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Mercury 80 hp running rough

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Dia75

New member
I have a mercury 800 80hp engine, that previously was in major need of a little TLC as it sat for about 7 years, I have only owned this boat for a short time and after replacing numerous parts... impeller, spark plugs, trigger, stator, fuel lines (now with inline filter), fuel pump kit, carbys rebuilt/blown out with compressor and new gaskets, new engine harness, starting solenoid, battery, fuel line from tank to engine and fresh fuel. It now runs, idles and I guess sort of drivable, no timing has been adjusted only the fuel mixture. Initially the idle mixture was set the about 1.5 to 2 turns ccw from seating position, after first test driving it did operated at WOT and hummed along as it should, I was rather impressed but after 20 mins use it started to run rough, when throttle applied it stammers to WOT, but can be driven at certain positions on throttle but not as smooth operating as the first 20 mins of operation. If I apply the choke button obviously dumping more fuel in it, it does improve power. I have readjusted the fuel mixture to make it run rich.
It starts, idles and goes into gear and drives, again it did operate at WOT and planed but something is a miss.

Where should I start fault finding.. carbys? ignition? or could this now be internal? so far I have not delved to much into the mechanical side just parts that were in need of replacing due to visible deterioration.

Also should reverse make a clicking sound before actually engaging or should it clunk like forward gear?
 
Outboards should be shifted with a quick firm motion.----If you try and ease them into gear you will do damage ( grinding / clicking ) noises.
 
Ok it does actually engage correctly with a quick action, but at times it would click with the quick action there is a distance from neutral to reverse don't think forward, would the selector need adjustment? as the impeller was replaced.
 
You need to adjust the carbs, not set them to X turns out--that's just a starting place. THat's your problem.

Do this:

1. Tie the boat to a dock (or leave it on a trailer, but with the motor level).

2. Warm the motor thoroughly by high idling it in gear.

3. Slow the motor to idle speed and start dialing the idle screws. You want to turn them IN (CW) very slowly, about a 1/4 turn at a time, and WAIT a few seconds each time to let the motor settle in. If the motor stalls immediately, meaning it was too lean, open (CCW) each idle mixture a turn and start again.

4. When you have found the best (smoothest) idle for both carbs--and you need to jump back and forth, getting each carb set properly, then open (CCW) each idle adjustment about 1/2 turn.

Jeff
 
Initially the carbs where set by running at idle with ear muffs, and initial test run on the water motor was running extremely well, WOT and power as expected but after 20 mins of driving, the motor started to run rough, as I said previously it was running as it should or so I thought, mixtures where adjusted as it did improve the stammering a little, I assumed as it is getting a richer mixture that timing or maybe something else was causing the issue, ill take it out tomorrow and check the mixtures.
 
If it was running well at first, I suspect you may have some debris come loose inside the fuel system. Is the fuel tank an old tank? It sounds like you have replaced everything else. I am guessing you have debris in your main jets again. Have you done a compression test? Before taking the carbs apart again I would test the compression and the spark on each cylinder. Then go through the carbs again.
 
If the motor was still cold when the carbs were first adjusted, then they are running too rich (which is what it sounds like from your description).

That motor is notorious for running too cold, which eventually ruins the bores (unless the motor is worked HARD all the time. Its water pump impeller is way too big for it, being the same size as that used for the Merc 1500.)

These motors were a favorite of northwestern US loggers, who could run them for an amazingly long time before a rebuild.

Jeff
 
well 3 out of 4 spark plugs loose.. lol, remembered my oldest was helping that day and I obvioulsy didnt tighten them once he put them back in.. link and synch done.. motor runs a dream.
Cheers all
 
That's one way to improve idle! Not too good at full throttle, though....

I recall the day my 3 year old daughter "helped me" work on a motor--dropping a couple of bolts down the intake!

Jeff
 
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