How to get more power Johnson 20 Jet

[ratherhunt]

I have a 2000 Johnson 20 Jet its suppose to have a 25 hosepower head . It came new from the Johnson dealer with the jet.I am trying to figure out what I need to convert it to a 35 horse so I have more power since I am running the jet. I really like the jet in the rivers I run, but with decoys and gear it just doest have the power, the motor is on a 14' Crestliner Jon boat. Do I change powerheads or just carb and timing??. Thank You.

[JohnsonM50]

Its hard to say, over the years these motors have have had subtle changes. I think the 35s have a different crank [same bore & stroke] some have different ex. tuners and intake covers. the time will likely be 30, some 25s are 34. And the carbs are probably different. you could try -BRP Parts Catalogue- and compare #s to see what can be done. The 35 powerhead should bolt in but might not be much faster, more mid range is the most likely improvement

[ratherhunt]

Thanks for your help.

[yamonster]

Compression always helps, bump the head .030

[JohnsonM50]

Compression always helps, bump the head .030
The OMC twin 31.8s -In good shape run 120 to 145 psi. Ive gotten stock powerheads near 150psi that have only a little more punch. Going more yet might require some new specs. Id also be very interrested if anyones done that and is willing to write . . .
The 76 to 80 year 35s did 5500 to make HP but after that the RPMs to make HP went down to 5000 then 4500. Whats the operating RPM of the jet motor? Ive also heard but am unsure of, if the power pack has an R in its #, it could mean rev. limited. These motors usually can run to 7000 Rs and stay together but the weak link seems to be the rod bearing cages. 7200+ and about 60 seconds is where you might find out that oil aint glue. [or maybe before]. :eek: and then there wont be any :confused:

[j nye]

In The Early 90s Had A 25 Johnson Got A 30 Intake ,carb,and Timing Ramp Set My 25 On Fire .sold It To A Freind Still Running Good Today ! Some Of These Guys Can Tell You If This Will Still Work Good Luck

[goldenshinner]

i may be wrong but isnt the 20jet actualy using a 30(tuned) motor.as the jet loses 30% in the jet.

[river_ratj40]

before id mess with the motor i would do a little work in the jet pump. a good sharp impeller is a very big key to performance. polish pump, liner, and impeller, and keep it shimmed tight. people say that you can thin the grates on the shoe of the jet and sharpen them to cut water better. be careful. if you have a good muffler shop, pull out the exhaust pipe in the pump and have him make one about 1/4 in bigger than the stock one. theory is, its the same as a water hose, if you put your finger in the end it will increase pressure and squirt farther.

[iwanajohnson]

Im with him. It will be alot cheaper to sharpen and polish the impeller. You wouldn't think something so simple would help much but it will. Just make sure you don't sharpen it backwards:eek: Have fun polishing it:D

[Bill Gohr]

Look for a 1982 35hp powerhead, that was the strongest one

[goldenshinner]

Look for a 1982 35hp powerhead, that was the strongest one
slightly heaver crankshaft otherwise nearly identical to the 79 series. if you swap the jet lower with a traditional prop lower you will increase %30 to probably 30-35hp which is possibly what your motor itself is. otherwise if you want 35hp jet y might have to move up to the next class in enjines which im not that familar with but i think the block is like a 50hp to deliver 30hp equilivalnt jet.

[JohnsonM50]

Look for a 1982 35hp powerhead, that was the strongest one
Thats good to know Bill, The 31.8s being my choice of motor, I have thought about the ratings, could you elaborate on what makes the 82 strongest? The later than 80 models rate HP at lower Rs, is that better overall or would that depend on your application? I look to get more than rated Rs from mine with fairly lite resistance and low load. The runnabout turns from 5400 to 6200 depending on prop and the hydro alot more with good results.

[ratherhunt]

Thanks for all your help.