Street use YZ400 questions - Please Help !


4 replies to this topic
  • demonicracing

Posted November 02, 2004 - 03:31 PM

#1

I am in the process of a motor swap project – stuffing a YZ400F motor into an Aprilia RS50 chassis and I have some questions and concerns that I thought some of you might be able to address for me:

I bought the motor used and right now it is torn down except for splitting the cases. I am awaiting a rebuild but I am undecided about a few things. Everything loks pretty good and I haven’t found any problems. I plan on replacing most of the seals and bearings as well as the piston and cam chain but is there anything else in particular that I should be weary of on this motor? I have plenty of experience wrenching on two-strokes and larger, multiple cylinder four strokes but this is my first thumper rebuild.

As far as pistons go, are any brands far superior? I am planning to go with a Weisco 12.5:1 kit but am considering the 13.5:1 kit. My primary concern is reliability because this motor will be run on the street, is the extra power from the 13.5:1 kit worth it? Does the 13.5:1 kit really need better than pump gas?

Anything special to look for when examining the head and valves?

I will need the motor to charge a small battery and power a few lights. I was given a stock WR450 flywheel from a friend and was hoping to be able to use it and a WR450 stator to run the electrical. Are the WR450 stator and flywheel compatible with the YZ400F motor? Are any special changes needed (timing, etc.)?

The exhaust system will be custom and most likely based on whatever I can find a decent deal on at the time I am ready to install it. I don’t care about it being loud (actually I prefer it) but are there any systems that don’t hold well under stop-and-go traffic or extended high RPM highway usage? Which systems don’t seem to be bothered as badly by reshaping the header and midpipe?

I do plan on using some kind of auto-decompression exhaust cam but am unsure about if I want to go with a YZ450 unit or a hotcams unit or hotcams intake and exhaust cams. Any input on what would be best for street use (I want lots of low end hit without losing top speed)?

Since the motor with be covered up with beautiful Italian fairings, I will not be able to get to the choke and hotstart knobs. Do you know of any kits to relocate these to the handlebar?

Thanks a lot !!!

  • PumpkinHumper

Posted November 03, 2004 - 09:19 AM

#2

I dont think the WR stator and flywheel will work. I know a WR426/400 will work. I think it has to do with the crank beeing diff on the 450's. www.electrexusa.com can hook ya up. YA need to rewire the WR stator to work with the yz cdi. Electrex can explain how or just buy from them.

Change the timing chain. Use a torque wrench in the cam caps. I think the high compression pistons need higher than 92 octane gas to run right.

The early 400 motors are fairly bulletproof. Not much went wrong with them that I can recall.

Dr D. or Zip Ty racing make hot start relocator kits. I dont think anyone makes a choke kit.

Oil was stored in the frame on those. What are you gonna do about that? I seem to remember someone making a DRY sump conversion kit for them to get the oil outta the frame. But I dont remember who. Dr D maybe.

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  • bushy

Posted November 03, 2004 - 12:47 PM

#3

Chrome-crf has did this conversion but with a yz426 motor.

http://www.thumperta...t/1#Post1252660

  • demonicracing

Posted November 04, 2004 - 02:01 PM

#4

I dont think the WR stator and flywheel will work. I know a WR426/400 will work. I think it has to do with the crank beeing diff on the 450's.


Thanks for the reply! I think you are right, I was comparing the two flywheels last night and it doesn't look like the WR450 one will fit.

Change the timing chain. Use a torque wrench in the cam caps.


Of course, those are givens!

Dr D. or Zip Ty racing make hot start relocator kits. I dont think anyone makes a choke kit.


Thanks for the leads. They look good, but pricey.

Oil was stored in the frame on those. What are you gonna do about that? I seem to remember someone making a DRY sump conversion kit for them to get the oil outta the frame.


I have been looking for an oil cooler to do the job, but I don't think any hold enough volume so I will most likely end up making my own custom oil tank. That wet-sump conversion is a nice idea, but it greatly reduces the oil volume and lifespan. Since this will be street use with lots of high RPM and longer times between oil changes, I will need more oil volume.

  • demonicracing

Posted November 04, 2004 - 02:05 PM

#5

Chrome-crf has did this conversion but with a yz426 motor.


Right, and knowing that he sucessfully pulled it off was the final deciding factor in me using the Yamaha thumper instead of the a 250 2-stroke as I originally planned. I have tried to get ahold of him a few times but haven't had any luck :cry:





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