top end gone, backfire, loss of power band 99 yz250

[lilk67]

I have a 1999 yz250. im a new owner of 250s. it seems like i have no top end power and almost sounds like a little bit of piston slap. the bike had the top end rebuilt recently. the power band seems to be cutting short and a light backfire under compression. what should i try before i rip the motor apart?

 

[Moparman1539]

Check the Compression of the top end. And also is the piston in right, and also check the power valve.

 

[elcamino12sec]

Get a manual and check the power valve system, if the system is not working properly the motor will not rev like its suppose to, thats what it sounds like to me anyway.

 

[bk250]

What would you look for with the powervalve? (I have the same kind of problmes with my bike and was planning on going through the valve this weekend).

 

[rmc_olderthandirt]

What would you look for with the powervalve? (I have the same kind of problmes with my bike and was planning on going through the valve this weekend).

To see if it is operating properly.

The power valve rides horizontally in the engine, right to left, just above the exhaust port on the cylinder. There should be metal plates that cover the access to the power valve on both sides. If you can get either plate off you can watch the operation of the power valve as you rev the engine up (it is okay to test run the engine with the plates off).

You may need to first remove the pipe to get to the plate, then replace the pipe so you can test run the bike.

Start the engine and watch the end of the power valve. As you rev the engine up you should see the valve rotate, probably about 90 degrees. If it appears to move smoothly then it is working fine. If it doesn't move properly then you need to take it apart and clean it.

On one side (probably the right) is where the linkage is to the governor. There is usually a spot to place a pin/nail/screw driver to lock the linkage so that you can remove the nut on the end of the power valve without putting a strain on the linkage down to the governor. Lock the linkage, remove the nuts, then CAREFULLY remove all the pieces making careful note of the order in which they come off.

The valve will separate in the middle and each piece comes out their respective side of the engine. The screw that holds it together is most likely straight down the center and accessed from the opposite side as the linkage.

If it was gummed up so bad that it wouldn't operate you might have a little trouble getting the power valve pieces out. I would start at the linkage side as you have something to grab ahold of. Just be careful that you don't mess up the threads on the end of the shaft. Once you get that side out you can poke through with a punch or screwdriver and push the other half out the other side.

Clean the power valve and the bore it rides in. Reassemble and things should be good again.

lilk67: Your problem doesn't sound like powervalve to me. I would not expect the powervalve to cause backfiring or a piston slap issue.

What grade of gasoline are you using? If you are not running premium already then I would recommend you try that. Your YZ-250 probably has a 12:1 compression ratio and needs the high octane gas. Running regular would cause detonation, which could sound like a piston slap.



Rod

 

[125#48]

I had a 99 YZ250 after a full strip & rebuild, I had the same thing, I checked the power valves etc, when I had stripped the carb I had moved the float height by accident, set that & all was ok, It was realy bad before, no top end & cutting out badly, maybe worth a try !

 

[chillrich]

Check your resistor spark plug cap, I had one open on my '98 YZ250 and the bike would idle, and low/mid was OK but if you hit it it would just sputter etc. This cap should measure 5000 ohms or close to that.