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Discussion Starter #1 (Edited)
I have a 1991 ski-doo scout 377 i have been trying to get going. I am having trouble 1/8-1/4 throttle. It will "blub" a little. It will idle o.k. and 1/4 to WOT in runs great. I have cleaned the carb (twice), checked float height, checked jet numbers, cleaned the fuel pump, and installed an inline fuel filter and new plugs. When i adjust the idle air screw (witch should adjust idle up to 1/4 throttle) i do not get much engine change. And does not seem to change 1/8-1/4 throttle. I also adjusted the needle, leaner one notch witch didnt seem to have any effect.
My next move is to do some plug testing and see what i get.

Something i did notice is there is a crack (pretty wide) in the air box right next to the carb flange. Could this be my problem? I

Any suggestions?

FYI: it has a mikuni VM34 carb.
 

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Are you adjusting the Air mixture screw, or are you adjusting the idle screw. They are two different adjustments. I have never seen the air mixture and idle screws be the same screw on any carburetor I have ever worked on. No matter if it was a four stroke quad, a 2 stroke snowmobile, or a 4 barrel under the hood of a car.

Now that crack would cause the air mixture to become leaner than you want. So that crack needs to be fixed. Also check the Carburetor mounting Boots as those like to dry rot and crack and cause a lean fuel mixture.
 

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Discussion Starter #3
i am adjusting the idle air screw. I know the difference. Havnt done any plug testing yet but it seems to be running rich at low RPM. Checked the slide cutout and it is factory spec. Carb. boot is new=no cracks on engine side. Starting to wonder if jets arent just wore out.
 

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My first thought while reading your post was idle air mixture or the small idle air mixture port located just inside the carbs entrance. Those little air ports get plugged with belt dust and what not rendering the ajustment screw useless, but you cleaned the carb several times.

I agree with the reeds being bad. Either cracked, chipped, streched or not sealing properly against the cage.
 

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Another thing to consider if all else checks out. Every manufacturer will have a jetting chart for elevation and temp. They always err on the safe side. Many times ther jetting recommendations are pig rich, especially from the 90s. The machine might seem fine during the coldest of days, but when the temp gets around 30 they run like slugs. There isn't a machine from the 90s I can remember not having to jet down from manufacturer recommendations, atleast from Arctic Cat.

Its been a while, but if memory serves, our 600 tripples had VM 36 carbs. We dropped main jets atleast 2 sizes usually 3. Air screws had a short range of adjustment, so they were all the way out and or pilot jet size was reduced by .5. Needles were either raised or lowered for 1/8th - 1/4 throttle performance. Oil injectors were also leaned down, but I wouldn't recommend anyone do this.
 

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Have you checked out the clutches yet? Something could be worn or binding.
 

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Another thing to consider if all else checks out. Every manufacturer will have a jetting chart for elevation and temp. They always err on the safe side. Many times ther jetting recommendations are pig rich, especially from the 90s. The machine might seem fine during the coldest of days, but when the temp gets around 30 they run like slugs. There isn't a machine from the 90s I can remember not having to jet down from manufacturer recommendations, atleast from Arctic Cat.

Its been a while, but if memory serves, our 600 tripples had VM 36 carbs. We dropped main jets atleast 2 sizes usually 3. Air screws had a short range of adjustment, so they were all the way out and or pilot jet size was reduced by .5. Needles were either raised or lowered for 1/8th - 1/4 throttle performance. Oil injectors were also leaned down, but I wouldn't recommend anyone do this.
When I worked for the A/C dealer we would automaticly drop the jets 2 sizes to prevent it coming right back with fouled plugs. I seriously doubt different plugs arw going to fix it. Its in the fuel / air mixture. What color are the plugs? Did you replace the diaphrams in the fuel pump? Check your crank seals with some starting fluid?
 

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When I worked for the A/C dealer we would automaticly drop the jets 2 sizes to prevent it coming right back with fouled plugs. I seriously doubt different plugs arw going to fix it. Its in the fuel / air mixture. What color are the plugs? Did you replace the diaphrams in the fuel pump? Check your crank seals with some starting fluid?
My dealer wouldn't touch the jets without voiding warranty. Myself and others were forwarned that factory jetting better be in the carbs if you happen to need warrantied engine work. I never said anything about plugs.

Salmon Bum could be onto something with worn, broke or binding clutching.
 

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Salmon Bum could be onto something with worn, broke or binding clutching.
Clutches get overlooked by many, but they need more attention than any other part on the sled.
 

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Discussion Starter #14
Thanks for all the replys guys. Havnt had time to do any plug testing yet but i believe it is running too rich. Plugs are usually black but that is throogh all throttle positions. It will smoke at idle a little bit and i believe through 1/4 throttle. Kinda hard to tell when you are moving. (havnt even had time to block it up and run it) I did tinker with the idle air screw a bit more and did get it to run a little better but still not completely smooth through 1/4 throttle.

As far as the clutch i havnt checked it out. Seems to be correct but will check it out when i get time.

With the reeds, would i notice a lean mix through all throttle positions if it where bad? Also doesnt this have a rotary valve? I cant seem to be able to find witch one it has, but i know larger rotax engines of the same year have rotary valves.
 
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