Mercruiser 250 6cyl 4.1l wont preform under a load.

[Biglou0212]

Need any help figuring this out. 78 stern craft ran great one time after father in law purchased it. Quit on him in the middle of the lake, right as it was trimming out, thinking due to old gas. Cleaned fuel filters which had hard deposits boat would not turn over. Back on land now and returned all wiring back to original. Replaced starter and ign coil, ign switch starter fuel pump. Got it to fire up in the driveway and it sounds like it has a miss. Will hit 70 on tac Did plugs and checked rotor and cap. Had light carbon deposits which I used dremel to polish. Did compression test on all cylinders and they all read between 115 and 123 psi. Rebuilt carb Get boat back on water and now it won't go over 30 on the tac full throttle. Need to know what else to try or check. Any help appreciated.

 

[alldodge]

Need any help figuring this out. 78 stern craft ran great one time after father in law purchased it. Quit on him in the middle of the lake, right as it was trimming out, thinking due to old gas. Cleaned fuel filters which had hard deposits boat would not turn over. Back on land now and returned all wiring back to original. Replaced starter and ign coil, ign switch starter fuel pump. Got it to fire up in the driveway and it sounds like it has a miss. Will hit 70 on tac Did plugs and checked rotor and cap. Had light carbon deposits which I used dremel to polish. Did compression test on all cylinders and they all read between 115 and 123 psi. Rebuilt carb Get boat back on water and now it won't go over 30 on the tac full throttle. Need to know what else to try or check. Any help appreciated.

:welcome: to iboats

Did you get all the old gas out of the tank and put fresh in or just add to it? Are the filters still clean?

 

[Biglou0212]

Been added gas to it, have cycled 2 additional tanks since. Filters are clean. The carb rebuild was the last thing I did which was yesterday

 

[achris]

Check timing and correct operation of advance mechanism. Check for correct HT lead order, check valve lash adjustment... Run it from a remote tank (like an outboard) with known good fuel.... And check the filters again.

 

[Biglou0212]

Timing was my next step. Are my compression setting in fair to good range? Not sure what you mean by advance mechanism. My original post may have made me seem much smarter than I am. Just been reading a lot of forums chasing my tail. I'm a tinker more than anything but very mechanicly inclined. My largest issue like many post before is creating an environment to test in without putting it in the water. Advanced thank you for your help.

 

[thumpar]

The distributor will have weights that spin outward as the distro turns faster. That is the advance since boats don't use vacuum like cars do.

 

[998cc]

What did you polish with the dremel? It sounds like it may have been the rotor or distributor cap. Any metallic or carbon dust could easily cause the high voltage to "track" to ground inside the cap causing a cylinder to misfire. A cracked distributor cap will do the same thing. If the cap and/or rotor needed work, it is better to replace them with new components.

 

[Biglou0212]

Did that today. Had it timed. Sounds great in the driveway. Found a gasket leak on piston rod cover. Seems like every time I look I find something else. Going to put it in the water next week to see if it will handle load now. Read somewhere that the prop could also be the issue. It's a 15 1/4 pitch 21. Any thoughts here?

 

[achris]

That's a LOT of prop for a 165, but if it's in a small light boat, it might be the right one. A 165 was the original engine in my boat (see signature) and I found it best with a 14-1/2 x 15" prop. What boat is it in?

Just re-read your first post. You said it won't go over 30 on the tacho? That's 3000rpm. You need to prop that engine to make between 3900 and 4300 at full throttle (WOT) with a 'normal' load. Any less (as it appears you have now) is seriously overloading the engine, and not just at full noise, through the entire rev range, and that's why the boat isn't performing. You need to get the revs up about 1000, so that would require a drop in pitch of 4 to 6 inches. Start with 17" and see how you go. If that gets you up toward the 4,000 mark, great. And you should be doing about 30 knots at that engine speed with that prop...