RPM's

[cullracing]

What is the factory rpm range for an '85 Evinrude 150XP? Model # 150STLCOS, it's on an '86 Hydrasports, with a 14.5 x 24 Raker 3 blade stainless prop. Right now I run 60mph at 4500 rpm's with the trim high.

 

[walleyehed]

Re: RPM's

The numbers you list figure out to a negative 5% slip...impossible. Tach or speedo (both) are off.
you're about 1200-1300 low "IF" your tach is correct..
And "if" your tach is correct, your speed is way off...about 52mph at best. are you using GPS? If you are, and 60mph is what it says then your tach is way off and you're turning about 5300, which is still too low, almost lugging.
This engine likes 5800 real well. The manufactures specs from 1985 say 4500-5500,(could be 4750-5750 on the GT/XP) but with the crappy fuel we have today, we find it best to get the crossflows on up to 5800 for best results. Your GT/XP 150 uses the same block as the 200 with different port-timing than a std 150. It also has a tad more compression from different heads than std 150's have.
I have the same basic engine as you with a few added features, and when I'm testing props, I found that when I get down close to 5250RPM, My cylinderhead temps rise quickly at that point. I have Boyesen reeds and these allow more RPM, but for the most part, only above about 6400RPM. I've ran this engine over 6000 continuously for many hours now and it's the most trouble-free engine I've had. For everyday use, 5800 is best. That 24 Raker is a hard pulling prop and it takes alot of engine heigth to get all that 24 has to offer.

 

[cullracing]

Re: RPM's

Yes, I run a gps, and you mentioned overheating, it overheated the other day. I had it trimmed pretty high, and when I slowed down to idle speed the warning alarm went off. As far as the tach, I'm gonna take it out again later this week and check it again. I run a jack plate that came on the boat, what should it be set at? I've never had one, so I don't exactly know how it should be set, but I know on the inside of the jack plate, where the numbers are, it's sitting on 1. Not to sound too dumb, but what are Boyesen reeds?

 

[walleyehed]

Re: RPM's

Reeds are the "valves (plates) that allow fuel/air mixture through to the ports but close as pressure builds behind them...part of the intake manifold.
You may be running higher than the water pick-ups will allow at high-speed and max trim.
How far back does the jackplate extend the engine from the transom? How high is the AV plate in relation to boat bottom?
It could have been getting hot at speed, and just by chance, it reached the temp that sets the alarm off when you slowed down. And yes, it runs hotter at 5300 than it will at 5800.

 

[cullracing]

Re: RPM's

It sits 7 inches from the transom and the AV plate is about 3 inches above the bottom of the boat. I'm assuming the AV plate is the part a hydrofoil would bolt to.(Not real knowledgeable on all the parts yet) As far as the overheating, I found out today my impeller is gonna need replacing, pumps with water hose, but not in a tank. I guess water hose forces water in, but isn't sucking in water in the tank.

 

[walleyehed]

Re: RPM's

Yea, better get that changed....
Post back when done!

 

[cullracing]

Re: RPM's

I'm gonna try to get to it in the next few days. Does the jackplate sound like it's set up right to you? Never had one until now.