Yz85 new top end 100 psi?

As title says, brand new cylinder and piston aftermarket brand. Compression slightly better than before. Main bearings feel tight, Will weak crank main seals be the culprit? Possible open reeds?

Ring end gap set to .016

Reeds look good

Reeds look good

What is standard PSI on this engine. PSI read is only from compression so rings would be the loosing end. Seals will be from leakdown test. 

Can't verify a yz85 but a rm85 runs 150psi on fresh rebuild, can't imagine much difference! Sounds way low in my book and maybe the wrong Piston/rings

It was a complete kit, so I doubt the wrong piston. I'm leaning towards seals, they do effect compression, correct?

The bike did not smoke excessively

Compression before complete top end was mid 90's

Yup,yz85 150 psi of compression, build a leak down tester,the instructions how to build one are in the sticky at the top of this forum.have broke the bike in?compression will be low till the rings seat.

Throttle is wide open when testing?

Is the power valve closed?

Squirt some oil through the plug hole and re-test.

Is oring for head cut or anything like that?  Is this a Chinese kit?

I believe it is Chinese, 90$ complete would suggest so. No power valve, throttle wide open

Using a Chinese piston in a 12000 rpm engine ?  

You really like to gamble!

1 hour ago, ickfinger said:

I believe it is Chinese, 90$ complete would suggest so. No power valve, throttle wide open

100psi may be it ;) put couple of those on big husqvarna saw engines didn't last a week.  But if was only used now and then probably be ok same goes for YZ. I noticed the ports much smaller on replica also. So don't expect much but may be ok. Put one on a blaster engine one time same thing owner got off ebay for 100. It does run still and pretty good. A wife rides it tho and gets babied. Goodluck bud :thumbsup:

So today progress...compression with dry side crank seal removed exactly the same.

Have you compared the new piston to the old? (height, diameter)

Did you measure piston to cylinder clearance, old versus new base gasket thickness.

 

You mention a new cylinder, brand new OEM or replated?

Again I wonder why cheaping out with a questionable Chinese piston if going to the trouble and expense of a new cylinder.

 

Edited by mlatour

Yz85's do not have plated cylinders, this kit did not say where it was manufactured, I can only assume at $80 it's Chinese. Everything measured out same as stock.

12 hours ago, ickfinger said:

Yz85's do not have plated cylinders, this kit did not say where it was manufactured, I can only assume at $80 it's Chinese. Everything measured out same as stock.

This is not true.

02-18 YZ85 is in fact plated

please see the Oct 17th post in this thread

 

Edited by R and D Marine

I did not see in that post where it mentioned it was a plated cylinder. You can usually tell by looking at them,right? My aftermarket kit is pretty much the same as the one used in that build.

Can someone give me a clear answer to the question about the Crank seals? Will bad seals cause low cylinder compression? As I stated, I took out the ignition side seal and it made zero difference. Still holding only about 90 pounds in the top end. I would think missing a seal would cause a very dramatic effect!

It just doesn't make sense to me that The original cylinder and the new one make exactly the same psi

 

The Engine took a fair amount of kicks to fire before new top end(15-20), so based on my initial compression test, I figured the top end was tired. I have never rode this bike, I am building it as a surprise Christmas gift for my friends son. The clock is ticking and I want it to be reliable.

 

See pic below.......

You mention in the Nov 20 11:18 post that you completely removed the crank seal and saw no change in the result of your compression test.   In a two stroke, the compression test only captures the sealing power of the rings against the cylinder after the piston has closed all the ports. 

I will perform a compression test on our 85 tonight and let you know what number I come up with.  

FWIW, 100psi is enough to run.  As for how healthy that is, let me see what ours makes. 

Is your gauge leaking?  Are you holding the throttle wide open while performing the test?

IMG_8702.JPG

Edited by R and D Marine

Those Chinese kits are notorious for having porosity and impurities in the bore, leading to lost compression. I looked into them for rebuilding my 350 twin, it looked enticing for $250 but people who used them didn’t like them at all. Some guys had absolutely no compression at all.

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