merc od ser# question

[Starflight]

A co-worker is thinking about buying a 71 23' boat w/ a 350 that was (by appearance) very well taken care of. The seller had the outdrv replaced in 10-04. The ser# is 6048829 lower unit # 8951r43. The seller said that since that was done it wouldnt get above 2200 rpms in the water. He thought they changed the gear ratio and he replaced the prop to try and fix the low wot problem. It has a 16x13 prop and still wont get near the rpms that it should. I told my bud that any change in gearing should give the opposite effect because the gearing other than 1.64:1 would cause it to over rev especially w/ the prop thats on it now. I suspect a low pwr issue ie fuel, tuning or compression problem. What are your thoughts? Do the #s I've listed tell for sure the ratio of the drive? also is it a alpha drive?
Thanks for your time and input.

 

[wewefirex2x]

Re: merc od ser# question

That is the old style alpha drive which was made in all of the gear ratios. I can't seem to pin down that serial number.

 

[Don S]

Re: merc od ser# question

The 6048829 number you have is an engine serial number for a 1981 Merc 260 V8, your outdrive serial number should have a number in place of the r you show in it. There were also some 1.32:1 gear ratio's out there that may have been put in the drive. But that shouldn't cause a 2000+ rpm drop.
Even the SN on an old drive may not tell you what the gear ratio is. Someone may have replaced the drive, or regeared it if it's the original 71 drive.
You can remove the drive, put the drive in forward gear and turn the ujoint shaft and see exactly how many revolutions it takes to turn the prop shaft exactly 1 turn.

 

[Starflight]

Re: merc od ser# question

Thanks guys. If it was regeared w/ the 1.32:1, what kind of rpm drop would be expected? About 4-500?
I guess the proper ratio for the 260 would be 1.46:1, would that be correct?

 

[Don S]

Re: merc od ser# question

Like I said before, I doubt the 1.32 gears could possibly cause a 2000+ drop in rpm, and you would also be able to get WOT with a prop change using a 1.32. you wouldn't have the performance because you have the wrong ratio, but you would get WOT rpm.
I have a feeling the Prev. Own. failed to give you the full story.
Some things to look for:

Perhaps Bondo will show up with his picture of the exhaust shutter laying in the drive bellhousing.

 

[Bondo]

Re: merc od ser# question

Ayuh,......... I can do that..........

 

[Don S]

Re: merc od ser# question

Starflight. That shutter in bondo's picture WILL kill 2000 rpm off the top end.
and it's the "Exhaust Restriction" in that list I posted.


Bondo
Did you notice how looks like the shutter

 

[Bondo]

Re: merc od ser# question

Striking Resemblence................

 

[Starflight]

Re: merc od ser# question

That looks as if it could keep a guy scratchng his head for a while trying to figure why its not running right, if not in the know! Great info guys, thanks a bunch. BTW is there a way to accurately check exaust pressure w/o tapping the exaust manifold? if not, Bondo, how did ya know to look there?

 

[Don S]

Re: merc od ser# question

Don't even think about trying to tap into the exhaust manifolds or risers. you would probably strike water and ruin something.

Bondo, how did ya know to look there?

It's just one of the things you check for low WOT rpm.
Review the list I posted.
A vacuum check on the engine can also point to it. But there is nothing that is going to jump up and tell you. But 10 Minutes pulling the drive will verify if it's plugged or not.