Mercruiser 305

[oldboat1]

I run a 1980 305 MIE inboard in an F26 Trojan. It’s original with a lot of hours (no rebuilds that I am aware of). I use it gently. But it has never run close to the rated 4400 rpm at WOT, and that should probably be addressed. I get about 17 mph at 3000 rpm, but that’s about it for both speed and rpms. The motor seems to be smooth at idle and on the top end.

The distributor appears to be a GM version with vacuum and mechanical advance. It’s my understanding that the vacuum advance is an idle advance (believe it works — canister holds vacuum, and I hear movement in the distributor when I apply vacuum). I’ve read that the mechanical advance mechanism can fail over time, however, and I’m wondering if that would affect rpm at WOT. (I haven’t physically checked the advance mechanism yet.)

I’m not a hands-on owner with this inboard — pretty good with old outboards, but not so much with inboards. I haven’t asked the boatyard to run a compression check yet.

Any comments or advice?

 

[Scott Danforth]

The distributor appears to be a GM version with vacuum and mechanical advance. It’s my understanding that the vacuum advance is an idle advance (believe it works — canister holds vacuum, and I hear movement in the distributor when I apply vacuum). I’ve read that the mechanical advance mechanism can fail over time, however, and I’m wondering if that would affect rpm at WOT. (I haven’t physically checked the advance mechanism yet.)

boat motors do NOT have a vacuum advance. someone dropped in a car distributor at a minimum, maybe the whole motor.

 

[oldboat1]

Yeah, never know. Thanks, Scott.

 

[Bondo]

Ayuh,...... Way to much unknown to guess why it can't make rpms,.....

Could be the motor, which obviously has issues, or it could also be the prop,....

​​​​​​​Seein's ya don't do yer own work, I'd think a full diagnostic tune up, 'n a new marine distributor would be the 1st step,....

 

[oldboat1]

Too old to be laying on my stomach and reaching down into an engine compartment. I’ll have the compression checked first, then decide from there.

 

[xltier]

ya gotta get some spark advance in there.summit racing products has a option if not here.