40 to 50 powerhead swap

[Gardog66]

I have a 1987 40 hp johnson and a1990 50 hp Johnson power head will the 50 fit on the 40? Also would prefer using the older style ignition and not the UFI that’s on the 90 can this be done as well? The 40 has a crack in the throttle side of the block after the intake that has been patched up but I think it’s leaking compression when it warms up.

 

[Chris1956]

if the crack is in the crankcase, it may be fixable with marine-tex and fiberglass cloth. I have patched a couple of blocks that way.

 

[flyingscott]

I believe they interchange. Look at the driveshaft splines and the base gasket Part #s. If those are the same should be good to go

 

[Gardog66]

if the crack is in the crankcase, it may be fixable with marine-tex and fiberglass cloth. I have patched a couple of blocks that way.

I think that’s what was done. It’s got really good compression 165 psi on both cylinders just has a hard time idling when warmed up and yes the carburetors have been rebuilt and I’ve done the link and sync Joe reeves high speed timing. It’s fine until I take it out of gear, just don’t want to increase the idle then I get a clunk going into gear. And the 50 is a better block but I just don’t like the UFI ignition I’ve had problems with it in the past.

 

[flyingscott]

I think that’s what was done. It’s got really good compression 165 psi on both cylinders just has a hard time idling when warmed up and yes the carburetors have been rebuilt and I’ve done the link and sync Joe reeves high speed timing. It’s fine until I take it out of gear, just don’t want to increase the idle then I get a clunk going into gear. And the 50 is a better block but I just don’t like the UFI ignition I’ve had problems with it in the past.

Retest your compression have never seen one of those that high. What is your idle speed? THat needs to be set in gear on a warm running motor. Not unusual to have a bit of a clunk when shifting.

 

[Gardog66]

Retest your compression have never seen one of those that high. What is your idle speed? THat needs to be set in gear on a warm running motor. Not unusual to have a bit of a clunk when shifting.

The idle is around 800rmp and the compression was wet just ran so it’s oily

 

[flyingscott]

That is a false compression reading. Let it sit overnite and try it again and your idle could be lower. look to make sure the rollers are in good shape on the 50.

 

[Gardog66]

The 50 I’m talking about has 150 psi compression dry my 30 tiller has 135 dry and I’ve tested with two different testers and they both come up the same

 

[Gardog66]

That is a false compression reading. Let it sit overnite and try it again and your idle could be lower. look to make sure the rollers are in good shape on the 50.

They are junk I found a friend with a 3D printer so I’m waiting for him to make me a few cam followers