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4.3 Merc runs rough after 45 min??

F

FULL-THROTTLE

Member
Its not a checkmate but there is alot of knowlwdge and experience here.

A friend has a 18' Regal lsr with a carb'd v6 4.3 and has a real strange problem. After 35-45 of towing a tube or skiier his boat drops in rpm to basically an idle, if you give it throttle in runs like *****, rough, skipping. He has to let it sit for 20 minutes and then its fine again for 35-45 minutes.

Work Done Already:
All filters
Cap, rotor, wires, coil, plugs, pickup in distributor
carb removed disassembled and cleaned, removed jets and cleaned by hand then the whole thing sat in carb cleaner over night
Carb fuel filter new
Brand new Tach
Checked timing
I have looked around the entire engine for loose or chaffed wires, all connections physicall disconnected, checked for corosion and reconnected.
Fresh 93 octane gas

The only thing not replaced is the ICM, usually when this goes bad there is no spark?

Any Ideas anyone


Thanks,

Fran
 
HOT?

We monitored the temp Gauge and it does not over heat, when we open the engine compartment it does not seem unusually hot, i forgot to mention the thermostat is new to???????????????
 
Vapor Lock

I was thinking the same thing, but the fuel line from fuel pump to carb is a good 1 to 1 1/2 inches from block. I thought about running a rubber line clear of the engine from pump to carb but when i looked at the distance of the steel line I feel comfortable it ok.
 
Vapor Lock

I just googled vapor lock with marine engines and I got a million hits. The ethanol fuel is causing alot of vapor lock problens due to the lower boiling temperature. I think I will go ahead and remove the steel line from the fuel pump to carb and install a rerouted flexible fuel line with insulation on it.
I will report my findings after the weekend.

Thanks for the input.
 
I think Corey is referring to lack of fuel due to Vacuum on the fuel system. Nexttime when it's acting up loosen the fuel cap to let air in and rule out a clogged vent.

If that's not what he meant it's something else to try.
 
Fuel filter or crap in tank clogging up the pickup tube screen would be my guess. The wait allows the crap to fall away from the screen and it runs ok until it reclogs.
 
RNGRDVE said:
I think Corey is referring to lack of fuel due to Vacuum on the fuel system. Nexttime when it's acting up loosen the fuel cap to let air in and rule out a clogged vent.

If that's not what he meant it's something else to try.
Click to expand...

I was thinking more about the fuel line being close to a heat source and the engine vapor locking that way. With the new crap fuels that are out with Ethanol in them, the temp for vapor lock is a lot lower. But the vacuum on the fuel system is a very good point to.
 
i would put a pressure guage as close to the carb as possible since you replaced the ignition parts since you didnt list that you replaced the fuel pump
 
Sounds like a heat issue change impelled, water pump and flush engine cooling system
 
I had a similar problem on a old four winns of mine ran great for about 30-40 min then lost power. There is a check ball coming from the tank that has a little spring, it was getting hot and blocking the fuel flow.
 
update

I found the fuel check valve coming out of the fuel tank had a 1/2" piece of aluminum thread keeping the ball stuck at 1/3 opened, also the shop that summerized it was looking around trying to find out what the real problem was pulled the coil wire and did not insert it all the way back into the coil and it was jumping from the coil output to the neg side of coil. Cleaned check valve and installed a new coil wire and it is running 95% better, but we have not had a chance to put some time on it yet, i will update later with findings.
 
Is the distributor electronic or does it have points and condenser?
Years ago I had a condenser give me a problem after the engine was running for a while. After sitting a few hours it ran fine again until it was in operation for a while. I thought the coil was breaking down but not the case. Never had that problem before or after that.

Maybe a possibility if it has points and condenser.

Far stretch but?
 
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