Compression on a 1978 Johnson 150

[Bretz56]

Anyone know what the compression should be on this motor? It has 90# on every cylinder. I have talked to some that say, put the money in it keep it going and others that say that low of compression is no good. Just want to know if I should spend the money to put new electrical components on it or look for another motor. It seems to push my boat good at 42mph. I just wanna be sure this motor is gonna last a couple more years if I replace things. Thanks in advance

 

[Bob_VT]

Re: Compression on a 1978 Johnson 150

Welcome to iboats

For a 6 cylinder of that year ..........you are fine. We are talking about a 2 stroke V-6 and not a 4 stroke.

Do a decarbonization procedure and it might come up 5-10 lbs per cylinder. Read this too http://forums.iboats.com/showthread.php?p=2648076#post2648076 and this too http://forums.iboats.com/showthread.php?t=158076

What kind of boat? What is your WOT rpms?

How long have you owned it? Have you ever checked the hull flotation for water infiltration?

 

[Bretz56]

Re: Compression on a 1978 Johnson 150

This s a big custom Jon boat that I bought the hull (an 1872) which is 18' long and 72" wide at bottom with a 103" beam. I Ripped all the guts out of the insideof the boat and put an 8' aluminum deck of front, 15" catwalks down each side and a 4' rear deck with a lot of bracing and the foundation for all the deckage is consisted of 2X4X.125 aluminum tubing and it was covered with .125" thik sheets of aluminum. All the deckage is covered in hydroturf for comfort. I am getting around 5300rpm with a 14.25X19 3 blade aluminum prop that is in pretty bad shape. I keep having cavitation problems at mid throttle, hoping a jackplate will fix this. I already added a hydrofoil and it helped a little.

 

[emdsapmgr]

Re: Compression on a 1978 Johnson 150

When new, the compression would have been around 110 lbs on each cyl. You may get the compression up a little by running some Bombardier "Engine Tuner" through the engine. Though the compression is a somewhat low, an important measurement is that the compression on each cyl. should be within 10% of the others. You are fine in that area.