Johnson GT 150: Should I buy it?

[Merc'Rude Winder]

I need some advice from those more experienced with these V-6s.

I'm looking to possibly sell my boat and start a new project. The guy picked it up and is offering me the whole thing for a good price, and despite the fact that it's probably the coolest looking motor I've ever seen, I don't know if its something I should be getting into.

It's a mid-80s GT and was put away three years ago and hasn't been run since. I checked the fuel tank, which was full and didn't have any water in it, but the gas was stale and foggy so I disconnected it from the motor before running a compression test. All the plugs were a good tan color and according to this guy, "it ran the last time he used it."

I sprayed WD into each cylinder before cranking it to get a good reading. I know these things are supposed to be low compression motors but the numbers I was getting weren't what I was hoping for.

Starboard top to bottom: 90, 80, 75
Port top to bottom: 90, 85, 80

Should I take my chances and just run some tuner through it with new head gaskets, or is that #5 gonna need rings?
Any input would be great! Thanks.

 

[TrueNorthist]

Re: Johnson GT 150: Should I buy it?

Rings can stick after long storage. The reeds can also be a tad leaky after sitting, especially if it wasn't layed up properly. It would be best to run some seafoam through it and do another test but I guess you are looking for some quick advice. I have seen engines with similar compression, even worse, and they have been good runners that are still on the water. I have seen some that had excellent compression but wound up as an anchor shortly afterwards, so there is no hard and fast rule really. It would be best to get it running before making a descision and that should not be terribly hard to do. Just get some fresh fuel and clean the plugs and give it a whirl. If it doesn't start you might be able to haggle the guy down even more. As long as it has spark and is clearly just a case of dirty carbs and reeds then it may well be a good deal. But I will deny everything in court...

Maybe you could rent a boroscope and peek into the cylinders? I have one that is worth it's weight in gold.

 

[emdsapmgr]

Re: Johnson GT 150: Should I buy it?

The GT/XP engines were the high performance 150 models. They sported 175 hp carbs, stiff engine mounts, vented cowls and high compression heads. It's best to have compression numbers within 10% of each other. Some of these compression numbers are outside that parameter and would be of concern. Agree, best to have a look inside the cyls with a pencil light. Look for cyl liner scuffing. See if there is any sign of water/moisture in #5. Also possible the engine has some excess carbon around the ringsets. An application of Bombardier Engine Tuner may bring the compression up a few lbs on each cyl. It's cheap and worth doing. You might want to negotiate this purchase from the standpoint that it will require an overhaul...

 

[Merc'Rude Winder]

Re: Johnson GT 150: Should I buy it?

The GT/XP engines were the high performance 150 models. They sported 175 hp carbs, stiff engine mounts, vented cowls and high compression heads.

Were these high compression heads similar to the early 80s 200s, that had smaller chambers, or are they just shaved shorter than the stock heads? I was expecting to see closer to 140lbs...(I got 135 on my V-4 from this same gauge.) But everywhere I've looked online, people are saying these only hold 90-110lbs of compression when healthy.

You might want to negotiate this purchase from the standpoint that it will require an overhaul..

I'm good with cleaning the heads and swapping head gaskets, but if it needs a full teardown I don't want to be negotiating this one at all. I had a flashlight, but I couldn't really see into the cylinders through the plug hole. Guess there's really no way to tell without pulling the heads.

 

[emdsapmgr]

Re: Johnson GT 150: Should I buy it?

I purchased a factory rebuild 150S circa 1986 vintage. It came with the low compression bathtub heads. It's stock compression was 98-102 lbs. I have also seen some 150S with heads are very similar to those early crossflow heads. Same face configuration. You'd have to pull the heads to check. The 79 150/175/200's used the 456/456 heads which made decent compression, around 110 lbs. The small bore 235 heads made around 120. The V6 heads never made the compression that those early 135's made, which were 135+.

 

[Merc'Rude Winder]

Re: Johnson GT 150: Should I buy it?

^^Great info. Thanks! A set of those 235 heads would be gold if I could find 'em. They only produced the small bores until 1980, I believe.

I'm thinking I should take my chances. New head gaskets, reeds, and some tuner might wake her up. If not...