Piston and Cylinder scoring on my LTD :(

[jimvw57]

I have always tried to vary the speed for the first miles, no constant speeds for an extended length of time.  After the first time they get up to temperature, it doesn't hurt to re-torque the heads.

[Mister Twister]

What is your ring gap and piston clearance in the cylinder? That wil tell a lot of where your compression should be. 90 still sounds low and it's going to start hard.  A little oil down the plug holes before you start it will bump it up temprarily to get it to fire for break in.

[Checkmarks]

When Twister says a little oil down the cylinders, I would add to his comment, one tablespoon through the spark plug holes.  90 psi is going to be hard to get started.  The extra oil helps the sealing and temporary bumps the compression up. 

[That Girl Racing]

I would strongly recommend you check the leakdown on that engine.  If it is less than 5%  you need to check the squish clearance . Google it, lots of info on how to do it. My guess is the aftermarket base gaskets or pistons are slightly different than stock. Probably should be around .070 stock. Someone else may know the exact figure. As far as that 90lbs going up... not a chance. You already have oil around the rings that will bump the compression in itself. Breaking in a motor doesn't pick up 40lbs of compression, at least not in my world.

[gixxer6]

Are you sure your compression tester is accurate? 

[shizznats]

Thank you all... just saw these replies now (I'm a little slow...) Doesn't sound too encouraging for our engine running with the new base gaskets, pistons, and rings :/ ... we're fairly sure the compression tester is accurate and we were getting the same reading on both cylinders (90psi). We are going to try to check the squish clearance and do a leak down test and post those results once we do. Thanks again.

Edit: Would this be the correct procedure for doing a leak down test on this motor?: Remove one spark plug, hook up compressor with a gauge in it's place, plug up the exhaust and intake ports, put piston at TDC, block the clutch so the piston can't move, apply approx. 100psi, and watch the gauge for about 20-30 min? Repeat with other cylinder. I read about this a while ago but want to make sure I'm remembering correctly.

Also... would a leak down test even be useful at this point... we know that the new pistons/rings have not seated yet so wouldn't we expect there to be too much leak down until we run the motor and they (hopefully) seat?

[jimvw57]

That will work. All it is going to tell you is if you have leakage past the rings or a head gasket leaking.

Oil in the spark plug hole will tell you the same thing, add oil, if the compression goes up. it's the rings. If it doesn't, it's head gasket.

A leakdown test will show more on a 4 cycle motor, like which valve is leaking. Since a 2 cycle Kawi motor doesn't use valves, why bother with a leakdown test.. (just my opinion)

[Lloyd (ljm)]

 Do a leakdown test for a two stroke engine.

 Plug exhaust and intake, leave spakplugs in, apply 10 lbs of air pressure to the impulse fitting.
 If it doesn't hold pressure   FINd the leak.

 Google  2-Stroke leakdown test or crankase pressure test.

  Not doing this will result in engine failure. BTDT

 Lloyd

[shizznats]

From what I understand, a crankcase pressure test and a leakdown test are two completely different things that get confused a lot. We did a crankcase pressure test, found that the MAG side crank seal was bad, replaced it, and re-ran the test... motor held 7psi of pressure for 10 minutes so we were satisfied. A leakdown test might be more appropriate to check if air is getting past the rings.

[shizznats]

Alright so I just went to check the compression again and here is what I got:

Cyl 1: 90psi before adding oil, 110psi after adding a tbsp of oil through spark plug hole
Cyl 2: 90psi before adding oil, 100psi after adding a tbsp of oil through spark plug hole

Idk if it matters, but it took quite a few pulls to get the readings above, not just 3 or 4. Again, this motor has not been started since the top end was rebuilt so we're hoping that the rings will seat and it will bump our compression to what it should be (120-130?) Possible based on the above?

[Lloyd (ljm)]

This is just my opinion  but  compression of 110 psi on each side or above should be fine.

 125-130 would be good   bur 110 or above should work.

 Lloyd

[Joel]

Just curious did you use a flex hone to cut a fresh cross hatch (lightly) prior to reassembly? Joel

[shizznats]

Just curious did you use a flex hone to cut a fresh cross hatch (lightly) prior to reassembly? Joel

We did a lot of reading about this... there are some who say to lightly hone nikasil plated cylinders and others who say not to come within 10 miles of them with a hone, and that they will seat without any cross hatching. After reading a lot of articles and threads on the topic, I decided that since I have no experience with honing cylinders, I'll leave them be. I simply put some muriatic acid to get some aluminum residue off of a few spots on the cylinders, and then cleaned them and reassembled.

What do you guys think... should these be honed prior to break in?

[gixxer6]

I know it's debateable...But I have honed several chrome cylinders lightly without issue. 

[Checkmarks]

Did you use OEM pistons?  If you did not then like That Girl Racing said, "check the squish clearance".  If you still have your old pistons, put one old and one new piston in the cylinders and measure.  Measure from BDC to the top of the cylinder deck on both cylinders.  With a good steel ruler you should be easily be able to see if there is a significant difference.  High tech measure, use a dial indicator.

On compression:  you can run the motor on one cylinder with the gauge in the other cylinder and the motor will only develop x amount of compression.  It does not matter if you have to pull the cord 100 times if it is still moving the needle on the gauge.  No offense intended but if you gave your sled little girl compared to burly manly man pulls the manly pulls just get to the maximum faster with less pulls.

Just a side note.  I have two compression gauges.  Gauge one will demonstrate 90psi as a maximum in my motor.  My second gauge (also bought from Autozone) will report 130psi in the very same motor.  The compression number your gauge/motor reports should not be directly compared to your buddies gauge/motor.  With your 90psi recorded in a note book and years from now is gauge checked again with 87psi then you have something.  You can monitor your wear.  Say in two years of racing your sled is becoming difficult to start and the compression is down to 80psi per cylinder.  You can expect with re- chroming the cylinders, new pistions/rings to get back to 90psi.

Altitude can also play an effect on compression.  My motors (@5280ft) run fine if they put out at least 90psi both cylinders within 5% and start great.