Compression issues... need some help:

H

hobbyz

X
I need some help understanding some compression issues.

Here's the deal: Front Cylinder at 120 lbs; Rear Cylinder at 90 lbs. Yeah, I know that's a problem :bigeyes: .

What would cause the difference in readings? Both were tested with no plugs in and WOT. I checked the reeds and they apear in tact (metal reeds) and the cages seem good too. So I took the head off... LOL... it's bad. The Rear Cylinder (left side in the pictures) with the lower compression seems to have a small flat spot on the side of the piston (top of pic) and both cylinders are SLIGHTLY loose.... so bad rings maybe?

I don't want to jump to conclusions though... could there be a bad bearing or seal blown out? But, if that were the case, wouldn't there be an equally 'low' reading in both cylinders... and shouldn't they have even readings? I just don't know enough about this part of engine work, and I have no idea what would cause two different readings.

I'll post pics in a few. Thank you all for any help!

Brad
 
Big Kahuna

Big Kahuna

Administrator
Location
Tuscaloosa, AL
what kind of guage are you using............

bearings are not gonna cause compression loss, neither will seals, compression is built from the top of the exhaust port up to TDC (Top Dead Center) where the piston has traveled to the top of the bore..... So, the only thing that will cause loss of compression is Head gasket or orings blown or broken or stuck rings.
 
kawirider00

kawirider00

BRAAAP
Location
South Jersey
there are no pics, but those numbers are decent, hard to say without pics, but it sounds tired, bore it one size up new pistons and rings, check the crank for excessive movement, its possible one rod bearing is bad, but you need the topend off to check
 
H

hobbyz

X
Big Kahuna said:
what kind of guage are you using............

bearings are not gonna cause compression loss, neither will seals, compression is built from the top of the exhaust port up to TDC (Top Dead Center) where the piston has traveled to the top of the bore..... So, the only thing that will cause loss of compression is Head gasket or orings blown or broken or stuck rings.
Click to expand...

The screw in type guage.. Auto Zone special... but it does screw in and has always worked right.

Your explanation is what I was thinking, but I have zero experience this deep in... time to learn :biggthumpup: . So, I'm thinking: Pisotons, Rings, bore.

Since I'll be doing the pistons/rings, I will be able to inspect the crank/rod bearings... correct?
 
H

hobbyz

X
kawirider00 said:
there are no pics, but those numbers are decent, hard to say without pics, but it sounds tired, bore it one size up new pistons and rings, check the crank for excessive movement, its possible one rod bearing is bad, but you need the topend off to check
Click to expand...

Shouldn't the numbers be around 130 apiece? But I'm not sure... 145 I thought was good for stock.

Bore/pistons/rings was exactly what I was thinking... thanks!
 
kawirider00

kawirider00

BRAAAP
Location
South Jersey
numbers should be 140-150, but his numbers dont signify a hole or a massive gouge, looks like a broken piece of the ring got into the top, and probly caused a vertical scrape on the rear piston, leading to some loss in compression, easily bored out, 120 psi sounds like a tired motor, it just needs to be freshened up
 
Speedfreak

Speedfreak

Location
So. IL
tom21 said:
the way it looks in the pic the top edge is rounded or burned away. Is it not?
Click to expand...


I agree it looks like the piston is eaten up at the exhaust port which is a lean conditon on the front cylinder.

You should also rebuild the carb or carbs when you do the motor to make sure that there isn't a problem there.

Also you need to check into it. I know that the tigershark 900 had nikaseal(sp) cylinders. If these have coated cylinders then you have to send the cylinders off to and have it drilled out and cast iron sleves installed or have the cylinder recoated after a bore job. A lot of money for an older ski.
 
Last edited:
H

hobbyz

X
Thanks everyone.

I do think the exhaust side has been "burned/melted/etc" away. And from what has been stated, that may be a lean condition. Also, when you would crank the engine, it would have a hard time 'idling down' when on the trailer; my friend said that would be tell of a lean condition as well. So: Lean = 2 / Brad = retard :Banane01: :shooter3: Should have caught that earlier.

If it is lean, which I think it is, the causes could be:

1: improper jetting (which wouldn't make much sense, since my friend ran it for years with no problems)
2: fuel pump / carb needs rebuild (seems to be more likely that the diaphram could be torn or have pin holes)

So hear is where I stand: You guys are awesome, thanks for the help. I should do the following: 1) Rebuild the Carb and 2) Bore and Top-end. Sound right? Total @ $350 (parts and machining).

Now, about the leakdown test, I don't know how exactly to do that. Is it a necessity?

Thanks again for everyone's help!
Brad
 
SkiDaddy

SkiDaddy

Just Havin' Fun!
Location
Orange City, FL
Crank seals wear, which can also cause a lean condition. I'd definately do a leak test.

Use the search feature; there are several threads detailing how to do it.:biggthumpup:
 
The other Alex

The other Alex

(Jetdude)
Location
Lake St. Clair--
If you are ripping into it anyways, I would just replace the crank seals anyways and skip the leak down test. Setting up a leakdown test would take more time anyways. If the motor wouldnt idle down, then that could be the cause of an air leak, so yes, lean, and quite possibly, crank seals.
 
Speedfreak

Speedfreak

Location
So. IL
If you skip the leakdown test and you "think" everythgin went fine when it went back together it "could" cost you a new bore and a couple of pistons within a couple of hours of riding.

I just rebuilt a kawasaki 750 and everything went fine new seals and everythgin. I thought it was an air leak so it got new crank seals and all and after a couple of hours on the ski it was running like crap...

Well skiping the 15 minutes to perform a leakdown test cost me a new bore and a couple pistons. The case had a tiny crack that you could not see. How it devloped I don't know when it devloped I don't know but both times I rebuilt it the front cylinder was leaned out and it ate the piston at the exhaust port just like the piston in question here but worse.

It will only take a little time and it is pretty easy to do. But the hour (I am over estimating here) it takes you to do a leakdown test is still less time and money than a complete rebuild

I learned my lesson every motor I work on gets a leakdown test before and after teardown. and mine gets one every winter wether it needs it or not
 
The other Alex

The other Alex

(Jetdude)
Location
Lake St. Clair--
Thats a good point, I never thought about the possibility of the crank case having a crack in it. I always assumed a leaky crank was leaky crank seals, and that what the leak down test was supposed to determine.
 
You must log in or register to reply here.
Top Bottom