Merc 225 rpm's fluctuating at cruise

[cdwill]

I have a 1996 225hp (carb) engine that the rpms are fluctuating +/- 100-200 rpms between 3800 and 4200 rpms. Above 4200 rpms or below 3800 rpm's engine runs smooth. The engine is also leaking a good amount of unburned oil/gas out of the drain holes at the foot once the engine is shut down. Plugs are black and wet and there is a very small water leak (slight salt deposits) near the two rear bolts at the lower exhaust plate gasket.Is this possibly a carb problem, air leak some where, bad thermostat...not sure how to troubleshoot.

 

[BW23]

Re: Merc 225 rpm's fluctuating at cruise

I run a 97 225 EFI. I experienced some "missing" and it took quite some time with a timing light to isolate which cylinder it was.<br /><br />Found a "marginal" CDM module or two. (spark plug coil) Cost about $65.00 ea.

 

[cdwill]

Re: Merc 225 rpm's fluctuating at cruise

Thanks for the response. It gives me something to start with. Did the problem cylinder show with the timing light only at a specific rpm or could you tell at idle.

 

[BW23]

Re: Merc 225 rpm's fluctuating at cruise

It was apparent at idle, went away from 3500-4600 rpm then missed very badly above that.<br /><br />It ran so rough I believe more than 1 cyl. was missing.<br />I finally had a friend drive the boat and I carefully moved the timing light from cyl. to cyl.<br />You could also try the guys on this site.<br /> http://outdoorsbest.zeroforum.com/zeroforum?id=16 <br /><br />Be careful!!!