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Low Compression, is this Possible?!

anotherrunner83

Active member
So, long story short, our Starliner was sent back to Checkmate to have it overhauled/refurbished. We made the decision to keep the 1988 Johnson GT200 on the boat as it still ran like a scalded dog. Fast forward 2 or 3 summers...

Boat had started to not charge the battery, slow coming out of the water and idled a little weak, as in it would die when idling without giving it more go go juice, thought this issue might be a result of the weak battery. Also thought the slow holeshot was a result of the stator winding melting and dripping on the timer base and locking it in the FOT position.

Before dumping more money into a 28 year old motor, we decided to check the compression on the old beast. What we found was a bit shocking and this is where you guys come in.

Every cylinder tested between 60 and 62 psi! We didn't trust the gauge so we hooked it up to another motor and pulled a 90 psi. So the gauge is good. We checked the compression with the motor cold and with it warm, same numbers. Motor starts dang near instantly and will idle on the hose for as long as you let it. Top end and mid range is still great.

Is the cylinder to cylinder similarity more important than the overall "low" compression. From what I'm reading, most internet "experts" say a motor is bad with compression under 70 and the things shouldn't run, let alone run well. What do you guys think? Is it toast? Does it make sense for it to run that well with that low of compression?

Any help is greatly appreciated.
 

ckaluger

Well-known member
60 psi is too low, but the close cylinder differential likely means all of the rings are worn equally, which is good, motor probably not damaged, but the motor needs an overhaul and also my experience has been that running a motor on a hose versus "in the water" can yield very different results. It is simply not making enough power when under load, and on the hose it is kind of "free wheeling".

Anyway, just one persons opinion.
Chuck
 

anotherrunner83

Active member
so we're looking around at rebulit powerheads. nothing obvious to say if there are any differences between a GT motor and a non-GT motor. i *think* it's that the GT runs the 225 carbs and tighter heads. most of these powerheads are all listed as 200/225, so can we just move the carbs and heads from the old motor over to the new powerhead?

where are Bill Gohr or Instigator at a time like this?! these two seem to be the knowledge pockets on these motors!
 

Bill Gohr

Active member
Right here

Bill Gohr 847 274 9378 847 356 3245, call anytime, It is strange that all cylinders are low, but possible, send it up I'll make t right, the 200 GT is the same as a 225 in those years just had 2 degrees less timing. They generally had 85 to 90 psi. The 200 TX had the smaller carbs, all the powerheads were the same in those years, I f I was going to build it, I would just go with the later closed deck block if it was affordable.
 

gbrodish

New member
I have a 17 foot open bow, 150 black max and it ran great with 2 people. With 4 it was kinda hard to get it up on plane and it wouldn't rev up until we shifted weight forward. I checked the compression, 58-60 across all of them. With me and my 7 year old we were 57 with that motor. I just picked up a rebuilt powerhead with 120 so I'm wondering what the difference is going to be. But otherthe motor with 60 fires right up, idles. Seems healthy.
 

MR.DETERMINATION

Well-known member
Low Compression

I wonder did any of you guys try spraying oil in the cylinder and rechecking the compression to see if the compression went. up. From what I read if the compression goes back up that indicates that the rings are worn.
You could have that power head bored out and the the heads milled that may give you a little more power or run it until it blows up. Or look for a parts motor or buy a new one.
 
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