670 power loss problem?

You should be getting near 8000 on the top-end....

Does it seem rich? Loading up at idle? Does it leave a black exhaust track on the snow everytime you hammer the throttle? Maybe your HAC has gone south.....

The HAC's are common to go out. When they do, the sled goes full-rich, and can make it a real boggy turd. If you think that this is a problem, I have covered removing the HAC several times here on the forum, do a search, and you should be able to find it. Simple solution, it only takes buying some main jets and some pilots... But, before you get drastic and rip it all off though, look over all the HAC hoses, making sure one did not come off, cracked hose, etc. If the system is not sealed, it does not work...

Another thing- Clutching on 670's is critical. If you have old, worn out or dirty clutches, it will make it a dog on the bottom and even worse on top. Go through your clutches thoroughly.

If your carbs are clean, adjusted properly, the float levels are correct, and your sled suddenly seems to be running "rich"... your HAC is bad... lol

Honestly, I know of no real way to check its calibration. It has a rubber diaphram inside, that if it gets damaged (by age, wear, exposure to fuel, etc), it simply quits regulating the air pressure in the float bowels, and the carbs then go full rich. You can either buy a new one (expensive), a used one (hope that it too, is not bad), or just get rid of the darn thing. I ride within a 2000' elevation spread, and for me, I don't need altitude compensation, so I have always removed them...

I have one, possibly two HAC assemblies laying around. Not sure if they work or not, but you are welcome to them for the price of shipping, if you wanted to give that a try...