Jump to content

Compression question


Recommended Posts

I just got an 09 250 XC and for maintence purposes checked the compression. Is 98psi low? My 05 yz 250 had 220psi according to the guy that just bought it. Just wondering if something is wrong, thanks in advance

oh and my YZ was bored to a 267 and had the head milled, but is it that big of a gap??

Link to comment
Share on other sites

It's a brand new bike! I did test it with a modified 20+ year old compression tester which may be the reason. It's really not even broken in completely yet, I've just ridden around my back yard. I used the estart button to turn the motor over instead of kicking it, would that cause it?

Link to comment
Share on other sites

It's a brand new bike! I did test it with a modified 20+ year old compression tester which may be the reason. It's really not even broken in completely yet, I've just ridden around my back yard. I used the estart button to turn the motor over instead of kicking it, would that cause it?

Well then your tester may be the problem... Also when doing the test

#1 remove the air filter

#2 open throttle all the way

#3 kick it 10 to 15 times without stopping...good kicks

#4 check gauge

#5 do it all over again to be sure...

Link to comment
Share on other sites

Well then your tester may be the problem... Also when doing the test

#1 remove the air filter

#2 open throttle all the way

#3 kick it 10 to 15 times without stopping...good kicks

#4 check gauge

#5 do it all over again to be sure...

Ok that's what I was hoping for, I didn't hold the throttle open, nor take the air filter off, nor kick it, nor use a quality gauge haha. Would it hurt to ride it tomorrow and break it in properly? Or should i check it again?

Link to comment
Share on other sites

Didn't get to test it again yet but I rode it for about 5 hours today. Decided on the red spring, need the tool to adjust it further though. It idles fine and starts easily, I think it was the tester not the bike that was producing that number. I'm going to test it again anyways though.

Quick question though, the bike kinda felt like I was on ice almost whenever I'd pick up speed on fire roads, is it just the suspension not being broken in or all they all like this? Tire pressure was 13 on front and rear. Hardpack with a loose on top, yes I know it's not high traction but my YZ felt pretty planted through the same stuff.

I love this motor so far, along with the super-brakes and hydro-clutch, oh and the e-start makes me tingle inside every time I use it haha.

Ah and my throttle got hung half way open in fourth gear on one of the fire roads! Will safety wiring the ends prevent this? A tree limb didn't hit it or anything, I was riding a wide open road through a field. Scared the sh*t out of me, I just slammed on the brakes and it came unstuck. I was reaching for the kill button though...

Link to comment
Share on other sites

ahhhhhhh it makes sense now... i did the same thing to my 250sx the other day (with like 30 hours max) and the thing said 50 psi lol.

so;

1. dad's gonna get a new compression tester, the other one didnt fit perfectly and was cracked up at the bottom so it would of leaked and was 30yr.old. :applause:

2. yeah i was kicking it like mad while dad was pushing the guage in.

3. no i wasnt holding the throttle wide open, but i understand why you would, then the motor can actually suck air in.🙂

4. taking off the airfilter.

^ this i dont understand:bonk: it sounds like cheating, i guess it'd help the flow of air, but the bike doesnt get this advantage while its running. why dont we just put on a supercharger while we're testing...🤣

can someone explain?

thanks, Mike. 👍

Link to comment
Share on other sites

ahhhhhhh it makes sense now... i did the same thing to my 250sx the other day (with like 30 hours max) and the thing said 50 psi lol.

so;

...................................................

4. taking off the airfilter.

^ this i dont understand:bonk: it sounds like cheating, i guess it'd help the flow of air, but the bike doesnt get this advantage while its running. why dont we just put on a supercharger while we're testing...🤣

can someone explain?

thanks, Mike. 👍

Good question,

The restriction of the air filter can change depending on how dirty and how much oil is in it. Somewhat like holding the throttle open vs closed. If you have a slight restriction in the intake your compression pressures will be lower. You don't what this to be a variable in the compression check results so you just get rid of it. I don't think it is normally much of a factor, but you don't want to tear down a top end because your air filter is dirty and removing it eliminates the possibility it is a problem. Also, you don't want your history of readings to be skewed by the filter condition, but rather to reflect blow by due to piston, ring, and cylinder wear and condition. For instance it would mean more to have reading unaffected by something like the air filter from when it was new, after break in, with various times since top end, after new rings, etc). As for history it would be nice to have used the same compression tester too.

You don't ever ride with a super charger, but you do run with various configurations of dirt and oil saturation on your filter. Maybe you could check your air filter condition by doing compression checks, I don't know, but it is probably easier to look at it and see it needs to be cleaned then it is to do a compression check for that.

Try a compression check with and without the filter and see if there is any difference. Let us know what you find. Again, even if you don't see a difference, you would want to remove the filter before using a compression check as an indicator that you need to do a top end.

Gary

Edited by Gary jp4
Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:

×
×
  • Create New...