1982 20hp Johnson mods

[burtco99]

First time posting here.

Looking for advise on a few mods I'm doing to my '82 20hp Johnson (31.8cid)

A bit of engine history:
Since new with a 14ft Princecraft aluminum boat, this engine would barely plane with 4 people and a cooler onboard. with 1 person it was reasonable. I played with it to see if it was an obvious tech problem, but nothing jumped out at me.
Original prop was a 9x9, so I tried a 9x8 with barely any difference. I advanced the timing stop to the max with little effect. We ran it that way until recently when I opened the intake manifold opening up to about 1" and tried a carb from a 25hp. Performance was now what I expected from a 20hp with the exception that I now noticed the engine RPM sounded much higher even with the 9x9 prop. I don't have any GPS speeds or actual RPM readings, but my "ears" tell me it is spinning somewhere in the 5500-6000 range. The boat is much lighter on the water and really flies. (I compare the sound to an '85 15hp Johnson i use on the same boat. The 15hp has always screamed in comparison to the 20hp, even though the boat was faster with the 20hp)

These results prompted me to research what other mods I could do and has brought me here.


Engine is over-hub exhaust. Engine has been totally apart and intake side has been deburred and any sharp corners slightly rounded. No other porting or exhaust work. Rest of engine is stock and in near new condition with no indication of wear. It only needs new rings and gaskets.

I`ve opened up the intake manifold to about 1-3/8" which matches the carb gasket and added a 35hp carb with the 1-3/16" venturi.

I run 94 octane pump gas from Petro Canada as it still is ethanol free around here.


Now the questions:

1) Should I run the 35hp carb with the jetting it came with, or should i expect to require some jet changes to get reliable performance?

2) Can I run with 35 degrees of timing, or should I back off to 30 degrees as spec`d for the 35hp?

3) Should I leave the exhaust tube as is, or can I gain from some slight tweeks??

4) Will a 9.25 x 11 prop be close, too much, or too little considering the upgrades I`ve done?

Thanks,
Les Burt
Montreal

[MWhite]

First time posting here.

Looking for advise on a few mods I'm doing to my '82 20hp Johnson (31.8cid)

A bit of engine history:
Since new with a 14ft Princecraft aluminum boat, this engine would barely plane with 4 people and a cooler onboard. with 1 person it was reasonable. I played with it to see if it was an obvious tech problem, but nothing jumped out at me.
Original prop was a 9x9, so I tried a 9x8 with barely any difference. I advanced the timing stop to the max with little effect. We ran it that way until recently when I opened the intake manifold opening up to about 1" and tried a carb from a 25hp. Performance was now what I expected from a 20hp with the exception that I now noticed the engine RPM sounded much higher even with the 9x9 prop. I don't have any GPS speeds or actual RPM readings, but my "ears" tell me it is spinning somewhere in the 5500-6000 range. The boat is much lighter on the water and really flies. (I compare the sound to an '85 15hp Johnson i use on the same boat. The 15hp has always screamed in comparison to the 20hp, even though the boat was faster with the 20hp)

These results prompted me to research what other mods I could do and has brought me here.


Engine is over-hub exhaust. Engine has been totally apart and intake side has been deburred and any sharp corners slightly rounded. No other porting or exhaust work. Rest of engine is stock and in near new condition with no indication of wear. It only needs new rings and gaskets.

I`ve opened up the intake manifold to about 1-3/8" which matches the carb gasket and added a 35hp carb with the 1-3/16" venturi.

I run 94 octane pump gas from Petro Canada as it still is ethanol free around here.


Now the questions:

1) Should I run the 35hp carb with the jetting it came with, or should i expect to require some jet changes to get reliable performance?

2) Can I run with 35 degrees of timing, or should I back off to 30 degrees as spec`d for the 35hp?

3) Should I leave the exhaust tube as is, or can I gain from some slight tweeks??

4) Will a 9.25 x 11 prop be close, too much, or too little considering the upgrades I`ve done?

Thanks,
Les Burt
Montreal

changing the carbs to 25 or 30hp makes a big differences on them motors, i believe the intake off of a 25 will fit that motor also, clean up around the reed block,,, u can shave the heads and increase compression which will help,, with all this done u going to have to find a higher pitch prop, as the little factory one wont have enough pitch, around 13 to14 pitch maybe more, cause with all this done she going to start pulling. lol

[burtco99]

Thanks for the reply. I already found most of that information in other posts here.

I still need answers to my 1st three questions though.

As well, Can some one tell me the fastener torque specs for this engine? I need the following:

a) Connecting rod bolt torque
b) Crankcase bolt torques (securing case halves together)
c) Cyl head bolt torques.

I am assuming that Loctite on the con rod bolts is recommended.

[JohnsonM50]

The carb should run good as is
Time is not critical to performance within 30-35 I choose 30 as its easier on the motor at speed & no slower.
The exaust tube is best left as is besdes some de-burring. I would test the prop you have & go from there. A prop of too much pitch/dia will rob both acceleration & top speed.
Check the hull for any hook or rocker. consider weight & its distribution too. The set up of the motor counts as well, if the prop's too deep it will have good traction but not reach potential rpm. Good Luck


First time posting here.

Looking for advise on a few mods I'm doing to my '82 20hp Johnson (31.8cid)

A bit of engine history:
Since new with a 14ft Princecraft aluminum boat, this engine would barely plane with 4 people and a cooler onboard. with 1 person it was reasonable. I played with it to see if it was an obvious tech problem, but nothing jumped out at me.
Original prop was a 9x9, so I tried a 9x8 with barely any difference. I advanced the timing stop to the max with little effect. We ran it that way until recently when I opened the intake manifold opening up to about 1" and tried a carb from a 25hp. Performance was now what I expected from a 20hp with the exception that I now noticed the engine RPM sounded much higher even with the 9x9 prop. I don't have any GPS speeds or actual RPM readings, but my "ears" tell me it is spinning somewhere in the 5500-6000 range. The boat is much lighter on the water and really flies. (I compare the sound to an '85 15hp Johnson i use on the same boat. The 15hp has always screamed in comparison to the 20hp, even though the boat was faster with the 20hp)

These results prompted me to research what other mods I could do and has brought me here.


Engine is over-hub exhaust. Engine has been totally apart and intake side has been deburred and any sharp corners slightly rounded. No other porting or exhaust work. Rest of engine is stock and in near new condition with no indication of wear. It only needs new rings and gaskets.

I`ve opened up the intake manifold to about 1-3/8" which matches the carb gasket and added a 35hp carb with the 1-3/16" venturi.

I run 94 octane pump gas from Petro Canada as it still is ethanol free around here.


Now the questions:

1) Should I run the 35hp carb with the jetting it came with, or should i expect to require some jet changes to get reliable performance?

2) Can I run with 35 degrees of timing, or should I back off to 30 degrees as spec`d for the 35hp?

3) Should I leave the exhaust tube as is, or can I gain from some slight tweeks??

4) Will a 9.25 x 11 prop be close, too much, or too little considering the upgrades I`ve done?

Thanks,
Les Burt
Montreal