Jump to content
Sign in to follow this  

Low compression, bike barley starts

Recommended Posts

I have a 2001 YZ250F. I went to the desert....o probably like 3 times and rode pretty nicely and had really high compression. I use my compression lever every time and have never had an issue starting it. A lot of people assume that since I have a old bike that uses a compression lever, that it is hard to start. Not that case for this bike. First or second kick it runs, and i mean this runs smoothly. Throughout the last day at the desert I felt a lack of compression when kicking the bike over. This lack of compression was to the point I did not need to use the compression lever, it was awesome. After letting the bike sit for a week and yesterday taking it to the track I found out it wasn't awesome. I tried kicking it over and the compression just was not there to the point where I did not feel a good kick. I would think that maybe the compression lever cable might be stuck, but it was not. The first thing out of every man's mouth on the track is "must be your valves". I took my bike home and bump started it down the driveway and the damn thing after a really good warm up (longer than the usual, it at first lacked to stay alive without bumping the throttle her and there, no biggy) idles and has no lack of power. I do not know, but maybe the guys are right, maybe it is my valves. Don't ask me when I had them done because the owner I got it from did that, so I do not know when they were done last. I do not feel like tearing the top end off and finding that the valves are fine. How long will it take me to inspect the valves, and are there and good sites (with videos) to walk me through? I am assuming that the valves must not be adjusted right so the compression is leaving the engine before I can get a good kick.

Thanks guys.

Share this post


Link to post
Share on other sites

maybe it is a bad valve and when it is cold it doesnt seal so you lose comperession but once it warms up the heat expands the metal and creates a good seal until ur bike cools down again

Share this post


Link to post
Share on other sites
maybe it is a bad valve and when it is cold it doesnt seal so you lose comperession but once it warms up the heat expands the metal and creates a good seal until ur bike cools down again

How would I know what valve would be the one that does not seal right?

Keep the ideas coming guys!

Share this post


Link to post
Share on other sites

Time a for a valve shimming or possibly a valve job.

Check the clearances to confirm it is a valve issue. If they have been shimmed before or you ingested dirt, it is time for a valve job. When you do one valve, you do them all.

Share this post


Link to post
Share on other sites
Time a for a valve shimming or possibly a valve job.

Check the clearances to confirm it is a valve issue. If they have been shimmed before or you ingested dirt, it is time for a valve job. When you do one valve, you do them all.

How much will it cost just for parts on average?

How much will it cost for labor average?

Share this post


Link to post
Share on other sites

CHECK THE VALVE CLEARANCEs!

we can talk "what if" all day, but you need to start somewhere. more than likely you need to shim them. if you cannot do it, then take it to someone that can.

Share this post


Link to post
Share on other sites

Start at the top. Since you went desert riding, you probably got dirt past the air filter. Confirm by removing the filter and check for grit in the air boot. Oil and grit in the bottom means it sucked sand past the filter seal or thru the filter. Next check valve clearances. Most likely the center valve is the tighest. In either case, if you found grit and or tight valves, then it has had dirt pass thru the valves. Pull the head and cylinder and do a top end, piston, rings, pin, clips; then get the valve seats cut and new valves installed. To prevent a future occurance, you can try what I do, and that is to use a TwinAir filter in conjunction with a Flo filter on top of that. My valves haven't moved in a year with that setup.

Share this post


Link to post
Share on other sites

Before you take anything apart, check the valve clearances. Then pull the air boot off and check for dust/dirt. Then you will know if you will be able to get away with a reshim or you must do the valves.

Cost is moot as either you fix the engine or you sell the bike as a non-runner. What it costs is what it costs. If you do need to have the head done, contact WilliamsMotowerx (nothing to do with me). Read your service manual many times before you do anything you are not 100% positive of. Be careful removing the cam caps and the bearing locator C clip.

Share this post


Link to post
Share on other sites

I opened the valve cover and all the valves are adjusted to specs and my compression lever is not stuck at all, it has some play in it, which is good. I am going to do a compression check, but if i have good compression I do not know wheat else to do.

Share this post


Link to post
Share on other sites

🤣

Start at the top. Since you went desert riding, you probably got dirt past the air filter. Confirm by removing the filter and check for grit in the air boot. Oil and grit in the bottom means it sucked sand past the filter seal or thru the filter. Next check valve clearances. Most likely the center valve is the tighest. In either case, if you found grit and or tight valves, then it has had dirt pass thru the valves. Pull the head and cylinder and do a top end, piston, rings, pin, clips; then get the valve seats cut and new valves installed. To prevent a future occurance, you can try what I do, and that is to use a TwinAir filter in conjunction with a Flo filter on top of that. My valves haven't moved in a year with that setup.

Nah RonBuell was close,

It wasnt dirt, it was Barley. Damn Barley farmers, why did they have to harvest this time of year ?????????

:moon::cheers::smirk::smirk:

🤣:p:p🤣

Share this post


Link to post
Share on other sites
A leak-down test is the correct test. With auto-decomp you can't do a compression check, the mechanism lets the pressure out.

01-02 does not have autodecompression cams stock

Share this post


Link to post
Share on other sites

So what ended up being the problem? I have an 02 with the same problem, about to change the cam chain and drop a new piston and rings.

Share this post


Link to post
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:

Sign in to follow this  

×
×
  • Create New...