Blue smoke at high revs - are my cylinders / piston shot?

[DaveG]

I've had 2 450's.

You can get a new cylinder and pistons for not two much $$. I rebuilt one and did not use new wristpin circlips likely $1 or so. don't make that mistake.

You just lower the cylinder over the pistons using the long studs.
There are no con bolts on the bottom end. The crank is pressed on the con rods.

You can hone the clyinders and use oversized rings. in that case you don't even need to take the pistons off of the rods.

 

[BMANN]

Thanks booya and daveg a pic is worth a thousand words, I see now, kinda like the 2 stroke rotax jugs on my speedster.

 

[Stewart]

have you checked the igintion coils with a strobe light at high rpms, if the coils are braking down, it will pump oil past the rings.

 

[boooya]

have you checked the igintion coils with a strobe light at high rpms, if the coils are braking down, it will pump oil past the rings.

Huh?
Please explain this, I don't get it.

 

[Tripletrouble]

You guys are dull! no one is curious about the corn flakes box? When working on a motor like this I use a corn flakes box (or similar) for the bolts. Often the bolts for a timing cover, rocker box, head bolts etc are different lengths and diameters and can be a pain to sort when reassembling. If you stick the bolt through the cardboard in the same pattern as they are on the motor it is a breeze to reassemble in the correct position.

 

[skapan]

We thought everyone knew that already, but good tip. And that short pieces of 1x2 work well to hold the pistons in place when you put the barrels back on. Use large hose clamps to hold the rings in place, easy to remove after install. Can't remember how many of these I've done, not a difficult job and very rewarding.

 

[Stewart]

Huh?
Please explain this, I don't get it.

if the cylinder doesn't fire, because of the igintion braking down, the piston will create a vaccum on the compression stroke, pulling oil up past the rings. there is no explosion driving the piston down, it's the other cylinders pulling it down drawing oil passed the rings. this is common on multi cylinder engines, with electrical (coil intermitted failure) or spark plugs. this test would be done after the leak down test.

 

[quirco]

Thank you for all the input this thread has received.

I ran the compression test dry a couple times and got a reading of 170psi on each cylinder.

I then put in a couple drops and got a reading of 180 - 185psi

I did it a third time and put in a bit more oil with a reading of 190 psi

Now I know boooya had mentioned that a higher reading with the oil indicated a buggered lower end. Based on my result, what do you think?

 

[boooya]

Thank you for all the input this thread has received.

I ran the compression test dry a couple times and got a reading of 170psi on each cylinder.

I then put in a couple drops and got a reading of 180 - 185psi

I did it a third time and put in a bit more oil with a reading of 190 psi

Now I know boooya had mentioned that a higher reading with the oil indicated a buggered lower end. Based on my result, what do you think?

170psi is still pretty good, 5-10psi difference between wet and dry test in the cylinder of that age is not out of the ordinary.

 

[Tripletrouble]

Those numbers are good and high. My guess is that the intake oil seals are not doing their job and the high pressure readings could be from carbon build up if those numbers are higher than the norm for that bike.