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1995 mercruiser 4.3

N

nperry_62

New member
just bought a boat with a 4.3 mercruiser. motor runs perfect for about 1/2 hour. after that time goes by when i give it more throttle it cuts out, surges and backfires. it will still run great at 25% throttle. has good gas, new fuel water filter and tried a new ignision sensor. i am out of ideas. anyone ever had that problem?
 
Re: 95 mercruiser 4.3

sounds like its going lean. items to check:

tank vent - open with no sags in the hose.

anti-syphon valve - usually at the outlet fitting of the tank.

crud accumulation on the tank floor - clogs the screen in the pickup tube
 
Re: 95 mercruiser 4.3

i'll look that stuff over. think it could be running to rich and the plugs can only keep up for awhile before they get to wet?
 
Re: 95 mercruiser 4.3

Just a thought; put a timing light on it when the problem occurs. The ignition module may not be advancing the timing due to a heat related electronic failure within the module.
 
Re: 95 mercruiser 4.3

Same thing happened to me last year and it turned out to be water and crud in the fuel tank that would get churned up after about a half hour and the boat would start running like crap.
 
Re: 95 mercruiser 4.3

No consider this when you start and run the motor the gunk and junk is on the bottom, then when it gets stirred up it clogs the filter.

Or the vent is clogged and is causing a vacuum in your tank.

So You can pump the gunk off the bottom of your tank.

you will need to purchase a cheap electric fuel pump from any auto store dc powered.
You will also need about 15 foot of cheap fuel hose from the auto store and clamps to fit the fuel pump.

Install the hose on the fuel pump both ends and cut about five feet on the sucking side of the pump just look for the arrow on the pump.

Remove the fuel pickup tube it's the one where the fuel hose hooks to the tank a 90 degree angel fitting, it's a anti-siphon valve. Unscrew it.

Crank the bow of the boat all the way up, and stick the hose connected to the electric fuel pump 5 feet into the bottom and back of the tank.

Into a clear container pump the water out till you see clean fresh fuel and stop the pump.

Make sure that the valve is clear and the screen on the bottom of the pickup tube is clear.

You could also run the motor on a external tank of gas to confirm it's the fuel, causing the problem. then pump the tank you pick your poison.
 
Re: 95 mercruiser 4.3

pumped tank. everything looked good. put boat in water, ran good until about 1/2 hour later it started doing the same thing. took it home pulled the spark plugs and they were soaked. took a flame to them and they almost flamed up. what next?
 
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Re: 95 mercruiser 4.3

Coil could be dying on you. Install a new coil or borrow one from a friend. You can check the ignition amplifier when it happens.

Tests for Thunderbolt Ignition:


W/ignition key on AND BILGE WELL VENTILATED OF GAS FUMES!!!


Check the kill switch first!


#1 - Connect your voltmeter neg. lead to the engine ground and pos. lead to the white/red wire terminal at the dist. It should read 12 volts.

#2 - If 12 volts are present, remove the coil spark wire from the distributor and connect it to a spark gap tester to ground. Remove the white/green lead from the dist. terminal. Turn the ign. key to on and strike the white/green lead to ground. If there is spark, replace the ignition sensor in the dist.

#3 - If there is no spark, substitute a new coil and repeat test #2. Now if you get spark, install a new coil. NO SPARK, replace ign. amp.

#4 - In #2, if there is No voltage present, disconnect the white/red wire and check it again for 12 volts. If 12 volts is present, replace the ignition sensor inside the dist. cap. If no 12 volts present w/it disconnected--ignition amplifier is shot and must be replaced.
 
Re: 95 mercruiser 4.3

had the same thing happening. Came down to thinking ignition module, replaced and now boat runs fine, had out for over a 2 hour run last weekend.
 
Re: 95 mercruiser 4.3

well took the boat out for four hours and it ran perfect. all i did was change the spark plugs and never really gave the boat a chance to idle. soon as i stopped i shut it off. did notice alot of the time when i shut the motor off it took almost 25 secs to shut off. what is causing that?
 
Re: 95 mercruiser 4.3

improper running of a carbed motor.

You must let it idle 4-5 minutes before shutting it down that diesling is the motor running backwards.

And it will draw water into the back cylinders.
 
Re: 95 mercruiser 4.3

Oh you must have done something else and didn't even know it.

Maybe the plug wires were not fully seated, crossed wire something. One wire grounding out caused it has a crack.

I'm glad it's fixed.

Run the boat in complete darkness and look at the motor see where it's sparking.
 
Re: 95 mercruiser 4.3

Misting water on the plug wires w/a spray bottle will give you a light show if there is HV leaking from the ignition.
 
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