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Discussion Starter #1
This was originally posted by Bc-Dan as a reply to a question in another section but was too good not to place here.

On a Mikuni carb, there really isn't a "choke". What there is instead is an enrichment circuit. The circuit adds fuel when you flip the lever. The amount of fuel being delivered is dependant on the velocity of the airflow across the opening in the carb. When you give it the gas when trying to start, the throttle lever lifts the slide, making the area where the volume of air is flowing a lot greater. Because the area is greater for the same amount of air, there is a lot less air velocity across the enrichment circuit opening and the amount of fuel being added is not great. Keeping the slide on the bottom (hands off the throttle) maximizes the velocity of the air and creates the greatest amount of fuel being delivered.

These carbs also have three main parts; the pilot jet, the needle jet/jet needle, and the main jet. When the engine is at idle to about 1/8 throttle, the pilot jet is delivering fuel. At 1/8 to about 3/4 throttle, the needle jet/jet needle is metering the fuel (the fuel that the needle jet/jet needle meters comes from the main jet). After 3/4 throttle, the slide lifts the needle so that it no longer is a player in metering fuel, so the main jet is what controls fuel ratio.

That said, if you look at where you are having problems, you can work on that system in the carb. If you have hard starting issues, the enrichment circuit or the pilot jet is where to start looking. It does not take much varnish at all before the air and fuel passages to the pilot jet get clogged, and that jet is VERY small. Disassembling the carb, removing the pilot jet and "flossing" it with a small wire (I use one strand of telephone wire with the insulation stripped, or sometimes monofilament fishing line). Cleaning the passages with the wire is sometimes helpful. Those jets have to be pretty much spotless to work properly. Aerosol carb cleaner in all the passages helps, just be careful when spraying, sometimes it sprays back at you!

You may also want to make sure the enrichment plungers are not stuck in the carb, if they are not coming up, no additional fuel is delivered when you "choke" the engine.

When I start my carb sled cold, I put the lever on full, hands off the throttle, it starts first or second pull, even in the summer. If it won't continue to run, you know where to look! And pumping doesn't help, as there are no accelerator pumps at all in sled carbs; all that will do is build up your thumb muscles
 

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Hard Starting

I read your instructions for cleaning out the guides for the needle valves with small wire am wondering if there are specific adjustments for the choke and throttle where they thread into the top of the mikuni carbs as the previous owner by visual inspection i can see they haver been moved and i only get the engine to start for short period when they are backed almost to the very end of there threads any help appreciated
 

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Discussion Starter #3
The choke cable should have about 1/4" of play when the lever is off.
The throttle cable should be adjusted with the lever held wide open and the slide cutout should be flush with the top of the carb bore.
 

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cannot get my 97 XLT Special to start consistently. It'll start one day..run great and won't start the next day. There is fuel in the line...spark at the plugs...what could be the problem...any ideas ?
 

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hub takes out the plugs and wipes them off...makes sure the connection is clear...runs a paper thru the top...etc etc....
like I said somewhere else (still learning to navigate this site)...he read to drain the crank shaft and he did...then it fired right up. So we ran both machines out on the lake...had a GREAT time until the troubled one...seized up. Hub thinks it might be a bearing...can't win ! Thanks for the reply !
 

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The Hard starting engine now seems to be more i cleaned the jets with small wire and will run but only with the left plug out and setting on top of the head i dont understand that and then afterpulling for a time last night i had no more spark at the plug is there such a thing as a CDI and or Coil being able to show spark but not start the machine I always thought they were either working or not no halfway thanks for the help
 

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I think i no what switches you mean they would have a wire connected to the bottom of the carb but mine doesnt so im assumming this doesnt have switches thanks
 

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my sled starts 1st or 2nd pull when cold, but seems to give me a good deal of trouble restarting after a short stop when shes still warm. Suggestions?
 

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On sleds with "warmers/heated" carbs its best to leave off the heat source unless you notice a ice buildup because in normal conditions, even real cold ones when warm you'll notice that you will have to pull a few times and give partial throttle to get the sled started, its the fuel in the carbs being to hot - try it and you'll be surprised at the difference in starting with heat source off.
 

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barg
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my sled starts 1st or 2nd pull when cold, but seems to give me a good deal of trouble restarting after a short stop when shes still warm. Suggestions?
I let all my two strokes idle down,before i shut them off.cool off the heads and the plugs.Do u just pull over and shut them down?
 

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Ok...I'm a newbee to this forum and am having a very hard time starting my 97 ski-doo formula 500...I had a needle and seat problem earlier to where if would flood the crank. I cleaned the carb's and replaced the needle and seats...no more flooding...Now I have to pull my guts out to get it to start. Once it starts it runs perfect... soon as you stop and the engine is warmed up it won't restart without pulling your guts out...so cold startup and warm its very hard starting... any idea's???? I seem to have spark...compression test was 130 on both and that was when it was flooded. Thanks to all who can help or point me in a right direction
 

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Spark is fine...coil is fine...I just don't get it because when it had the flooding problem it always started first or second pull until the crank filled up...I'm wondering if I got an air leak in the impulse line, or if it's a carb problem. I don't understand why it runs so good but starts so hard....
 

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Ok..I checked the primer, thats fine...I have noticed that the easiest way to pull your gut's out if there is such a thing is to flood it than hold the throttle and it seem's to start but still hard....is it possible for the coil to slowly go or do they just quit and no in between???
 

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carburator problems seems to be haunting these days

Took my 87 Eltigre sled out today for an afternoon ride started fine rode for about 10 min. and the carburators filled with gas and flooded out. Runs great at high speed and will even idle. But as soon as i give it the gas bog bog bog untill it clears out. I noticed the outside of the carbs were wet. Ideas??
 

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Discussion Starter #19
Needle valves are apparently not closing completely.
This can allow flooding at low rpm and hard starting. Higher rpm can draw enough fuel to keep up with the condition.

Check that the valves are not sticking or worn.
If the needles are worn, replace the seat and needle with the rubber tipped ones.
 

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Thanks, Im going to pull the carbs and give them a good cleaning tomorrow, Ill check the needle valves. Any special instructions on carb cleaning?
 
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