Crankseal inspection/ Leakdown test

[Dirtiki]

Aloha,
I have a 98 Suzuki RMX 250.
It seems to run OK. Now that I am back to stock jetting specs. It has New piston, new rings, new topend everything and new power valve assembly.

But at idle, and it does idle fine at 700 elev. higher elev it is a bit dodgie, it sounds kinda knocky or rattlely or some noise that I cant quite put my finger on.

I rode it yesterday in the rain at 45 degree F from 1000 elev to 2000 elev for 50 miles and it ran fine but i think I needed to go down on the slow jet and up on the main.

I rode for the first 30 mile on flat surface 2nd and third gear 1/4 to 1/2 throttle and broke in new topend.

But........the idle knocky sounds is worrying me.

I have read in the Jetting 101 and the Eric Gorr book that this could be a crankseal leak issue, to lean that is because of extra air coming in. So I did a crankseal inspection.

Removed magneto cover cleaned motor and magneto put baby powder everywhere, reed intake, cylinder base and case seam. I fired it up and I saw nothing. Honestly I wasn't quite sure what I would see.

So........I used WD40 and carb cleaner and sprayed it at these areas while it was running to see if the idle would change. NO CHANGE! The knocky sound is still there. I rode it around and it ran fine.

Should i do a leak down? And if so, how do I do that? Please be really specific if you can. I am more a visual person.

Or do you think it is something else.

Should i ignore it?

Is it me just ? Should I just get better ear plugs?

Thanks
Eric

 

[BigRedAF]

You need to seal off the exhaust port with a rubber expandable freeze plug. Then seal off the intake with a plug or steel plate over the reed cage area. You will need to have a pressure gauge in the spark plug hole and an air valve in the fabricated intake cover so you can pump about 20psi into the motor. Be sure the piston is at the bottom of the stroke. The engine should hold the 20 psi. If the left side seal is leaking it's easy to hear with the ignition cover off. If the right side seal is leaking you can hear it with the oil fill plug removed or from the tranny case breather hose.

 

[whenfoxforks-ruled]

You do have the exhaust valve tension and the choke/air screw properly adjusted. Have you done plug checks?

 

[AV8R]

The engine should hold the 20 psi.

No way! nothing over 8 psi should be used. 6 is plenty.

Here is an excellent post for a leak down test and tester that you can make from standard hardware.


http://www.thumpertalk.com/forum/showthread.php?t=506699

For "knocky" sound check that none of your power valve assembly bolts are loose.
Does the sound go away at slightly higher RPM's? If so it could be piston slap on a worn out cylinder. Did you measure the bore when you had it apart?
You can still measure it by taking off the head and using bore guages and a micrometer to see what the cylinder is at and comparing that to your manual.
Did you check the connecting rod wear? I assume you used a new wrist pin and bearing. If not this could be worn out.
If it is the crank main bearings then you will note movement on the stator or on the clutch side when you grab it and try to move it. There should be no perceptable movement.

 

[BigRedAF]

AV8R:

Thanks for the PSI correction, I pulled the number it out of my a$$. I haven't had the need to do a leak down test since the mid 1980's. The process is correct but sealing the intake and exhaust gives people the most grief when trying to set up for a leak down check.

After looking at the links that you posted I wish I would have been more creative in my youth. I used the same expandable freeze plug set up however, I used a Snap On screw in gauge in the spark plug hole.

 

[whenfoxforks-ruled]

Let it idle for a minute and pull the plug. Wet or dry? How many turns do you have on the exhaust valve tensioner? Would an oval bore make the piston knock?

 

[Dirtiki]

You do have the exhaust valve tension and the choke/air screw properly adjusted. Have you done plug checks?

I have the exhaust valve tensioner turn about a quarter turn. But I am not sure how much it is supposed to be turned.

I have the air screw turned out one turn. The choke idle is about 8 turns from the bottom.

Are these wrong?

Eric

 

[Dirtiki]

No way! nothing over 8 psi should be used. 6 is plenty.

Here is an excellent post for a leak down test and tester that you can make from standard hardware.


http://www.thumpertalk.com/forum/showthread.php?t=506699

For "knocky" sound check that none of your power valve assembly bolts are loose.
Does the sound go away at slightly higher RPM's? If so it could be piston slap on a worn out cylinder. Did you measure the bore when you had it apart?
You can still measure it by taking off the head and using bore guages and a micrometer to see what the cylinder is at and comparing that to your manual.
Did you check the connecting rod wear? I assume you used a new wrist pin and bearing. If not this could be worn out.
If it is the crank main bearings then you will note movement on the stator or on the clutch side when you grab it and try to move it. There should be no perceptable movement.

Ok. I will check the power valve. But I just reassembled it when I rebuilt the motor and powre valve. I tightened it really well with lock tight.

The sound goes away as revs go up. what does that mean?

No I forgot to measure the bore.

Unfortuenlty I dont have a manual and the bore looked great when apart.

I didnt check con rod wear. I did use a new wrist pin bearing and pin.

The stator has no movement accept rotation.

Now what?
Eric

 

[Dirtiki]

Let it idle for a minute and pull the plug. Wet or dry? How many turns do you have on the exhaust valve tensioner? Would an oval bore make the piston knock?

I let it idle and pulled the plug. it was wet. What does that mean?

I have the tensioner turned one quarter turn. But I am not totally how much it needs.

Eric

 

[whenfoxforks-ruled]

On my 00 it was about 2 turns on the exhaust. If the plug is wet, then I would not worry about being lean, sounds rich on bottom. The air screw at one turn confirms it, drop your slow jet one size. Putting more tension on the exhaust will make the power band hit harder, and could take the excess slop out. Could have been 2 and a half turns.

 

[AV8R]

The sound goes away as revs go up. what does that mean?

Could mean piston slap. Only way to tell is to take the head off and measure the cylinder. Cylinder to piston clearence only runs around .002" max. More than this and everytime the piston goes up and down it rocks on the wrist pin and "slaps" the cylinder walls with the piston skirts. This could lead to excessive piston wear and piston skirt failure or uneven wear of the bore.
Was this sound present before the top end? If the bore is worn, I would guess that it was. Although with the lower compression of a worn out top end it may not have been so loud or noticable.

I didnt check con rod wear.

Con rod excessive clearence (at least in the up and down direction) would be heard throughout the rpm range but at different pitches that would be directly related to the rpm of the engine. The side clearence is a different matter. There is a max limit for this also as the side thrust washers wear out.

 

[Dirtiki]

Could mean piston slap. Only way to tell is to take the head off and measure the cylinder. Cylinder to piston clearence only runs around .002" max. More than this and everytime the piston goes up and down it rocks on the wrist pin and "slaps" the cylinder walls with the piston skirts. This could lead to excessive piston wear and piston skirt failure or uneven wear of the bore.
Was this sound present before the top end? If the bore is worn, I would guess that it was. Although with the lower compression of a worn out top end it may not have been so loud or noticable.

I just rebuilt the top end. The piston that I removed was in perfect shape. Except were it kissed the broken power valve. Very little discoloration anywhere on the piston or noticable wear.


Con rod excessive clearence (at least in the up and down direction) would be heard throughout the rpm range but at different pitches that would be directly related to the rpm of the engine. The side clearence is a different matter. There is a max limit for this also as the side thrust washers wear out.

Nope there was no movement in any direction to the crank/ magneto.

Thanks AV8R,

Eric