Another newbie needs advice on low compression

[Greg Mason]

Hello to everyone, new to this forum and it is great!

I have a 1996 GTX that has just quit this weekend. I did a compression test. Number 1 has 145 and number 2 cylinder has 40 psi. There is oil in the bilge as well. Engine has 205 hrs. I never heard any knocking or foreign noises but I know it can't be good. Should I go ahead and yank the engine out? How many hours should a 787 run assuming it was maintained? Could anything minor cause a low cylinder? Thanks in advance.

 

[derek]

ok. this operation has to be done in a very specific order. drink a beer. now drink another beer. now take the head off. now you see why i had you drink a couple of beers first. GTX. it is the back one. it's always the back cyl in a heavy ski. crank torsion. the rear cyl is coming up a couple of degrees late under load. the ignition doesn't know this. result? it is too advanced. burns the top of the piston up. if the bore isn't too bad, you can get away with a new piston for about 70 bucks. get the WSM's. make sure of the bore diameter before you buy. or, pull cyl-get it bored and get a new piston to the new bore. just be sure what you are doing. i bought a 650 motor for cheap. guy got 1st over pistons. (.25 mm.) and 1 over rebore. (1.0 mm.) needless to say, it didn't run long. your bore is probably standard. you may well need a head too. chunks of piston went out the exhaust. they pinballed around in your combustion chamber first. that head could look as bad as the piston top. oh yeah. the cyl change can be done in the ski. but it is easier out on a bench. only about an additional hour to remove engine.

 

[Greg Mason]

Crank twisting under load is normal?

Thank you Derek, So do you think the oil in the bilge is a separate problem, maybe bad oil line or connection to the pump is loose? You are basically saying the crank twisting under torsion load is normal, nothing can be done to correct the late timing in the second cylinder. I will pull the head and look around and report back. Thanks again!

Greg

 

[derek]

the oil is fairly normal. the crank is flexing-not twisting. it flexes back. about the best fix is to retard your timing 1 or 2 degrees. makes the ski a tiny bit slower. but it is a GTX. my idea of performance; she can ride her grandson around the back bays all day and not scream because it died. That's performance!

 

[Greg Mason]

I finally got around to pulling the head. Both pistons look good, no holes! Bores look fairly good too. The o-rings looked bad, crushed, not broken, but there is a lot of white crusty corrosion around the dome, o-ring grooves, etc. Could I be losing compression here? Do you think I should install all new O-rings, clean the head, etc and put it back together, or should I go further such as checking the piston rings?

 

[cubanfisher]

lol off topic, when i get asked questions like this at the marina i work at, i tell them to go to napa and buy a can of compression. most of them beleive it

 

[Greg Mason]

That sure would make it easier, huh. Give me a quart of compression and a gallon of blinker fluid.

Greg

 

[jmartin915]

oh don't forget

Don't forget the pipe stretcher

 

[derek]

put in o-rings and bolt head on and check compression. if it is good, finish buttoning it up. but i doubt it will be.

 

[Greg Mason]

Derek, the cylinder wall doesn't have any scratches but is certainly not perfect, kinda glazed looking. If the motor has over 200 hrs is the engine ready for a rebuild anyway? I have no idea how long these Rotax's should live, so if you guys tell me it would be a good idea, I will go ahead and rebuild it. I HATE doing stuff twice, but if its possible to go another 200 hrs on this short block I will take a chance. I am used to working on much bigger stuff than this so O-ringing it, compression test is not that big of deal even it doesn't work. My biggest problem is I have too many projects going on so I don't want to make something worse than it is.

Thanks for all your good advice,
Greg

 

[derek]

you can use the head O-ring over anyway. it is only a few minutes time to pop on the head and check compression. but, i am really surprised at 40 PSI compression you don't see chunks of piston missing. the head is off? pull the cylinders. the top ring is probably stuck tight into the ring groove. put a pair of pistons in it.

 

[Greg Mason]

I pulled the engine out of the boat, pulled the cylinders off, found a nasty PTO piston. The side of it had a 1.5" wide gouge from bottom to top, and the ring was jammed into the piston. Derek is right, but the top looked normal.
So should I go into the bottom end or leave it alone?

I also damage to the flywheel teeth so I can take it off and flip it over. Right?

The bottom of the oil pump appears to be leaking. I couldn't find any other leaks so have you ever seen the pump itself leaking? I was planning on a block off plate anyway.

 

[derek]

if the connecting rods feel free with no up and down play AT ALL and very little play if you rock then side-side then i think a top end job would do the trick.