4500 RPMs/~30-35MPH/200 HP Johnson/21 ft 1969 Stamas boat

[fch2cxr]

Guys I'm having this issue with my boat, it will not even reach 5000 RPMs, I currently have a 14-1/2 x 19 3 blade propeller on my 200 HP Johnson, I can only get to about 4700 RPMs and to ~30-35 MPH, the motor seems to have enough power left on it when is at WOT however it will not go over 4700 RPMs, my boat is an old 1969 Stamas fiberglass walkaround boat I don't have the exact weight on it due to it being so old but it is a solid boat, I'm guessing the boat weighs at least 2500 lbs and it is 21 ft in lenght, also as I reach 4700 RPMs the motor is so trimmed up that starts causing propoising

 

[steelespike]

Re: 4500 RPMs/~30-35MPH/200 HP Johnson/21 ft 1969 Stamas boat

while it appears that the 19 may be too much for your appliccation
it seems that something else is at work.I think a 17" pitch should work but it is only going to add ABOUT 400 rpm.A 15 would put you at ABOUT 5,500 A 15 seems extreme.Perhaps boat is heavier than you think .A 71 Apollo 21 is listed at about 2,000.You might weigh yours.Is the prop in good shape? maybe a motor problem.Check compression,is throttle opening all the way.Is the bottom clean and smooth.

 

[fch2cxr]

Re: 4500 RPMs/~30-35MPH/200 HP Johnson/21 ft 1969 Stamas boat

Steelespike: Throttle is opening all the way, the prop is in perfect shape, few nick and scratches on the hull but nothing to be concerned with, I?m guessing that with motor/gear/fuel/people the total weight has to be more than 3000 lbs, I'll guess 3200 lbs total, I did run some bad fuel at the begging of the season, but I drained and cleaned tank, the motor idles good and has good low to mid range power and good mid to top end power, the problem is at WOT where it fails short of about 800 RPMs...

 

[fch2cxr]

Re: 4500 RPMs/~30-35MPH/200 HP Johnson/21 ft 1969 Stamas boat

One more thing the plate on the boat states that the boat is rated for 280 HP motor (max) so that 200 HP with a 19 pitch prop might be just a little weak for that size of boat (200 hp is right about the 70%) I might just have to settle with what I got...

one thing that I observed at one time was that while working on the motor at the lake one time I ran it w/o the air silencer and that time the motor RPMs actually went all the way up to 5300 RPMs and reached 45 MPHs (GPS) I'm not sure if that info is of any relevance but there it is...

 

[steelespike]

Re: 4500 RPMs/~30-35MPH/200 HP Johnson/21 ft 1969 Stamas boat

The various performance reports I've seen 200hp pushing about 4,000lbs.
are running 17" pitch stainless at about 5,450 low to mid 40s.
I think there's no doubt you need to go to a 17.You might check out the Turning Point props.Sometimes a prop just doesn't work with a motor.
Are you running an OMC prop?

 

[CharlieB]

Re: 4500 RPMs/~30-35MPH/200 HP Johnson/21 ft 1969 Stamas boat

one thing that I observed at one time was that while working on the motor at the lake one time I ran it w/o the air silencer and that time the motor RPMs actually went all the way up to 5300 RPMs and reached 45 MPHs (GPS) I'm not sure if that info is of any relevance but there it is...

Try that again , if it makes a noticable difference it could indicate either an air flow problem or a fuel flow problem.

May I also assume that while running at WOT that you have tried squeezing the primer bulb, and tried momentarily 'touching' the enrichener/choke? And neither of these made a positive difference?

 

[v1_0]

Re: 4500 RPMs/~30-35MPH/200 HP Johnson/21 ft 1969 Stamas boat

Guys I'm having this issue with my boat, it will not even reach 5000 RPMs, I currently have a 14-1/2 x 19 3 blade propeller on my 200 HP Johnson, I can only get to about 4700 RPMs and to ~30-35 MPH, the motor seems to have enough power left on it when is at WOT however it will not go over 4700 RPMs, my boat is an old 1969 Stamas fiberglass walkaround boat I don't have the exact weight on it due to it being so old but it is a solid boat, I'm guessing the boat weighs at least 2500 lbs and it is 21 ft in lenght, also as I reach 4700 RPMs the motor is so trimmed up that starts causing propoising

What's the history here: when did this start?

 

[fch2cxr]

Re: 4500 RPMs/~30-35MPH/200 HP Johnson/21 ft 1969 Stamas boat

Steelespike: I'm not sure what brand of prop I got, It came with the motor when I bought it...is there a place on the prop where I can find the brand????

CharlieB: I just installed a brand new Fuel pump, I doubt its a fuel flow issue, however, I'm betting on air flow but what can be causing it????

v1-0: this has been the issue since I bought the motor, I can't tell if it just need a lower pitch prop or there is a motor/malfunction issue, since the motor idles and really runs good other than the lack of ~500-700 RPMs at WOT that it should have.

 

[steelespike]

Re: 4500 RPMs/~30-35MPH/200 HP Johnson/21 ft 1969 Stamas boat

The prop may have a part number someone here can identify.May be on the barrel or under the prop nut.You might try a run with the hood off.
When it ran good with the silencer off; logic says maybe it is running a little rich and the slightly better air flow helped.Perhaps under strain the throttle linkage is binding a little.Maybe against the silencer.
Are we sure we aren't losing a cylinder?

 

[CharlieB]

Re: 4500 RPMs/~30-35MPH/200 HP Johnson/21 ft 1969 Stamas boat

Or you may have a small exhaust leak, partially suffocating the motor at throttle with the cover on.

 

[fch2cxr]

Re: 4500 RPMs/~30-35MPH/200 HP Johnson/21 ft 1969 Stamas boat

I saw some numbers on the prop I'll post them tomorrow, with the hood on or off it runs the same, the air silencer is what makes it act different...I don't think I'm losing a cylinder because as soon as the silencer came off it gave me the extra RPM's, last time I did a compression check, all cylinder were for the most part even, one or two PSIs of difference

 

[CharlieB]

Re: 4500 RPMs/~30-35MPH/200 HP Johnson/21 ft 1969 Stamas boat

Curious, how long have you had this boat, and how long has this problem existed?

Did this problem recently develop or has it always been this way since you acquired the boat?

What I'm wondering is if the carbs and/or silencer has been switched/replaced. We will need model and serial #'s to look up OEM parts for you to compare with what is actually installed.

Or you could take a drill/die grinder/exacto knife and modify the silencer to allow more airflow, add or enlarge the inlets.

Before making ANY changes I would be certain to double check the carbs, set trim level and while squeezing the primer bulb look to see if there is even the slightest fuel flow, even a hint of flooding, which could cause a demand for more air.