1975 Johnson 75hp stinger 2

[Robert E.]

Hey guys, just bought a 19ft lowe v bottom. It has a 1975 75hp stinger2. Took it out on the water today for the first time and it would start and run good but when you put it in gear and accelerate the boat will only go about ten mph. Prop is spinning fast but it's not gaining speed. I've seen this before with a short shaft on a 21in transom. But that is not the case here. Is my prop too small? My prop says vortex C03 15. Any help will be appreciated

 

[Lightwin 3]

Rpm??

Need that to determine if engine is ill or prop hub is slipping.

 

[flyingscott]

Compression and spark test need to be done.

 

[jimmbo]

The Michigan Prop selector shows Vortex 3-Blade (992112) Vortex B 13 1/4 X 13 as one of the choices. More digging shows a Vortex C 13 pitch but only a 12 1/2 diameter.
If your engine is a long shaft(20inch) it will use the Vortex B series. If it is a short shaft (15inch) it has a smaller gearcase and would use the Vortex C series. Using the Vortex C prop on a long shaft motor will have exhaust gases flowing over the prop blades as well as going thru the prop hub, resulting in a lot of slippage.

You need to verify which version of the 75 you have, and you may very well have a prop that is physically too small for the engine.
While the WOT range for that engine is 5300 -5800, it really needs to be propped as close to 5800 rpm as possible to get the best performance, especially with a heavier boat. At 1 1/2 hp per cubic inch displacement that 3 cylinder was designed for very light and fast hulls so it's not the best choice for a 19 footer. It will have a soggy bottom end when accelerating compared to say a 85hp of the same era. But the 85hp had almost double the displacement

 

[Robert E.]

I have good spark, new plugs. 110psi on all three cylinders. The model number of the motor is 75eslr75b. I do not have a rpm guage. I really don't know a lot about boat engines, and am not expecting this motor to make my boat fly across the water. But I noticed the prop is very similar in size to the prop on my 40hp Johnson.

 

[jimmbo]

If the 40hp is one of the 44 cubic inch looper models, i think the props will swap. The 40hp I'm familiar with is older crossflow 1960 -1976, which used a prop dia 10 - 11 inches.

From the model number I am going to assume it is a 20 inch model and has the bigger gearcase and your prop is for the small gearcase 15 in model,

 

[Robert E.]

From what I have been reading there are 4 blade props available as well. Is there any benefit to use 4 blade or should I stick with 3?

 

[racerone]

Post a picture of this motor and lower unit.--

 

[Robert E.]

I don't know how to make the pictures less than 2 mb, but it is definitely the wrong prop. 3 1/4 instead of 4 1/4