carb m7 lean at idle, not a jetting issue. any ideas?

[Vern]

It starts fine but after turning the choke off a few seconds later it will start to high idle, if you just leave it it will hi idle for a bit then it will die. When it starts to die if you flip the choke lever on then back off it will recover then repeat the hi idle til it dies again. When riding it runs like a champ, probably one of the better running sleds in the group, but when you come to a stop and let it idle the idle hangs at about 3k rpm for a while before settling down. I've just got in the habit of flipping the choke on for sec. to settle the rpms before shutting it down.

The jetting is 40 or 45 pilots, can't remember off hand,(stock is 35) air and fuel screws are both currently at two turns but I've taken them both in and out from there trying to figure this out. Carbs are clean and I've sprayed ether around the carb boots and crank seals to check for leaks with no change in rpm so they should be good. Sled is pushing 4500 miles and I'm wondering if maybe the pump is getting weak and can't pump enough fuel at idle but still flows enough when rpms are up when riding. Had the belly off a week or two ago and pulsation line appeared to be in good shape with no signs of damage. Can you test the output of these mechanical pumps? Does anyone know the specs? Or does anyone have any other ideas to check? Like I said it runs fine while riding its just a pia to baby sit it while trying to warm it up and get dressed in the morning.

Sorry for the long post, just trying to get as much info out as I can.
Thanks for any help

 

[Rixster]

Back your fuel screws out 1/2 turn. And I would bet you could turn your air screws back in 1/4 to 1/2.

 

[Vern]

I'll give it another shot but I've played with both screws many times with no benefit. Just out of curiosity what is the purpose of both a fuel screw and an air screw? What do they do differently that requires both where from what I remember older carbs only had one screw?

 

[2010Catman]

The fuel screw controls the amount of fuel going in, and the air screw adjust how much air it mixes into the fuel.

 

[CATSLEDMAN1]

you want to find an idle setting with these mikuni's where you have the slide as low as possible, they idle better and start better.

so............don't go toooo rich on the gas screw and compensate by raising the slide up to maintain an idle, that is usually a poor setup.

make sure you have same setting on both gas screws, pain to do and keep rechecking, but has to be.


also keep in mind the erratic idle is one reasons the rack mounted mikuni were generally replaced by efi, by all companies. the rack mikuni tend to leak excessive air around the slide and worse when well worn making for poor idle.

 

[Vern]

I know after riding for a while and it was good and hot I had to turn the idle up quite a bit to get it to idle, you think if I turned the fuel screw in some and backed the idle off it would make any difference? I'm not against screwing with it but if you've ever looked at the carb/throttle body area on an m you know it f-Ing tight in there.

I was also beginning to wonder if it was just the nature of the beast after fighting it for so long.

 

[Pinned W F O]

Have you pulled the pilots and checked them for blockage? They are what controls the idle curcuit.

 

[Vern]

Have you pulled the pilots and checked them for blockage? They are what controls the idle curcuit.

Yep, pulled the carbs off and went through them at beginning of the season. Pulled all the jets and fuel/air screw out and blew them all out and visually inspected them.

 

[Snowmaker10]

I had the same issue. My carbs were not pushed into the reed intake boots far enough. As soon as I redid that and set air and fuel screws to factory setting for 6-9k it ran like a dream. Also, make sure if you are running a fuel filter that it is not obstructed. I had bad fuel at one time and the filter was plugged up with ice and crap.