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1978 Merc 800 Idle Problem

M

MickeyShowers

New member
Hello all,

Any help you can give me with my old Merc would be greatly appreciated.

Symptoms/Background

Last spring the motor got to where it wouldn't idle unless I had the spark advance lever at mid position. One day I put the boat in the water and it absolutely wouldn't stay running unless spark advance was up. When I put it into gear, it would act like it was going to die unless I hit the throttle. At full throttle, it would surge. I suspect this was a come and go miss on one of the cylinders.

I took it to the shop and the mechanic said that he found that the wire to the #1 ignition coil was bad. Rather than replace the whole harness, he just built a jumper. I ran the motor a couple of times last season after that and it ran better but still didn't seem right. This season, I got the boat out from storage, and it's back to the same old story. I haven't taken it to the lake because I can't get the thing to idle in the driveway. I followed the mechanic's lead, seeing the deteriorated state of the 40 year old harness and built 3 more jumpers to the other coils. No change.

Long story short, does anyone with experience with these motors recognize the symptoms? Am I in the right ballpark thinking it's ignition that's causing the problems?

What I really need is a good troubleshooting manual for this model. A buddy of mine told me that I would need a bunch of special tools to work on the ignition. Is that true?
 
If your turning over the motor and you have good spark. May not be a spark issue. There is a low end and high end jet in the carburetor. If you had fuel in the carbs when set up in storage, may need to take apart the carburetors and go through the jets to clean out any blockage or tarnished fuel.
 
perry_merc said:
If your turning over the motor and you have good spark. May not be a spark issue. There is a low end and high end jet in the carburetor. If you had fuel in the carbs when set up in storage, may need to take apart the carburetors and go through the jets to clean out any blockage or tarnished fuel.
Click to expand...

Would that explain the no idle condition? The 800 has dual carbs, one above the other. Does anyone know where I can get my hands on a rebuild manual? I've never taken one apart.
 
Many folks want to jump on the carburetors first to fix an idle issue.-----I always start with a compression test.----Followed by a test of spark intensity on all leads.-----Those cost zero $ to check.------If those are good then look at fuel issues, tank , hose , fuel pump and carburetors.-----Has the water pump impeller been replaced recently.----Should be done every 3 years or so.
 
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