Archive for the ‘Top End Rebuild’ Category

This is a customers quad that was brought to me with a seized top end.  I was hoping to find something like a cooling problem or a damaged piston, but instead I found that the crank bearings had failed.  This machine is an 87 and I would guess that the crank has never been changed before.  On most any engine, there should be no vertical play in the connecting rod.  This one has about 1/4″.  In addition to that, all the pieces and fragments of the old bearing are now in the crankcase.  Unfortunately, the parts list and associated labor for this repair is too expensive for the customer at the moment, so I am returning it to him in its current state.

[View with PicLens]

I got the head back from Reeves racing engines and I would have to say that he did an excellent job of cutting the valve seats and installing the new valves.  I highly recommend him and his pricing is very reasonable.  His number is listed in an older post if anyone is interested in having him do some work for you.

Here is the head assembled and ready to be installed:

Installing the head on the cylinder and re-shimmed the valve lash:

Then install cams, chain, tensioner and cap:

After all that, Its time to test it out:

So far, the top end of the bike is great.  No strange noises or vibrations. However, I do have a problem in the carburetor.  It bogs if you snap the throttle coming off an idle.  Apparently, this is a common problem on the stock Yamaha 250f’s and what I have read is that it typically is a problem with the leak jet being too large and letting fuel leak into the bowl instead of being shot into the intake from the accelerator pump.

I’m going to pick the new jet now and installing tonight.  I’ll update soon.

I just picked up this bike on Friday.  It is an 06 YZ250f 50th anniversary edition.  Its mostly stock, with the exception of some black excel wheels.  It has a valve problem where the intake valves wore so thin that they chipped off. causing a loss of compression.

Upon doing some research, I found out that Yamaha recalled all 2006 YZ250f’s and replaced the intake valves, so I am going to give my local dealer a call and see if there is any chance of getting this work done for free before I spend my own money.

The scope of work for this bike will include new top end gaskets, piston, rings, intake valves, exhaust valves, and a new timing chain.

Here are some shots of the bike itself before doing any work.

I bought this bike with the top end already disassembled, which I prefer not to do since there are usually missing parts when it comes time for re-assembly, but the previous owner convinced me he had everything, and as nice as this bike looks, I just couldn’t pass it up.

50th Anniversary hurricane graphic kit.

Here is a side by side of a good cylinder and a bad one.  The one on the left is from an 02 250f and the 06 is on the right.  Its amazing how the entire 250f motor has had only minimal changes since its introduction in 2001.

The head seems to be in great shape.

Here is the bad valve.  Notice the 2 chips in the edges.

Old tired piston

There are some slight scuffs in the skirt.

Thats all I can do for now.  I’ll be calling the dealer to inquire about the recall tomorrow, then I’ll make my parts order and I’ll post back once I have them.  Check back soon for updates.