1989 Johnson GT150 Pro Series WOT RPM'S

[bnurczyk]

1989 Johnson GT150 Pro Series
Mod. TJ150SLCEM
S/N R 08266527

Hi, I'm new here (and to boating) but already got a ton of advice off these boards just reading but now have some questions.
I have and old 1983 Sea Sprite Fish & Ski, Iboats says it's 950 or so pounds plus three batteries etc. etc.
So boat, engine, gear, is around 1600 lbs.
I read some ware that the loaded rpm for this engine should be around 5200. Please correct me if I?m wrong.
Wide open it's turning 4200 @ 55miles per hour. It currently has a Raker 14-1/2 X 24 prop. (setup by prev. owner)
How much less pitch should I try?
I should also mention; compression is 105 average, all within 5 psi. New timer base, power pack, and VRO.
Carbs professionally rebuilt, and all linked and synched, running great.
Thanks, Bruce

PS I?m 49 years old, don?t really need to go that fast, just want to get it all balanced.

 

[Dhadley]

Re: 1989 Johnson GT150 Pro Series WOT RPM'S

Your target rpm is 5800 with an average laod. If you use ethanol fuel you may be looking at 100-200 less. Theoritical speed with a 24 at 4200 is about 51 with 0% slip. Maybe the tach or the speedo is off.

 

[bnurczyk]

Re: 1989 Johnson GT150 Pro Series WOT RPM'S

Thanks for the info, I took the speed off the GPS so the tach is most likely off. It was a little choppy that day so I had it trimed a bit nose down, that was the only wot test I did.
I'll try it again on smooth water and trim it up, see what happens with speed and rpm. Also, I have to double check but I think the power pack rev-limits @ 5800.

 

[Dhadley]

Re: 1989 Johnson GT150 Pro Series WOT RPM'S

Agreed, the tach is a likely suspect. Especially if it's not an OE tach. Some aftermarket tachs can cause a real headache, not all but some.

 

[bnurczyk]

Re: 1989 Johnson GT150 Pro Series WOT RPM'S

OK, I did some looking around and my power pack has a 5800 rpm limiter. So I?m assuming WOT with the proper prop should be some ware below that number.
I do have an inductive tach that will work on a two stroke, so I will double check RPM?s on my next outing.

Bruce