1981 Johnson 7.5hp too slow

[Perry125]

I have this 7.5hp on a 12ft aluminum jon and only get to ~10 mph but it does plane with just me.

It really seems to lack top end. I think it should be faster. It runs fine..

I cleaned the carb and that did help, definitely gained some speed. As far as i know there is no high speed adjustment in the carb so im not sure what to do.

My thoughts are this could have to do with prop size or something with ignition or fuel.

Any ideas?

 

[Chinewalker]

Re: 1981 Johnson 7.5hp too slow

Sounds like a classic case of running on one cylinder. Pull the cowling off. Start the motor and while wearing thick gloves or using insulated pliers, remove one plug wire from its plug. Note any change in how the motor is running. It'll do one of three things. Either it will stall (indicating it was only running on that cylinder); keep running with no change in sound (indicating it was running on the other cylinder); or it will drop in RPM (indicating it was in fact running on both).

If it does turn out to be running on one cylinder, it leaves you with either an ignition problem or a compression problem. Have you done a compression check? Any sign of water on the spark plugs (indicating a blown head gasket)?

Let us know what you find...

 

[the machinist]

Re: 1981 Johnson 7.5hp too slow

You also could be asking to much from that motor.

 

[Perry125]

Re: 1981 Johnson 7.5hp too slow

Well it is running on both cylinders. Rpm dropped when either plug was removed. Maybe this engine is just not enough, but i have heard people reporting 15-17 mph with the same setup. I only weigh 130lbs so its not me.

I have not done a compression test, i have been told that is useless on 2 strokes as long as it runs.

The old spark plugs looked ok, just black.

Also there is some water shooting out of where the shaft cover meets the gear case (don't know proper names). Is that something to be worried about?

Thanks

 

[nwcove]

Re: 1981 Johnson 7.5hp too slow

i dont think a compression test is useless on two strokes! ......as long as they run? where did this info come from? if comp is low, you are loosing much of your hp to "blow by". compression is one of the three ingredients to having any combustion engine work properly.

 

[Perry125]

Re: 1981 Johnson 7.5hp too slow

That's what i thought as well, it was from a parts counter person. The spark is very strong, it revs high and doesn't bog.. What else could it be besides piston rings?

 

[nwcove]

Re: 1981 Johnson 7.5hp too slow

could be a few different things, is the prop low enough in the water? is it slipping on the hub? (not sure if that year motor has a pin or pinless setup), or maybe you need more motor for what you want?

 

[Perry125]

Re: 1981 Johnson 7.5hp too slow

Im not sure how i could tell if the prop was slipping, but yes it's low enough. i think maybe yes i just need more motor.

 

[nwcove]

Re: 1981 Johnson 7.5hp too slow

a slipping prop could do it , but it should be quite noticable .....engine revving inder load, moreso than ecelleration, i agree with you......more hp is what yer looking for!