Small Engines (Lawn Mowers, etc.): snowmobile, rodent nests, air leaks

Question
-------------------------
Followup To
Question -
I ride a 600 xt yamaha vmax
when the moter is up to operating temp and I sudenly let off the throttle I get a loud bang in the exaust like it is loading up and detonating in the mufler.
I figured the carbs are dirty.So I cleaned them.
I sinked the carbs and I put new plugs in.
I checked the exaust for rodent nests and replaced the gasket
I checked for leaks.  I am stumped. I have not had this problem before and we have 3400 miles on this sled.
Answer -
A loud bang in the exhaust is indicative of excessive fuel (that accumulates in the exhaust).  When you let up on the throttle a temporary lean condition occurs.  This can cause the exhaust temperature to rise enough, momentarily, to ignite leftover fuel in the exhaust.  I hope you have been thorough in cleaning the carbs, set the float levels properly, made sure the inlet needles and seats are in good shape, etc.  You didn't mention if the plugs come out black, but I have to assume they do because of the nature of this problem.  Plugs should come out tan to slightly black when the outside temperature is around freezing (colder requires more fuel so a wee bit of black is OK)  I don't suspect the fuel pump because a bad diaphram in the fuel pump will usually load the engine with fuel while it sits rather than when it's running.  I would also check with a Yamaha dealer to make sure there is no service bulletins for this problem.  There could be a carb issue that Yamaha is aware of.  Hope this helps.


Thanks for the help but this was not the problem. Here is something else you might want to try just in case you or someone else runs into this problem again. After thouroughly going through the obvious and knowing the things you have mentioned are not the problem, take some carb cleaner and spray around the case gaskets and reed gaskets for air leaks.  If there is an air leak the engine will drastically change RPM. What i have found was a bad crank seal.  Which apparently has been a known problem occasionally on this particular motor.

Thanks again for your input.
Kevin


Answer
Thanks for the feedback.  You are correct that an air leak will be found by spraying carb cleaner or WD40 as noted by a change in RPM.  It is unusual that a lean condition (crank seal) was causing this, although this will increase the exhaust gas temperature significantly.  But being lean, there should be very little fuel accumulation in the exhaust available for combustion.  Lean conditions generally cause running problems and can be found by viewing plug colours (to give an indicator that is the problem you are looking for).  However, if you have found the problem and corrected, that's terrific.  Out of curiousity, was the crank seal bad on the clutch or the mag side?  I don't work in the industry and am not privy to the latest bulletins, so I like to keep this information in case someone else has the same problem.  Happy sledding.