I can only get 3000rpm...

[DukesFin]

GM L6 250 cid with a stringer outdrive pushing a '74 Grady White 212 Chesapeake with a 19 pitch prop...

No load (neutral in the water and running "on the hose", I can get more RPM than I need, but when I'm running in the water, I can only get up to 3000 (full tank and only about 250 - 300 lbs of people/tackle/ice on board). I'm hitting 33.4mph (GPS speedo).

I have installed a new (used) lower unit and I'm using the prop that came with it. With my old lower unit, I could push her over 4000, but not with this one.

I'm not familiar with how pitch works, so any suggestions would be welcome.

When I'm at 3000, she runs like a top: Quiet, smooth, etc, so I'm not thinking I've got a fuel or carb issue here. (Especially since she won't stutter at all in neutral but again, I can run the rpm up way high.

Thanks guys!

Should I post this on the prop forum? Never been there. THIS is home!

 

[Don S]

Re: I can only get 3000rpm...

First, let me say I know nothing about stringer drives. But if you could get the proper rpm before the change, and not after, then you have a gear ratio or prop problem. Try your old prop. If the rpm comes up, then it was a prop problem. Was your replacement drive the same ratio as the old one, if not, then you found your problem.

 

[Boatist]

Re: I can only get 3000rpm...

I do not know about the 74 models but the 80 models each have several gear ratios. The same outdrives supports several engine sizes and they each also have a high altitude gear set.

 

[DukesFin]

Re: I can only get 3000rpm...

I do not know about the 74 models but the 80 models each have several gear ratios. The same outdrives supports several engine sizes and they each also have a high altitude gear set.

Yep... I've got the same ratio as before (one thing I DID check!), but if I'm correct, the gear ratio is in the upper unit and I've got my "old" upper unit in her, so this shouldn't be an issue. If I'm wrong, PLEASE correct me, but for the '77 model year with a 165 L6 motor, they used the same ratio as my '74 model (19/21).

Anyway, to raise my RPMs, do I go higher or lower on the pitch? Old prop is too worn to read what pitch it was!