Maintenance & Technical > Kx Frankensteins

YZX500 Project

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scottysg:
Hello, this thread will attempt to document the process I go through to install a KX500 engine into my street legal 2002 YZ250 dirbike.

Purpose: The goal of this project is to have a true, no compromise dirt bike (2 stroke) that also performs reliably for consistent road use, and in motord trim gives quick road bikes a run for their money. Budget has not been decided. In my experience, these projects always break it so I may never set one. Thriftiness will be required.

Motivation: I bought the YZ250 to get back into the off road scene after being away for 5 years. The kicker was I also wanted to get to work on it legally. Something about starting the day smelling that castor oil... I guess we all have our addictions! (another one is boost, so that will come to this bike one day, but not now)

The YZ's been great to get to and from work (17" wheels, 15-49) since the max speed limit is 45 on my route, but it really doesn't have the torque to pull the highway gears (14-40). And don't even get me started on lack of wheelies...Needless to say, for my application there is no replacement for displacement. Enter K500!

The 1988 KX500 motor I bought appears to be in good condition. It shifts nicely, piston and cylinder seem brand new, and it was sold as a fresh rebuild, though with unknown porting, and high compression piston. The inside of the cylinder that I can see looks good, with totally fresh looking crosshatching from hone. I need to get a little mirror to see the other side. The engine did not come with any external electronics, but the flywheel is there with wiring harness coming out from behind it. I have no more info on it, so I should probably take it apart to check everything. If anyone knows how I could do that without needing new gaskets, please let me know.

I have been looking at an ecotrons efi kit, to solve the lack of CDI and it also allows me to upgrade to a turbo setup when I get used to this bike how it is. Its very reasonably priced, around 500. Need to do more research here. I feel like this would be a great possibility for an arduino, but I don't know if I have time for that now. Open to suggestions for this problem.

Check it out!

umberto:
Is the KIPS valve assembly in place and not cracked?  That's one of the issues I had with my 88.  Also, does your engine have the KIPS cast onto the left side of the cylinder?  For some reason your cylinder looks different than mine, but I can't find a decent picture of mine from that side.

scottysg:
Umberto, KIPS is cast into the left side of the cylinder.

Tonight I will get a light up into the exhaust and look for cracks in the valve. Then I will remove the side cover for the kips and see if I am able to actuate the valve.

I noticed the rings where the exhaust springs hook onto the cylinder are pretty worn out. The springs have acted like a knife, sawing away at the cylinder material. If I can find a stainless steel grommet that size to put into those holes to prevent it from getting worse, I think it would be a good thing for me to do. I will get some pics of that as well.

I got an email back from ecotrons. They suggest their 2 stroke kit with a 38mm throttle body. They corrected me, telling me that their unit does not control spark, only fueling, so I will need to get the CDI and coil anyway. $799 was the quote price for EFI. Then I would need to upgrade the stator to supply enough power to run it all, another $500 ish. Looking at 1500$ to go EFI yikes.

scottysg:
Here is a shot up into the exhaust port. Nothing seems cracked in there. Using my fingers, I can rotate the power valve drums, not sure if that is how it is supposed to be.


This shot shows how worn the exhaust spring retainer is!


Using a 10mm on the power valve arm bolt, the two drums are operated and open to just the right location. No visible cracks in them.

Check out this last shot. If you look real close, you can see someone tried to chamfer the edges of the ports. They are not done evenly at all, especially the one left center. Is this really bad?

umberto:
Can you see if the entire KIPS linkage is in place?  You shouldn't be able to rotate the drums with your fingers.  You may be missing a part or two, which isn't a big deal (I bought several parts for mine on eBay and Rocky Mountain).  The important thing is that the drums are in place and in good shape.

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