Low compression after top end

[MWEISSEN]

We rebuilt my kids' '01 YZ250F over this winter, taking a lot more time than I thought. We repainted the frame, re-did the forks, etc at the same time.

Anyway, after all is said and done, it was time to kick it over. The first thing I notice was the relative lack of compression. Youza! Nasty things went through my mind of bent valves, broken rings, or other obscure stuff. A re-check of valve timing, ignition timing, head torque indicates all is well and good.

I've worked on bikes for 34 years and am careful with my work. This is the first Yz250F Iv'e done, and I'm perplexed by the relatively low comparession. Should I expect this until the bike runs and breaks in the rings better? Oh yeah, the run part - it won't start.

I feel silly asking nebulous questions, but I'm just looking for good advice on what to look at next.

 

[Rich Rohrich]

Mark - I'm assuming you already verified, fuel and spark so a simple place to start might be to look at the compression release first, to verify that it's not hanging the exhaust valve open. Then you might want to move on to dropping some oil down the sparkplug hole (two stroke oil if possible) in case the cylinder walls are washed and the rings don't want to seal yet.

A leakdown test would give you a good idea where the compression is going, but if that's not an option, it might be worthwhile to pop the cams back out of it and check that you don't have a valve cup hanging up. You can tap the cups with a small plastic deadshot hammer to verify that they are seated. Once you are sure the valves are closing all the way, you can hold up the cam chain and try slowly cycling the engine to see if it is still lacking compression.

If you need some specific help feel free to PM or e-mail me. Good luck. :thumb:

 

[MWEISSEN]

Rich, thanks for the great advice!

Spark and fuel are OK, and I checked the compression release clearance. However I realized after posting I didn't re-verify valve clearance after re-torquing the cam caps. I'm leaning toward a valve partially open due to an errant shim or other reason. I'm also thinking about the oil trick you mentioned. I had oiled the rings, piston and top end well during assembly, but with work and all going on that was two months ago! I hope to have time to look again tomorrow, will post what I see.

Thanks again.

 

[smb_racing]

kind of a dumb question because it seems you know your stuff, but you didn't somehow align the ring end gaps during assembly did you?

 

[Hick]

Originally posted by MWEISSEN
I'm leaning toward a valve partially open due to an errant shim or other reason.

I managed to leave a shim wedged a bit sideways in its "hole" on top of the stem, forcing the bucket to ride high enough to take up all the clearance and leave the valve hanging open.

When I got it back apart I had to look twice to figure it out, it only takes a small amount to take up the clearance, and the shims fit snugly in place.

I'm thinking if it isn't this or some other similar "oops", it must be something simple.

 

[MWEISSEN]

smb, nothing's "dumb" when there's something like this, but yes I staggered the ring gaps. Thanks for the feedback.

Hick, that's exactly what I think happened, and I suspect one of the exhaust valves. When kicking the bike over and listening to each end, the exhaust doesn't "sound right", so I think the shim is hung up.

A hint of advice to YZ/WR owners - if you need to do a valve clearance adjustment, take the time to pull ALL of your shims and write down the value of the shim. I didn't do this on this bike (I had on my 400), and found that two of the shims are now unreadable after two seasons of wear. I just put a micrometer across them to get the reading, but if you don't have one of those having those numbers when you have to do another adjustment later helps get things in working order quicker.

 

[MWEISSEN]

Rich, Hick, your advice was dead-on. Thanks a bunch. A quick .10mm feeler check verified NO GAP on #1 intake valve. I pulled all the buckets to make absolutely sure all the rest were set, buttoned the bike back up. It started right up on the first kick. I wish I had my camera to get a picture of my kid's grin when his bike fired up. Priceless! :thumb:

 

[Rich Rohrich]

Cool. Glad to hear it was nothing major. :thumb:

 

[BadgerMan]

:yeehaw:

Mark,

So how's it run? I am interested to know how much of an improvement the Power Now device makes.

TF

 

[MWEISSEN]

Tim, as you know we were in the middle of getting 4" of snow! So, it didn't get its maiden voyage yet, but should this weekend.