Mercruiser 2.5

[mechanic95]

Hey guys, I am working on a 2.5 mercruiser. Compression is 160 on all cylinders, it has new ignition coil, new points and condenser, and a rebuilt carb on it. The issue that i am having is that it starts very hard and at 1200 rpms it begins to shake and shakes violently down to about 900 rpms where it stalls off. Ive adjusted the timing advance and retard but nothing seems to help the shaking. 3 spark plugs were fouled when i took them out but one was perfectly clean. Do spark plug wires ever go bad?? It has the mercarb on it. Any help???

 

[Rick Stephens]

Recheck firing order. Double check timing and dwell. All else fails, do the carb again, most common job needing redoing.

 

[achris]

Perfectly clean spark plug could be water in the cylinder. Check your exhaust elbow.

Chris.

 

[Scott Danforth]

160 psi is either water in the cylinders or a bad gauge. that 153 cubic inch should be about 140 psi with new motor

did you file your new points (yes you must file new points) and verify your dwell

what is the timing (with points, you must re-time every time you adjust the points)

agree with Chris, the clean plug means water in that cylinder. my guess cylinder #3 or #4 (where water will run into from a leaking exhaust manifold)

 

[mechanic95]

Ok oil looks great. Would i be seeing milky oil if water was running back into that cylinder??

 

[achris]

Ok oil looks great. Would i be seeing milky oil if water was running back into that cylinder??

Not necessarily... If it's leaking in through the elbow, the water will only be present while the engine is running.

You need to remove the elbow and check it and the gasket surfaces...

We can discuss this ad nauseam, but you'll still have the remove the elbow at some stage. It's a 10 minute job and will cost you a gasket.

Chris...

 

[mechanic95]

Ok awesome! I've always just replaced the manifold and riser when in question for people but this guy doesnt want to spend the money! Probably shouldnt own a boat!! LOL! I will pull both the riser and manifold and let you guys know

 

[achris]

Leave the manifold, just pull the elbow. Easier, quicker, same (observational) result..

Chris......