[bigalexpresscruiser]

I'm very curious as to what you found with your boat and it's back firing under load , my 5.7 litre is doing the same thing but not at WOT just like yours. I have changed fuel filter , carburetor, plugs , wires, rotor , cap,pick up coil, ignition coil , bypassed safty stop switch as well as shift cut out switch, I set timing , checked the stand pipe in the tank as well as the anti siphon valve . things I did not do yet are compression check, timing chain and gears , or check for broken valve spring or possible play at the distributor. if you have any answers for me I would appreciate it

 

[greno]

Welcome to the forum, bigalexpresscruiser!
There is such little overlap in marine cams that for a backfire to take place it almost has to come up through a leaking intake valve OR the timing chain has developed so much slack that it has changed the cam timing to such a degree that the intake valve is closing late or opening too early allowing a combustion charge back up the intake runners. A rich charge can hold this off but a lean charge allows the burn the proceed rapidly up the runner. Contributors are bad timing chain, carbon buildup, rust on valves/seats, improper timing, lean mixture, too low octane, bad camshaft, broken valve spring.

 

[GA_Boater]

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