Hey guys,
I’m chasing my tail a bit on compression readings and wanted to see if anyone else has run into this.
I routinely check compression and leak down on all our bikes and have a pretty consistent process.
My son moved up to a YZ85LW last year. Before the season, compression checked at 110 PSI, so I went ahead and put in a Wiseco piston and ring set. Honed the cylinder, verified ring gap, replaced all top-end gaskets, and leak tested the cooling system.
Did 3 heat cycles at idle, followed a proper break-in (15:1), then switched to 40:1. Jetting is 145 main, 45 pilot, 1.75 turns out.
Bike runs great. No issues at all.
Here’s where it gets weird:
On both of our YZ85s, I’m consistently seeing low compression readings, around 90–100 PSI. The bikes feel strong, start easy, and don’t show any signs of low compression.
I’ve tried:
- Different compression testers
- Testing hot vs cold
No change.
On all my other bikes, compression has always been a reliable indicator for engine health, but these 85s just aren’t lining up.
Does anyone else see this with YZ85s? Is this just normal for these motors, or am I missing something?
Well , i personally dont own any yz85s at the moment but i would not be running 15:1 ( lean actually) regardless what anyone told me or what i read somewhere....
I will leave it at that because there will be alot of opinions on this....
3 heat cycles riding the bike or on the stand in the garage?
Seating the rings is so important at initial start up and to perform this properly you need to put a load on the motor , ie; riding the bike . Higher oil content (15:1) could actually prevent initial seat.
Maybe youve done 100s of rebuilds and your comfortable with that process , but if im having compression drops that drastic immediately after break in id be pursuing other "break-in" methods.
32:1 , 40:1 and heat cycling by putting a load on the motor immediately after first start-up ...imo.
I am not implying what you have done is wrong , but what i would be doing if they were my motors....
Hope you are able to get to the bottom of this.
15:1 is just asking to lean seize the motor. I know some of the manuals recommend this but it is not necessary at all and a big risk. Next thing I would look at - what type of honing are you doing? You may be creating an improper cross hatch pattern or excess piston clearance. Are you measuring the piston and bore precisely after the hone?
I suspect there may be something odd going on with your testing method. Most little 2-stroke engines will not want to kick start when the compression numbers get that low and will need to be bump-started. Numbers in the range of 90-100 would typically give an immediate "WTH?" reaction when pushing the kicker with your hand.
Hey, Walent, thank you for the advice. 15:1 is the recommended mixture for 1 hour of riding after a rebuild, according to the service manual. The full break-in process is as follows:
After Rebuild:
I typically run 40:1 VP T2.
So I tested this on a friend's son's YZ 85 at the track this weekend and ended up with the same results. 85PSI. This makes the third YZ85 I have tested that produces the same results (approximately).
Here is my son riding it this weekend: https://www.youtube.com/watch?v=ckDCoHPHI4w
Compression (feels) great and the bike runs really good. Just can not get the numbers right on these 85's. I feel like I am missing something on the testing side.
I have also performed this on:
KTM 350 XC and a YZ250 and compression come in line with what the calcualted range should be. YZ250 for example was 190 PSI. So I believe that my instrumentation is good.
Dropping oil down into the cylinder head does not change the pressure reading. Which should be a sign that blow by is okay.
Hey pal, thanks for the feedback. Yeah so I have always just followed what the service manual say as far as break in.
On the honing, I do get the inner diameter of the cylinder wall and three locations and the compare it to the piston 20mm above the skirt. Which is what the manual has recommended.
When I hone I typically run it in and out several time until a good cross hatch at 45 degrees is achieved.
But here is the strange part. I tested another a friends son's yz85 at the track over the weekend and it produce the same results. Around 85 PSI. We went right over to my YZ 250 and it was 190 PSI.
So that makes now the 3 third YZ85 that has produced those results. Do you know of any videos or articles where people are actually achieving those results?
The bike runs great and compression feels strong. I just cant get the data to agree with that.
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The compression testers you are using are failing you or the method you are using the tester is incorrect. I personally have spent years building the yz85 and yz65 and i can tell you that an 85 with 85 psi of compression will NOT be a good running and easy starting engine, that just doesnt even make sense. A new (fresh) yz65 will show consistently 130-135 psi, an 85 will be around 145. Also in my opinion, you are wasting your time with these compression tests, and break in procedures. Regardless of the manual, 15:1 is not how a success engine break in is done. A compression test on these engines should be performed as follows; throttle open 100% and kick until the needle stops moving. For what its worth, I have rebuilt dozens and dozens of these 85's and ive never once seen a fresh top end produce just 110 psi on my gauge,
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