1986 GT150 compression

[Rev.]

I recently recieved the motor (along with a 86 19ft HydraSport) from a friend who thought it had a broken rod because the motor wouldn't turn over without locking up. He parked the boat in 2003. I got the boat home and figured out the lower unit gears were trashed, and the motor itself turns freely. I went ahead and ran compression checks on all 6 cylinders to make sure it had good compression before I spent the money to repair the lower unit. Keep in mind this motor hasn't ran since 2003, anyway, all of the cylinders had approx. 100lbs of compression except for the bottom 2 which had exactly 85. I was told by a friend that the bottom 2 were always alittle lower and given the motor was dry and hadn't ran in almost 5 years the compression was ok. I would like a few more opions on this subject.

 

[HighTrim]

Re: 1986 GT150 compression

Your friend is somewhat right. Junk always flows downwards into the bottom cylinders. Run some de carbing agent . Ill bet the compressions get closer to levelling out.

I think i would invest in a can of seafoam, in a portable tank, mix 3/4 gallons of premix gas, and 3/4 can of seafoam, (assuming you don't have the vro connected). Put the remaining seafoam in a spray bottle, run the motor up to temperture, remove the plugs, one at a time, spray some seafoam into each cylinder and replace plug. let sit 15 minutes, remove the air silencer from the carb while waiting, restart motor, and spray seafoam into each carb, until it start to stall. repeat till seafoam is gone. run out the rest of the gas. it is going to smoke like it is on fire, and drip goo on the ground. replace the plugs with new ones, and then do the compression check again. this procedure burns the carbon out of the motor and been known to free up rings, and increase compression. let me know the results.

 

[emdsapmgr]

Re: 1986 GT150 compression

The GT150s did run a little higher compression than the stock 150's. 100 is just fine. I'd follow high trim's advice on decarbing the engine. Take another compression reading after and see what the numbers are. 85 is getting low, but the engine may run fine for some time the way it is. I'd probably take the carbs apart and clean out any old gas residue before running. Also put in a new water pump impeller.

 

[Rev.]

Re: 1986 GT150 compression

thanks for the info guys. will let you know as soon as I get my lower unit back, and get er fired up.