I have my 250 xplorer running great with a cyclone cylinder head shaved .020" and 34 mm carb
only thing it it makes a sharp metallic ringing sort of noise when I decel maybe 5 or 6 times then sounds normal mostly as the engine comes down to idle
only when the engine is hot (riding for more than 10 minutes)
other than that it is running quite strong I still need to do more clutching and need some suggestions on what RPM to run this cylinder at (stock pipe)
right now it is pulling 6350 rpm

 

This sound is pinging my 250 had this problem after milling my head. Try richening your air screw 1/4 turn in at a time this may fix it or at least help. also if your not running 92 octane try that. Only pinging on return to idle is not that bad you can get away with it for along time. But if you ever hear that sound under power your asking for trouble if you don't correct it. If the air screw helps but won't take it all out you can put a larger polit jet in.
But under no load and only pinging a little 5-6 times I wouldent worry to much about it. also adjusting you rideing style may help try letting off the throttle fast and then getting back on it a little and letting her come down to idle slow I can work around the pinging on mine just with the way I use the throttle.
Be careful if your running wide open for awhile and then start to ease off the throttle this can create a heck of a pinging. you can pull the choke a
little to help with this when riding wot and mid range at high speeds for extended times.

Jeff

 

ok good info
it is a very loud ringing so if it is pinging due to lean mixture a larger pilot should fix her up
now that you mention it the origional low performance cylinder had a 30mm carb running a 40 pilot
now with the cyclone cylinder i'm running a 34mm carb and it seems to run the best with a 25 pilot I will bring it up to
a 30 this weekend before I go out maybe switch to a UFO which in my sled pulls alot of fuel when you let off the throttle.

work in progress still I guess

change pilot
back off secondary spring
install heavier clutch weights

 

In mine with a 30mm carb. is runs best with a 30 pilot but pings on decel. With the 40 pilot no ping but it's not as responsive. Mabey I need a UFO. What is a UFO?

Jeff

 

ufo is an incert which covers the underside of your carb slide
it is designed to smooth out the airflow across the slide improving fuel atomization
one of the side effects of using it is that on decel the carb goes quite rich you can fix this by drilling the hole through the ufo that the needle passes through a little larger but I might try using it as they come and richening up my decel,It only goes rich during decel but quickly clears and runs normal
I "think" that they are designed this way for safety so if you run WOT really hard and are on the lean side to really squeaze out the last bit of power the engine will quickly cool as soon as you back off the throttle.

 

Cool thanks Baxy sounds like a UFO may be the answer to are pinging probs.

Jeff

 

only thing is that when using UFO's the fuel economy is GROSS!!!!!!
I'm going to try it its hard to find a single UFO they like to sell them in kits for a twin snowmobile also you need to know what your cutout of your slide is
I will write mine down this weekend when I change pilots (testing purposes)
if it runs without pinging then I will install a UFO
I might try lifting my needle one clip first as well