'85 mercruiser 3.0 1500 rpm in drive

[roktman]

haveing troubles with motor. just swapped a '71 straight 6 mercruiser with a '85 mercruiser 3.0. same mercruiser outdrive. getts full range of rpm's in neutral and reverse but can only get 1500 out of drive...really stumped on this one. needs a tune up but dumping gas fine and firing ok. thinking it has to do something with the drive assembly. any suggestions???

thank you

 

[crdpoker]

Re: '85 mercruiser 3.0 1500 rpm in drive

Certainly much more knowledgeable people on here than me, but if I was gonna guess, I would say it's the drive ratio. Guessing your old inline 6 was a 1.65, all of the 3.0's run a 1.98. Even if you change your prop pitch, I don't think it's going to be able to compensate for the difference.

If it's not stamped on the drive, pop the top cap and count the teeth.

-Chris

 

[Don S]

Re: '85 mercruiser 3.0 1500 rpm in drive

1. Boat is too big for a 3.0L engine
2. Wrong gear ratio
3. Wrong prop
4. Less HP
5. WAY less torque

I wonder what could possibly cause your low rpm.

 

[wire2]

Re: '85 mercruiser 3.0 1500 rpm in drive

.... gets full range of rpm's in neutral and reverse but can only get 1500 out of drive...

You can run at 4600 rpm in reverse???