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Hi. I'm new member ac bigbore. Yes, people familier with me from Snowmobile Forum, I can be a big bore. But please
bear with me on this.

I have spent many hours reading and learning from lurking this group and reading/posting/getting responses on Snowmobile Forum.
As any good student, the more I learn, the more I realize I don't know.

Almost totally ignorant of snowmobile engine repair I put a 790 Wiseco top end kit on a 1988 Wildcat 650. The engine had issues
before I put the top end on. Five minutes into its first field test a crank bearing blew and destroyed the crank. I don't know if
the bearing was going before I rebuilt, or if there was something wrong with the rebuild.

That sled got parted out and the shell taken to the junkyard. Next I tried the 790 kit on my 89 Wildcat 650. Compression was
very high even with the modifed heads. Measured about 150 on my guage when my shaved head 900 tcat minutes apart measured high 120's. I couldn't run the sled without it knocking and overheating. Maybe it was the compression, maybe it was carburetion.
I took the kit off that sled and put it on my 92 Wildcat 700. It was very happy there and has given me thousands of miles of
mostly happy sledding.

Thinking that there might be something with the 650 crankcase that didn't like the 790 top end, I bought a junker 91 Wildcat 700, took the engine out of it, put the bottom end in my 89 Wildcat, then put the 790 top end on it. Finished it today, 12/30/06. Checked the compression. Damn. Still measured 147/145. This is very much on the high side in my limited experience. I even had the
heads re-shaped to give it more squish room after the first mod. The sled idled very nicely and had a lot of snap when blipping the throttle. but due to 40 degree temps and heavy rain today, no field test.

My questions: Does anyone know if there is any difference between the 650 and 700 AC case reed crankcases? Volume difference, bearing difference, strength difference? Does squish shape and size have anything to do with how high compression you can run without knocking? Why can some sleds run 140 plus compression and not blow up?

PS: Weatherby 460 is running the kit on his 650 crankcase and says he has had no problems.
 

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Just checked my compression for the first time ever with a brand new compression gauge. I read a consistant 130-135 on both cylinders checking each cylinder twice. The instructions said to warm the engine first, I did not do that, and I did not hold the throttle open either. Not sure if that makes a big difference.

So far no problems. Last year I scorched a wall of a cylinder beacuase my oiler quit working on the 1 side. This year I unhooked the oiler and am running straight 50:1 premix, no problems so far.

This sled is snappy, and very fast, especially in short runs. If you ever make it down this way, you are welcomed to drive it.

One bad thing....this engine is very LOUD! I hope thats normal. I don't hear anything abnormal however. Just LOUD! Once I close the hood and take it out of my garage it isn't near as bad.

I would like to point out that Eric Gorr did the head job on the stock 650's and also the cylinders were from a 700, not sure if that makes a difference. According to him, everthing is the same as far as the case. But, would like an arctic cat person to cofirm this also.

I don't remeber my jetting for sure, but I think the only thing I did different from stock was move my needle 1 position to richen it up. Which Eric recommended.
 
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