Compression / Engine condition

[riverman]

Trying to figure out the condition of the engine of my donor bike for my conversion. Before removing the engine from the steel frame I done a compression test 115psi. The guy I bought it from said he had just done the top end (doesn't everyone). It does look to have new gasket on the base of the cylinder. But looking up the exhuast port the cylinder looks like it is worn pretty good. So what should the compression be. I need to do a leak down test also.
The motor was hard to start but ran good and would start easier after the engine was warm.

thanks

[AlisoBob]

You can do a negro leak down test by cycling the kicker through the compression stroke.....

This is kinda hard to describe, but once you start feeling compression.... you "bounce" the kicker as you turn the engine over.

If the leakdown is good, the kicker will bounce back at you.... like the engine turns 15 degrees, and bounces back five... turns another 15, and bounces back another 5... and so on, and so on.

If the leakdown is bad, the engine will turn 30 , 60 , 90 degrees and the compression stroke is over.

You'll get the idea

[Nodge]

I just bought a harbour freight compression tester off ebay for like $20.
It gets the job done....

[AlisoBob]

Remember to hold the throttle WFO and ground the spark plug on something.

[100hp honda]

gauge shows cranking pressure. i see no way to translate that into how much the topend is worn on a unknown motor. if you knew 2 years ago that particular motor had 180psi for example, and now 2 years later it has 115psi, then it would give indication of possible worn parts. but to throw a gauge on a unknown motor and use that as a judge to whether anything is worn just seems silly. if you understand how many factors affect the gauge reading then you would see what i meen and believe me theres alot of things that cause the needle to go up or down. good strong motors can register 125psi..... because of all the factors that affect the gauge like i said before. to most people that would indicate something is worn out . is the piston slopping around ? crank bearings shaking like a crack whore ? thats what i would be concearned about.

[2strokeforever]

or whoever sold it to you coulda shaved the head until it made a good #
id pull the jug and measure the bore (oval), check clearance, check the rod big end, see if the piston is cracked and check reeds
that way you know for sure whats going on

[riverman]

I understand compression is a relative number and many things could change it , porting bad gaskets and seals. But i would think there is a base number or ball park number for a stock bore. That what i was asking. When I rebuild I always check it to have the base line number in the future.
The engines getting rebuilt this winter if it was really low compression I was not going to run it any more before the rebuild.

Thanks

[dubious01]

or whoever sold it to you coulda shaved the head until it made a good #
id pull the jug and measure the bore (oval), check clearance, check the rod big end, see if the piston is cracked and check reeds
that way you know for sure whats going on

doesn't everybody mill the head? I mill the head on every 500 I ever owned. Best bang for the buck mod out there, makes a pipe or reeds look like a waste of money.

To answer his question, a stock cr500 without the decompression eyebrows is wore out if it reads below 135 psi .... thats my experience anyway. Stock exh port, stock head, wfo multi kick test.

[plynn41]

Remember to hold the throttle WFO and ground the spark plug on something.

Sorry to hijack the thread... Why should the plug be grounded when checking compression?

[AlisoBob]

When the CDI fires the coil, the high voltage jumps the gap of the plug.

If your kicking it over with the plug out and dangling, or removed from the plug cap..... this high voltage spark has no where to go.

It might arc over from the coil tower, inside the coil itself, or damage the CDI. All are bad scenarios.

Whenever you kick the bike over, ALWAYS ground the plug, or push the kill button.

[100hp honda]

probly best not to dry fire the ignition. i usually hold kill button

[seanmx57]

The last stock headed motor I built was at 90.5mm so it had no scallops in the exhaust. It turned 165 with a fresh bore and piston (vertex) at .003 cylinder to piston clearance. Turned 135 with a decompressor head and the valve enguaged.

I've always read that once you loose 10% of your compression it needs a rebuild.

So 165 - 16 = a lot more than 115............but it still might run well after all it is a 500 that is broken in. I'd pull the reeds and check the piston for cracks on the intake or you could loose the cases if the piston takes a dump

[100hp honda]

many variables like i said before. dont forget altitude aswell. the stock 550 is 125psi up here and runs strong as a sherman tank, fresh pc-9 around .0025 clearance, brandnew reeds, good gap on the rings, no air leaks, good seals etc etc. at see level it would be around 150-160psi. previous owner slightly modded the head most likely because it was a bear to kick over and that has decreased pressure slightly. probly have a extra 10psi if head was unmodded.

[seanmx57]

I'm around 1500'

[100hp honda]

im bumpin 6500'. at 10,000 its doubtful i would be much above 100psi, but obviously nothing in the motor is wore out. tons of places in colorado near 10,000. must be a bitch losing all that power

[riverman]

Yea I'm at 500ft above sea level. I figured 115 was low it's been a while since I checked a 500 could not remember what was a ball park number. I have 2 banshee's stock is about 135 but I have a milled head on one and a cool head on the other with 19cc domes they are at 175 to 180. I'll be pulling the top end off and checking everthing Rod & crank. I always use wiseco piston but it sounds like most people here do not like them?

Thanks

[100hp honda]

never had a prob with wiseco pistons. doubtful i would ever use their cranks or rods but the pistons treated me well

[AlisoBob]

I always use wiseco piston but it sounds like most people here do not like them?

Nope.....

[100hp honda]

hey roostius you got the weight and clearance specs for the BRANDNEW cr500 from wiseco ?

[Roostius_Maximus]

hey roostius you got the weight and clearance specs for the BRANDNEW cr500 from wiseco ?

the weight is posted in the wiseco thread i think, i dont have my book at home today