Low WOT RPM's

[KStoon]

I'm trying to decide which way to proceed at this point. What I have is a 1973 85hp Johnson which I bought last year. It's on a 20' TC pontoon boat. I'm correcting problems and lack of care by the previous owner(s). I recently installed a new Teleflex tach and it is set to 6P. I have verified my throttle is now opening all the way and my carb linkage goes to the WOT stop when running it. I have the OEM size prop on the engine. My issue is this. While crusing across the lake at WOT I'm taching 2,100 rpm. It doesn't seem to sound like it's at 5,000. Per my OEM service manual I should run at 4,500-5,500 rpm at WOT. Being a noob I'm not sure which way to go now. The motor doesn't seem to be running poorly at wide open. No sputtering, missing, or anything. If the high speed jets plug up will it act like a rev liniter when crusing? Is it possible to be this far off with a prop? Timing issue? Any direction will help, thanks!

 

[Jeff_G]

Re: Low WOT RPM's

This is a case in which a trip to a good service shop may be in order. Pontoon boats are a different animal when it comes to props, you will usually need a much smaller prop to get the proper WOT RPM range.
Ask around any other pontoon owners with similar engines, if you can find any.
Your engine may not be turning up all the way as well.
Have the engine checked out. It shouldn't be too expensive, well under $100, usually around $50 at most shops.
There are also some prop calculators on the web, I don't have many links offhand but plug in the info and see what is recommended. I would bet the prop is too big for your application at the very least. Good thing is there are tons of used props for that engine.

 

[petryshyn]

Re: Low WOT RPM's

While crusing across the lake at WOT I'm taching 2,100 rpm.

I think you have more than prop issues unless its specs are waaaay out of the ball park. I think a compression test and spark test are in order. Pontoon boats can be hard on outboards due to aeration of the water from the pontoons. If there is no working overheat warning, piston damage is common.

Check the pitch of the prop and engine health....

 

[KStoon]

Re: Low WOT RPM's

I do plan on checking out the prop forum for info and using a prop calculator to see if a optional size is make to fit what it tells me. I was just concerned that being 3,000 rpm off seemed odd. I know a pontoon is a different critter vs. somthing like a bass boat. Checking the compression sounds like a good idea. The engine runs strong and pulls good out of the hole so that makes me think it should be sound. But, being a noob I will check it out just to ensure I'm not skipping a basic problem. I'm an auto tech and know all about skipping the basics and getting lost looking for answers in the wrong places. Keep the suggestions coming! Thanks!