are powerheads interchangeable

[delsol]

i have a 1976 135 hp johnson my compression is 155 in 3 cyl 125 in one ...i have a used powerhead 1977 johnson 85hp v4 it looks the same has 120 compression each cyl can i just change the power head and electics and carbs ..and is my difference in compression a concern 155 all 3 - 1 at 125lbs

 

[Bear]

Re: are powerheads interchangeable

"..and is my difference in compression a concern 155 all 3 - 1 at 125lbs" <br /><br />Some say it is some say it isn't:<br /> http://marinemechanic.com/New-Folder2/comp-test.htm <br /><br />Check it out.

 

[Dhadley]

Re: are powerheads interchangeable

I'd do the Engine Tuner treatment and / or pull the heads. Most likely its time to change gaskets and deflectors anyway. Either way you've lost nothing. If its a broken ring, then you can address that issue. <br /><br />As long as the 77 85 was on a 20" midsection (not a 25" or 1978 and later) it will bolt directly up. The 135 carbs would be a bit of an over kill but they will work.<br /><br />Good luck!

 

[delsol]

Re: are powerheads interchangeable

i failed to say that i put 1 new piston in lower cyl.125 lbs comp.. ran for awhile cannot get higher reading honed cyl and new piston and rings

 

[Dhadley]

Re: are powerheads interchangeable

That would explain a lot. Glad you mentioned it! If you only honed that cylinder and didnt measure it theres no way to know how much it was worn to start with and how big it is now. Then if the other 3 pistons were original and high ring pistons that would add some to the difference too. <br /><br />Good luck!

 

[delsol]

Re: are powerheads interchangeable

i mic. cyl. it is round and with in clearences but the compression is 125 in my new cyl. 155 in the other 3 do you think that would be a problem or do you think it would run ok but rough idle i took the boat for a ride it had good power but was having problems under a load its a minor prolem but i think its carbs... i have 85 hp 1977 johnson that has 120= compression was thinking about swaping just the power heads and changing the carbs what do you think??? i would hate to lose the power but if compression is a problem i would rather run great and slow than fast and poor

 

[Greg Lawrence]

Re: are powerheads interchangeable

dont cylinders build compression as the rings seat? Before my rebuild i was at 150 in all cyl. after i broke a ring from water in the gas, (now I will always use a water seperator) the cyl. were at 125, 123, and 120. I was worried they had installed a THICK head gasket. well after about half a tank of double oil, im at 140, 130, 135. so im thinking after another tank ill be at 150 in each. Im sure breakin has alot to do with it. btw why did you only re-ring and hone one cylinder?

 

[delsol]

Re: are powerheads interchangeable

i put new pison and reringed new piston ...its the right piston top rings ...had no compression now have 125 - 155 all other 3 had cly that had light csoring that measured well with mic