Mariner outboard power loss

I have a 1984 40 hp Mariner outboard on a small fishing boat. The engine runs fine on the ear-muffs, but has no power once in the water under even a minor load. It even has diffculty idling once the outdrive is in the water. It'll go in gear on the ear-muffs, but quits running when put in gear in the water.

Put a new fuel pump on the engine to no avail, also checked the spark to both cylinders with an inductive timing light, cleaned but did not fully dissasemble the carbs. Compression is 95 psi on both cylinders (with throttle closed). It seems the engine exhaust has difficulty pushing the water out of the prop once the ouitdrive is in the water.

Has anyone experienced this type of problem with their engine?

Re: Mariner outboard power loss

The idle may be set too low. The link and sync idle settings should be done with the motor in the water. If not, it will be set too low for it to idle in the water OK. Of course, if the motor has other problems or is not tuned correctly, it will not idle well even if the link and sync is done right but it is worth checking. Just my thoughts...

Re: Mariner outboard power loss

At 100lbs you might be able to tune around it for another season. If I have a motor testing at 95 lbs the first thing I do is grab my second gauge and check it again. If it's correct then I recommend a rebuild.

Based on the symptoms as you describe them, the backpressure on the engine is still what it was when it was new but the low compression is keeping it from developing adequate pressure, properly clearing the cylinders and drawing in fuel.

You could try leaning out the idle and messing with the timing but in the end you'd probably just end up paying for that when you finally decide to rebuild it.

For about 1200.00 and a few weekends you can rebuild that motor or you can opt out and just go with a new Tohatsu... (Same motor just about and several $$$ less)

Just my thoughts...

Steve

Re: Mariner outboard power loss

Cylinders are likely scored on exhaust side and still cheaper to rebuild as this is the Yamaha import engine

Re: Mariner outboard power loss

Thanks, Found the problem. I had a clogged carburetor inlet. Took bothcarbs off to clean them, and found a small piece of rubber/plastic clogging the bottom carb inlet. Cleaned it and the engine runs very well. Might look into a rebuildsoon, but this engine is on my son's boat - he might like the extra power, but I might be a little happier without it....