1977 Johnson Stinger 75 hp

[edquade]

Hi, I have a 1977 Johnson Stinger 75 hp long shaft model mounted on a 1977 Sylvan Sportster, just curious as to the proper rpms that it should run wide open, I recently installed a tachometer but haven't had time to make it to the lake yet to see what rpms it runs. It currently has a 13.5x15 prop on it, and came with an extra (dinged up) 13.75x15 prop also, both 3 blade aluminum. It usually runs okay, especially if I'm alone or with my girlfriend plus fishing gear, trolling motor, two 6 gallons tanks of gas, etc. but if we ever try to pull a skier or a tube with more than two people in the boat it has a difficult time getting on plane and up to the speed it normally runs wide open.

 

[flyingscott]

5800 rpm is what you are looking for. You are going need 2 props one for skiing/tubing and one for normal operation. That motor is not known for it's low end power so for skiing/tubing you will need a prop with less pitch. When using that prop you will want to watch for over-revving if that prop is installed with no skier/tube attached. Call a dealer there is a service bulletin for ventilating the prop on those motors for helping with holeshot.

 

[jimmbo]

The WOT range for you engine is 5200 - 5800 rpm. I agree with flyingscott to aim for 5800 +/- 100 rpm, for the reason he mentioned. How long is the Sylvan Sportster. 15 - 16 ft?

 

[edquade]

Its a 16 ft, I did take it out tonight alone and if my tach is reading properly its only running 4700-4800 rpms wide open. This is with a 3 blade aluminum 13.5x15, what would be a good prop to look for?

 

[jimmbo]

What was you speed at WOT?
Before you start changing props, make sure the engine is in good shape, properly tuned and the throttle cable is adjusted so the engine is actually wide open. Also make sure the tach is accurate and set correctly. I say all that because according to my OMC charts a 17" prop is for a 14' - 17' boat. and the 15" is for 15' -19' boats. The 13" is listed for 17' - 21' boats. Mind you those are just guidelines but were not too far off in 90+% of the time