Snowmobile Repair: 79 Yamaha Exciter 440, yamaha exciter 440, worst case scenario

Question
Hi,
 Need some help.  I just picked up a 79 exciter 440.  It has been sitting for a couple years, but put some new gas in(oil was already full) and it fired right up and ran fine for about an hour.  Then, under acceleration, mild and strong, it would intermitenly start to die out, but keep running.  Today, I started it up, ran for a few minutes, and the problem is getting worse.  Runs so bad now that I had a tough time getting it to build enough RPMs to engage the clutch and get it going to get back to my house.  Pulled off the fuel lines and started it with what was left in the bowl, and it seems to be pumping okay, not sure how much pressure there should be, but it was flowing.  Changed plugs, didn't help.  Not sure what to try next.  I don't see any inline fuel filter.  Any chance after filling the tank it picked up some junk from the tank and is clogging the pump or bowl?  Thanks for the help.  -JIM

Answer
Hello Jim,

  Once that sled is on the trail again, it will be a good one, those motors are nearly bullet proof. As far as the poor performance, that is almost always due to the carb being gummed up. The jets may have gotten clogged, and leaned out the mixture, causing engine damage, but that is a worst case scenario. I would recommend starting with removing the carb, removing the float bowl, and soaking it in solvent all night. Next, remove the inlet needle and seat, and also the jets. Use carb cleaner and a air compressor to clean the carb and all of it's passages thoroughly. Next, install new jets and a carb kit that includes a new inlet needle and seat. If the inlet needle and seat look ok and can be cleaned, that would be ideal, as most carb kits include very poor fitting inlet needles and seats, unless the are Mikuni kits. www.denniskirk.com has nearly any part for the sled, for very cheap. The sled has a Keihin carburetor, and they are probably the hardest carb to tune. Be sure to use the correct spark plugs for the sled, they are NGK BR9EV, if the sled reads 1978 on the VIN plate and BR9ES if it reads 1979. The correct gap is 0.5-0.6 mm. Also, the fuel pumps on these sleds are very low pressure, so it sounds like it's working good. I hope this helps you. Let me know if you have any other questions. Good luck with the sled! --Arcticatman