1986 85hp force low RPM at WOT

[Bissen]

I have a 1986 Bayliner Capri Cuddy 1600 with a 85 hp force on it. Last year everything worked great I could cruise at 4900-5100 RPMs no problem and then go WOT and bring it up to around 5500 RPMS and get speeds around 35 mph. This year I decided to get a new prop because the old one looked a little worn. The old prop said 13 x 19 Michigan on it so I got the exact same one 13 x 19. I took it out for the first time this year and could only get 4000 RPM out of it at WOT. I put the old prop on then went out and could only get 4200 RPM out of it. I do want to add that the motor idles fine and revs up fine it gets up on plane with no hesitation but I do not feel like I am getting the same power out of it that I did last year. Any advice?
Thanks guys.

 

[Bissen]

Re: 1986 85hp force low RPM at WOT

I checked the compression and it was 120 all three cylinders.

 

[pnwboat]

Re: 1986 85hp force low RPM at WOT

Make sure the butterfly's in all three carbs are opening up all the way. They should all be perfectly horizontal at WOT. Check to see if your primer bulb gets firm when you squeeze it multiple times with the motor off. If it does get firm, this confirms that the needle valves in the carburetors are closing properly. When you do hit WOT, try squeezing the primer bulb to see if this has any affect. If it helps the motor pick up some RPMs, I would suspect that you may have a fuel supply problem. Fuel pump diaphragm, clogged fuel filter, partially clogged fuel line, etc.

If the fuel system checks out OK, check your rectifier. A shorted rectifier can affect ignition system causing lack of performance at higher RPM's. Radio Shack has a replacement for around $3. Exact same rectifier sold by Marine stores that charge $40.

 

[Bissen]

Re: 1986 85hp force low RPM at WOT

Thanks pnwboat I will check this out and keep you posted.

 

[Bissen]

Re: 1986 85hp force low RPM at WOT

I checked everything out diaphragm was good, fuel bulb got firm, fuel filter was clean. I looked at the butterfly valves and all three were not opening up all the way at wot. I adjusted those and now heading to the lake. hopefully that will do the trick.
I will post results.

 

[Bissen]

Re: 1986 85hp force low RPM at WOT

Well I took it to the lake yesterday. wasn't a good test large waves so I had to turn around. Lake Superior is pretty unforgiving. It still seemed like I could not get enough rpm's. I may try a different pitch prop. On the boat is a 13 x 19 prop and I ordered a 13.5 x 17 prop. Hopefully that will help. I am going to get a new rectifier today.

 

[Bissen]

Re: 1986 85hp force low RPM at WOT

I got the boat out yesterday on a calm day so a good day for a test. I changed the rectifier. At WOT I was getting 4400 rpm with gear and myself in the boat doing 37 mph. She was running great, at cruising I was doing 4000 rpm and 28 mph. This is with a 13 x 19 prop. I am going to try a 13.5 x 17 prop. hopefully that will get my rpm's up to proper rpm range of 4500 - 5500.

 

[pnwboat]

Re: 1986 85hp force low RPM at WOT

Sounds like your motor is running pretty good. The RPM/MPH numbers that you are seeing for your sea trials this year are pretty much in line with what you would expect according to the propeller calculators that you find on the WEB with a 19 pitch prop. The biggest variable is your hull design and weight. The 17 pitch prop will help bring the RPM's up and may even increase your top end MPH a little bit. It will also help your hole shot and getting on plane even faster.

 

[Bissen]

Re: 1986 85hp force low RPM at WOT

I finally got the boat out with the new prop 13.5 x 17 and it ran great I got 4600 RPM WOT and 36 MPH. I could cruise at 4000 RPM and 30 MPH. I think I am happy.

 

[jerryjerry05]

Re: 1986 85hp force low RPM at WOT

There is still a problem.If you ran good with a 19 and then something happened and it's not performing right.Something is wrong!!
Do a spark test.Then go to outboardignition.com and check your stator/trigger.
You should be able to hit 35mph and 5500rpm ALL the time.
Running it that way can cause problems.
Don't accept poor results.J