Only getting 4200 rpm

[jnk111]

I had made a thread a couple/few weeks back, but have done somethings since then. Here goes.

I have a 1988 force 85hp on a 17' bayliner capri cuddy. I can only get 4200rpms out of it. I have replaced fuel pump and cleaned the screen. Checked and adjusted mixture and timing. I have made sure that the butterflies are fully open at WOT. Compression from top to bottom is 125, 125, 130. I do not have any help at the moment and do not have anything to ground the plugs, so I verified spark with the timing light a idle. All three were blinking. It runs great. I have the boat on a slip on a river and after cleaning the bottom it would get to about 32-34mph on a speedo (I know its not accurate). I am not so much worried about how fast I am going as to how well the motor is running (optimum rpm range)

Just today I rebuilt the carbs. I was sure it was the carbs because ALL the floats were out of adjustment and the bottom one had quite a bit of sediment in it. As you probably figured out because I am posting this, I am still only getting to 4200rpm.


Where do I go from here?


Thanks,

Joe


P.S. prop is new and a 13 X 17 pitch

 

[jnk111]

Re: Only getting 4200 rpm

Couple of other things I forgot. The fuel tank vent is open. The primer bulb does not stay fully hard when running. By that I mean I can pump it a 1/4 of the way then it is fully hard. When I do pump it while the engine is running it has no effect on the way it runs.

 

[MikDee]

Re: Only getting 4200 rpm

I suggest you clean the piddle valves, like I just recently did, it does help a bit.

 

[jnk111]

Re: Only getting 4200 rpm

I suggest you clean the piddle valves, like I just recently did, it does help a bit.

I actually did that also when I rebuilt the carbs.

 

[MikDee]

Re: Only getting 4200 rpm

Couple of other things I forgot. The fuel tank vent is open. The primer bulb does not stay fully hard when running. By that I mean I can pump it a 1/4 of the way then it is fully hard. When I do pump it while the engine is running it has no effect on the way it runs.

Take the hose off the fuel tank, unscrew the fitting, and check the antisiphon valve, and screen underneath for dirt, or debri.

 

[jnk111]

Re: Only getting 4200 rpm

Well I checked for spark again with a real spark tester, it was good and would jump a 7/16 gap, that was yesterday. Today I thought I would use a portable tank and rule out an obstruction in the on board tank. I had a spare 19 pitch prop that came with the boat (not mounted) I thought I would give it a try it just too see what happened. There was a 17 pitch prop on when I bought it, and I replaced it about 2 months ago because I bent the original (replaced with brand new 17 pitch). I threw the 19 on, got past the no wake zone, hit the throttle and could tell right away something was different. The rpms were climbing much faster, it made it to 4700 rpm. HURRAY!!!

Anyway I could actually see that the washer under the locknut was grinding into the prop housing on the 17 pitch I pulled off. This seems a little strange, I thought that the rubber would give away from the splined sleeve before it gave way from the whole housing. If I am thinking of this in the right way the "outer" part of the prop was spinning while the rubber and sleeve held with the motor.


What is also strange is with the 19 pitch I was going 30mph at 4700 rpm(GPS measured, the boat has growth on the bottom again).
With the "bad" 17 pitch I was going 29 mph at 4000rpm. Those numbers seem strange strange to me. I almost wonder if I didn't get a mislabeled 21 pitch instead of a 17. I don't know.

 

[roadrunnr]

Re: Only getting 4200 rpm

Joe
I have a similar setup. 85hp Force on a 17' 89 Maxum. It has a 15 pitch prop and with just me in the boat I can turn 5500 rpm @ wot and about 36-37 mph on the speedo. I think this setup came from the factory with a 15 pitch prop. I could be wrong but I always thought that when you went up in pitch you gained speed but lost hole shot. I don't know what happens to rpm. This maybe a good subject for the prop forum!

 

[bowhuntrrl]

Re: Only getting 4200 rpm

I agree with Roadrunnr. It sounds like a pitch problem. I'm going through something similar on my SportCraft 16' CC and 85 Suzuki. I started at 19 pitch(4600 rpm) and 25 mph, dropped to 17 pitch where I get 4900 rpm @ 34 mph, but I have a 15 pitch on order. There can be differences in props with the same pitch. If your 17 pitch is stainless, it is probably cupped more than your 19 P. Cupping and efficiency will drop your rpms even though you dropped down 1 prop pitch. I would go with a 15 pitch and get it up into the power band.

 

[jnk111]

Re: Only getting 4200 rpm

I looked up the number that was stamped on the prop and it says it is a 17 pitch. The more I think about it though I believe that I got a mislabeled prop. I think it was a 21 pitch. If you look at the speed I was at and the rpms (29mph @ 4200rpm) versus the 19 pitch that was the spare (30mph @ 4700rpm) I think it makes more sense than a spun prop. I think a "real" 17 pitch would put me right where I need to be, around 5000. I am just glad it is nothing with the motor. It might not hurt to try a 15 pitch either. I wish I could try them out first.

 

[bowhuntrrl]

Re: Only getting 4200 rpm

It might not hurt to try a 15 pitch either. I wish I could try them out first.

Check out the PropMD.com , I just bought a prop from him and he has a policy of exchange as long as the prop returned is in "as new" condition. My 17P is going back for a 15P. There are other places that will also do that. Check with a local prop shop or dealer that might have a prop for you to try.

 

[jnk111]

Re: Only getting 4200 rpm

Yeah, there is a place here local that has a 17 and a 15, I just need to ask him if He would let me make a quick run with it. I would know right away if it is the right one or not.