Mercury 850 rough running

[Andrew1346]

I recently inherited a Starcraft super sport, late 60’s. Powered by a late 70’s mercury 850, it’s hasn’t run in probably ten years. I put a new water pump in it, plugs, fuel lines and resealed the carbs. Tried adjusting the mixture screws on the carbs with muffs in the yard. I’ve taken it out on the lake, it doesn’t want to idle at 1000 rpm, which is still too high I know. I go to get on it and it bogs out, I richen the mixture screws out until it accelerates good, it will run decent at the higher rpm’s, above 3000. But as soon as it comes back to an idle, it will not accelerate again, almost acts like it’s loading up-too rich, but I can’t seem to get it tuned back once it’s warm to get it to run right again. Haven’t checked the timing yet, other things to check?

 

[Chris1956]

Initial carb settings are likely 1-1/2 turns open. Final carb settings are made on the water in gear, with the motor warm.

You are looking for good acceleration and a reasonable idle. Carbs will likely need to be set at about 2 turns open, but your settings will vary.

Did you install new floats and inlet needle and seats when you "resealed" the carbs? New gaskets? Carbs are clean and float height set properly?

Check the fuel pump(s) for leaking diaphragms, as they will cause the motor to load up at idle.

 

[Andrew1346]

New needle and seats in carbs, new floats. New diaphragms in fuel pump. New plugs and wires. I can tune it on the fly in the water, but as soon as you bring it back to an idle your done, runs like it has a miss, won’t throttle up at all just dies.... thinking of checking the lower crank seal to see if that’s leaking

 

[Chris1956]

Inspect the spark plugs for water. You will see real clean spark plugs or grey ones or droplets, if water is present. If they are black or brown, the seals and exhaust covers are good.

There is only one adjustment on the carbs. It is for idle mixture. I am therefore not sure how you "tuned it to fly on the water".

 

[Andrew1346]

Adjust the screws to where it will accelerate good without bogging out

 

[Andrew1346]

Cylinder four plug was clean, possibly had water on it. The other three were black and oily

 

[Chris1956]

OK, sounds like water from leaking lower crank seals is getting into cylinder #4. Time to pull powerhead and replace crank seals, and powerhead base gasket.

Spark plug wires are not arcing to ground, right?

 

[Andrew1346]

That’s the next step, tried new plug wires, did find one not getting clipped in tight into one of the coils.