Low WOT RPM's

[FreeBeeTony]

I am still having problems with low WOT RPM's. I am only getting about 3200 RPM's at WOT. The boat is a '78 20' Grady White Gulfstream with a Mercruiser 160 IL6, 1.65:1 outdrive and a 19" pitch prop. I just completed rebuilding the engine (bored 0.030 over, new pistons, rings, bearings, oil pump, fuel pump, lifters, recon head, electronic ignition conversion kit (Pertronix)and rebuilt the carb). I checked to make sure the carburator is opening all the way when the throttle is pushed forward all the way and it is. The engine seemes to run smooth and strong but just not at the proper RPM's at WOT. I don't think its the prop because I am so far off. The boat gets on a plane and cruises at 2800-3000 nicely. Can it be a problem with the spark advance or the pertronix conversion kit? The weights in the distributor seemed free to move. Any other ideas? I sure could use some help here.......I'm kinda stumped right now.

 

[akriverrat]

Re: Low WOT RPM's

restriction in fuel delivery? overheating problem fixed?

 

[FreeBeeTony]

Re: Low WOT RPM's

It looks like the overheat problem has been fixed, I removed the thermostat and its running at about 180 now. How can I tell if it's a fuel problem? It doesn't choke or sputter. It runs very smooth. Can it be a carb problem?

 

[DJ]

Re: Low WOT RPM's

FreeBee,<br /><br />180 degrees is pretty warm, without a t-stat. That leads me to the next ideas.<br /><br />Is your timing advancing properly. Check it with a light.<br /><br />Do you have an exhaust restriction?<br /><br />Remember, you are only going to get about 4300, tops.

 

[stub756]

Re: Low WOT RPM's

my 88 2.3 omc wasnt reaching proper rpm either... Timing was too far retarded and the points werent gapped right.

 

[FreeBeeTony]

Re: Low WOT RPM's

Djohns,<br /><br />I had a difficult time setting the timing, the timing flange on the block is almost illegible. Originally I think I had the timing set too far advanced. I think this also contributed to the overheating problem. I put a light on it and increased the idle (in the driveway) and the timing did look like it was advancing. Is this what you meant by checking to see if the timing was advancing? After I retarded the timing it seemed to cool down. It's only getting that hot when I try to open her up. I think the boat calls for a 17" prop, but this will only bring me up another 200 RPM's, right?

 

[Doug Durako]

Re: Low WOT RPM's

Free Bee---it's 200 rpm per inch of pitch, so you should gain 400 rpm going down to 17 pitch.<br /><br />A larger diameter blade also reduces rpm.--if you are over 15 inches in diamter on your present prop, I'd try to borrow a stainless prop at under a 14.5" dia. by 17" pitch and give it a test.

 

[FreeBeeTony]

Re: Low WOT RPM's

Thanks ddboater, I think I am going to go that route. My current prop is a 14x19P, what if I went to a 16x15P? Is it safe to rev the motor on the muffs in the driveway to over 3000? I haven't tried this yet but was wondering if I should.

 

[FreeBeeTony]

Re: Low WOT RPM's

bump

 

[Doug Durako]

Re: Low WOT RPM's

FreeBee----I had your exact engine and drive on my 1976 Cobalt, but it was only an 18 foot boat and I never had to go to the larger blade. Try both a 17 and a 15 pitch if you can.<br /><br />I used to run the rpm up in short spurts in the driveway, with plenty of water running to the hose muffs, of course, but that won't tell you what is happening under load.<br /><br />One other note, when my wrench-buddy put the magnetic advance in the distributor on that straight six I had, it would not idle. The only thing he had to do was barely turn the distributor base to get it to idle right at about 700 rpm. Never adjusted anything else.

 

[FreeBeeTony]

Re: Low WOT RPM's

ddBoater, will is there siuc a prop as a 14x15P?<br />Or once you go to a smaller pitch does the diameter go up?

 

[Doug Durako]

Re: Low WOT RPM's

Props come in any and all dia. and pitch combinations.<br /><br />When the numbers start to be the same for dia. and pitch, I think you are getting into the territory of cruiser props that turn higher rpm, but are used on boats that do not really get on plane.<br /><br />I have no experience with those. Someone else might jump in here and comment??

 

[FreeBeeTony]

Re: Low WOT RPM's

I am going to try and find a 15' pitch prop to borrow, or find a cheap one on ebay! How do you think a 16x15P would work?

 

[Doug Durako]

Re: Low WOT RPM's

I'm guessing from a 14 diameter to a 16 diameter is going to reduce your rpm by 200 or so.<br /><br />From 19 pitch to 15 pitch should increase your rpm by 800 or so.<br /><br />Note: Make sure you have clearance on your drive to turn a 16-inch wheel--that's the max on Alpha and if your drive is the older Merc model with the torque adjustment tab the prop might not clear.

 

[FreeBeeTony]

Re: Low WOT RPM's

How about a 15x15P?

 

[DJ]

Re: Low WOT RPM's

Free Bee,<br /><br />Your timing should move (advance) as the engine accelerates.<br /><br />A mistimed engine can run hot.

 

[FreeBeeTony]

Re: Low WOT RPM's

djohns, it is advancing.........thanks. The more I think about this the more I think its the prop.

 

[fishing2hightide]

Re: Low WOT RPM's

Tony i hope this will help you. I have a 1977 cruiser made by mirror marine it is a 21 ft. fishing boat that is heavy like your grady white. It has a 165 with a 1.50 mercruser outdrive on it 2002 out drive low hr rebuild on motor.It had a 13.75x23 2800max rpm. I then put on a14.5x17 3300 rpm then went to a 14.25x15 s/s 3800-3900 rpm. Look for a prop shop that will let you try one out and bring it back if it is not the right rpm and try another one. that is what i had to do. good luck!

 

[daveswaves]

Re: Low WOT RPM's

Tony, you are OVERPROPPED, find a 15 by 15.<br />You cannot run a 16" dia, it will hit your trim tab.

 

[mathiasmorgan]

Re: Low WOT RPM's

"Thanks ddboater, I think I am going to go that route. My current prop is a 14x19P, what if I went to a 16x15P? Is it safe to rev the motor on the muffs in the driveway to over 3000? I haven't tried this yet but was wondering if I should."<br /><br />I have the exact same engine as you (250 cu. in. 160 HP inline 6 cyl, GM). I'm not an expert on props, and can't give you much feedback on WOT RPM yet (I just bought my 1970 Penn Yan and I'm still getting it ready for the water). <br /><br />I can tell you that in the original owners manual I have for the boat/engine, it says never to run the engine on muffs higher than idle speed. <br />Also in terms of timing it should be 6 degrees BTDC. But you probably knew that.. <br />George...