WOT RPM's too high
 

First of all let me thank everybody for the very useful information I have found throughout the forum so far. I just bought my first boat last week, a 1993 Diablo 180 with the 4.3 Mercruiser Alpha One Outdrive Thunderbolt IV ignition. I didnt know a thing about boats when I bought it, but all the information here is helping me learn quickly.

When I first bought the boat, I got a WOT RPM of 3900-4000 max and 28 mph on GPS speed. It sputtered and hesitated on acceleration and dieseled when you tried to shut it off too quick. After reading all the info on how to diagnose low WOT RPM's, I discovered the ignition timing was WAY off. It was set at 2 degrees ATDC when it should be 8 degrees BDTC. Fixing that and replacing the plugs, wires, cap and rotor did magical things to the performance of the boat. No more sputtering, hesitating, or dieseling. I got about another 10 miles per hour out of it, 38 mph GPS speed. My carburetor idle speed and throttle cable had to be adjusted to accomodate for the change in timing, but all is in order now and she runs like a champ.

My concern now is that now my boat may be over-performing. The spec for my boat on WOT RPM's is 4400-4800. I get a WOT RPM of 5200-5400 at a top speed of about 38 mph with 2 people in the boat on smooth water, trimmed all the way down. I have a 15x16 4-blade stainless prop. With this prop, I get a great hole shot and quick plane.

I know the prop plays a role in the WOT RPMs. But I have also read about 3.0 Mercruisers going 45 mph on the water, so it seems like 38 max in my 4.3 may still be low. Prop theory is still new to me and I'm trying to wrap my head around it all.

Questions: Is my prop the prime suspect of my high WOT RPM's? Other than the prop, are there other things that will make my WOT RPM's too high? Does my particular prop (that gives a great hole shot and quick plane) cause me to suffer on the top speed?

Re: WOT RPM's too high
 

Uhm don't be revving that 5k mark to often with a 4.3 or any other un modded chec engine except the 496ho. Other part sounds like you need to go up about a pitch maybe 2 on the prop there are some prop gurus in the prop section that are better at that then I. Good luck with it.

Re: WOT RPM's too high
 

I've never heard of anyone running a 15" x 16" prop on a 4.3L 180 and yes it is very likely the primary reason you are revving way too high. Your boat / motor combo should be running a 19" 3 blade or a 20" 4 blade.

In general..... the lower the pitch results in great hole shot, higher rpms at WOT and lousy top speed> Higher pitches
do the opposite. The trick is to find the middle ground. I'm not sure what the prior owner was using that 15" for. Maybe he was pulling ocean going ships into dock.

btw.. glad to hear a little work brought your motor back to running like she should!

Re: WOT RPM's too high
 

Thanks for all the advice. I re-posted in the prop section to get some of that expert feedback :)

I think the prior owner adjusted lots of things on the boat to compensate for the improper timing. The prop is probably one of those things. I was a little overwhelmed when I first bought the boat and learned all the things that weren't right on it, but with all the guidance here the issues are getting tackled one at a time :wink_thumbup:

Re: WOT RPM's too high
 

There was no need. I'm the one who will answer it over there too...

This isn't rocket science. Just follow my suggestion on props and you will be just fine.