4.3 engine miss ??

[stiteler22]

I have a 89 baja with a 4.3 mercruiser. It runs good for about a half hour then starts to miss and backfire through the carb. It seems to have good spark from the coil to the cap but from the cap to the wires it is hit and miss. This only happens when the motor is up to temp and i assume the temp under the cover get pretty warm. My firts thought would be the coil breaking down when hot but seems okay. Ignition amplifire ?? or sensor in dist. Any help please !!

 

[Don S]

Re: 4.3 engine miss ??

Sounds more like a lean fuel condition to me.
Might try these tests.
http://forums.iboats.com/showthread.php?t=283269

 

[stiteler22]

Re: 4.3 engine miss ??

I would think lean fuel would do it allthe time. This only happens after a good half to 45 mins. of running

 

[JustJason]

Re: 4.3 engine miss ??

your ignition system has nothing to do with it... as there are no spark plugs inside of the intake manifold.

You have fuel pre-igniting in the intake, or blowing back into the intake.
Couple of reasons.
1. lean fuel condition or poor quality fuel. It may be not lean enough to run good on a cold engine, but once the engine is fully hot is when the lean condition shows itself. Poor quality fuel will do this to. Old fuel, water in the fuel, it all lowers the fuels octane, which is the resistance to pre-ignition/spark knock.
2. incorrect timing. If the timing is to far retarded for a given RPM the engine will not make power plus possibly start popping through the carb.
3. burnt or semi burnt intake valves. The combustion process is leaking through the intake valves into the manifold, and igniting and fuel up there as well.
By the way, an engine that has had a lean condition slowly develop will burn up its intake valves, as it's fuel that cools them.

 

[stiteler22]

Re: 4.3 engine miss ??

Changed distributor sensor and adjusted timing. Former owner had the timing at 12*. Took it out Sat, and ran like a champ

 

[sethjon]

Re: 4.3 engine miss ??

It could be several things. The carb might need an adj due to running to rich. The plugs could be fouled. I would also check the points. These are for starters. Doesn't sound like anything major though.