Sluggish 25 Johnson

[Tullibee]

My motor is a late 80s 25 hp Johnson. I have owned this motor for about 3 years now and have always been very happy with its reliability and ease of operation. It always starts with little effort and idles very smoothly and at low rpm.

My only issue has been the taper off of the top end performance as the air temperature rises. At temps below 60ish, it runs very strong at top end pushing my 14 ft. Lund around 27 mph with two people and a dog in it. When temps rise above that 60 to 70ish range the top end suffers to around 22-24 mph.

Now I am not real mechanically savvy but i have tinkered with carbs and other general maintenance on snowmobiles on occasion.

My guess is that it needs different jets in the carb when it gets hot out???

What do yall think?

[Surface Tension]

Its and air to fuel mixture thing.

The cooler the air temp is the leaner your engine will run. You'll notice an increase in performance.

The opposite happens in warmer temps. You'll run richer and see a decrease in performance.

That is assuming your carb is tuned and clean. You'll notice this with most engines with a carb but snowmobiles are one where the operating air temps can easily range 60 degrees and very noticeable.

Fuel injection can take those temps difference into account.

[mnguy152]

I would try running seafoam in the gas may help too

[RiverFish]

Its and air to fuel mixture thing.

The cooler the air temp is the leaner your engine will run. You'll notice an increase in performance.

The opposite happens in warmer temps. You'll run richer and see a decrease in performance.

That is assuming your carb is tuned and clean. You'll notice this with most engines with a carb but snowmobiles are one where the operating air temps can easily range 60 degrees and very noticeable.

Fuel injection can take those temps difference into account.

Very interesting. Is there anything that can be done to correct this or is it just something you have to live with?

[broken_line]

you can adjust the fuel air mix on the carb if you want to keep getting max performance at any temp but that would be a pain. your best bet would be to tune it at a mildly hot:/ humid day also keep in mind water is less dense when warmer same with air. making it easier for the boat to push but less for the prop to grab

[Surface Tension]

Best thing to do is be sure the carb is clean for proper air/fuel ratio. That way in those colder lean times your getting enough fuel.

Switching jets to account for air temps/humidity isn't practical at all. Altitude yes.

If your into racing for top performance or big changes in altitude a dial a jet is one way to go.

[delcecchi]

I am surprised that there would be that much difference with only a 20 or 30 degree temperature difference. 5mph is a lot. My money is on something else besides mixture.