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1994 Mercury 50 hp problem

C

CometOTY

New member
Hi,

I have a problem with my 1994 Mercury 50 hp 2 stroke outboard. When running in gear at or below 1/2 throttle it is fine but when more throttle is used (up to WOT) it runs well for 30 seconds or so then dies off a fair bit. At this stage it runs as if it were at 1/2 throttle. Every now and then it will pick up momentarily then back to slow again. It seems to do this more in rough water.

I have so far tried: Cleaned out fuel tank; Replaced split pick up tube in fuel tank; Removed the fuel filter and used primer bulb to flush clean fuel through the lines and fuel pump; Placed a new fuel filter; Drained fuel from carby's by the mixture adjustment screw cover plugs; Pumped primer bulb with engine running in said condition (bulb was firm before pumping); Compression test (125psi in all cylinders when warm); Checked spark plugs (clean with a light film of carburised oil and sqare shoulders on electrodes).

The oil injection (pre fuel pump mixing type) was disabled and I have been running 50:1 premix for quite some time.

Has anyone out there solved a similar problem or got knowledge of such problems? Your help is greatly appreciated. Thanks.

Brian.
 
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..." Pumped primer bulb with engine running in said condition (bulb was firm before pumping); "

That could be the problem--it should be soft! I suspect your bulb has a stuck outlet check valve that's choking off the fuel flow. (Common problem.) Try another bulb and you might get lucky.

Jeff
 
Thanks for that Jeff, I'll give it a go, it make sense as there was a few small globules of hard white almost calcium like substance around 1 to 2mm in diameter in the fuel that was flushed through. Maybe this stuff has been caught in the check valve.

Brian
 
I've changed the primer bulb, pulled the carby's down(they were pristine inside) and dismantled the fuel pump to check the diaphragms (which were in good nick with no splits or cracks) and the problem is still there. The fuel running through the system is high octane fuel that is never left to go stale.
The motor was running well then suddenly started to do this so it is not likely to be a float level or mixture problem(for 3 carby's to develop exactly the same fault at the same time is highly unlikely)
So in effect I have I ruled out the fuel system which leads me to the next question: Is there a separate ignition circuit or such for high and low speeds? Would the high speed circuit malfunctioning limit the speed to the low speed circuit? Am I on the right track.

Any help greatly appreciated. Thanks.

Brian
 
Dave took my idea away! Do NOT rule out fuel problems until you do a high speed plug test. To wit:

Warm it up, run it full blast until it screws up, then throttle down as fast as possible and switch off. Next--after it cools down a bit (unless you're really impatient and brave!)--pull the plugs and eyeball them. (If this motor uses those "permagap" center electrode plugs, this might not work--they're hard to read.) If you see any sign of running lean, it's still a fuel issue. If they're kinda dark, start looking for ignition problems.

Jeff
 
Thank you both very much for the replies. The Plugs looked ok when I checked them with a healthy mid brown light coating on the ceramic and outer electrode and light film of oil over the rest of the plugs. None seemed to be too clean or too oily so I don't think there is any water passing the seals. On the weekend I'll do the suggested test.

Cheers.

Brian
 
Just got back on here after a few years, sorry I never updated the situation. I fixed the problem by changing most of ignition system. I replaced the stator, voltage regulator, ignition module, coils, leads and spark plugs. the parts were all CDI and NGK (plugs) they have worked beautifully since. I premix fuel to avoid the aging oil pump dying.

Hope this helps future queries.

Brian
 
Well, that'll do it!

Reminds me how today's "mechanics" fix car problems: change everything in sight until they stumble onto the problem.

Jeff
 
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