The motor is a 1975 three cylinder two stroke. Mercury 650. Good compression on all three cylinders, good spark on all three cylinders.
The problem we are having is that after running for about three seconds or so the motor dies out, wont hold idle and dies.
While checking to verify that all three cylinders we took out the top spark plug and cranked the motor over and it idled fin running on the two bottom cylinders.
We all found this very odd, haha.
We noticed that the spark plug on the non-running cylinder was real dry like it wasn't getting fuel.
Next we tried screwing the top plug back it without the lead connected to it. Wouldn't run this way either.
The motor only idles when the top plug is completely removed from the cylinder head.
Our fuel pump is good. Starter motor is good. Spark plugs are new all with big spark. Compression good on all three cylinders. Coil seems to be good. Fuel line is all new. Brand new battery.
A 3 cylinder will normally run OK on 2 cylinders and strangely enough they usually just feel flat or lazy and sometimes a little rough. A twin can run on one cylinder but not as easily.
Your motor appears to run OK with one plug removed so it's running like a twin, but can't sustain itself if the third plug is in the head. That normally would not be enough to kill it unless both of the remaining plugs are not firing all the time. That was the logic behind that answer.
Hillsy suggested crank seals and that is very possible. Leaky seals do cause a motor to run lean and maybe that's enough to stop it from running. Do a leakdown test and see if it will hold 6psi for 6 seconds on each cylinder.
Is yours a 3 cylinder, 2 carb model? Maybe one of the carbs is starved of fuel or has blockages.
More Mercury outboard motor Problems
Posted - Jul 28 2010 : 7:09:30 PM
