79 Scorpion Sting Carburetor Adjustment for Altitude

[sdsnocop]

Does anyone know what main jet should be used for a 79 Scorpion Sting 440 Mikuni Carb at 6000 feet elevation? Factory specs a 310 main which should be for sea level.

[weverat]

Hi elevation will cause a rich mixture, and a loss of power. If the 310 works well at sea leval, you could possibly work it down like this, 280, 260, and finally 250. Ever so carefully !

Rich on the main is safe, especily when racing your buddy all the way across the lake on a groomed path.

These little motors lean out when they get hot.

Only race for a mile at a time. I often shut it down at wfo, and then check the plug (plug chop), this way you are checking main jet mixture.

Usually we only adjust the needle, and live with the rich main. I am in southern Mn, adjusting for temperature.


The very best is them there old tunnel mount Scorps, with a nasty walbro. I like to put a big thumb srew on the high speed mixture screw, and crank on it all the time. One hand on throttle, the other on the screww. Tight woods in 1/2 turn, open smooth trail back er out till it stinks, then back in as far as you dare!

[scorp11]

The stock main jet for a 1979 Sting would have been a 280 main. I am going to guess your looking at 250 or 260 by the chart. But. . like weverat says, I would start rich and lean it out step by step.

Be sure you have the powerjet carb still on it too. If it is a non power jet carb, you would have to be considerably richer. I run 280 / 290 main in my powerjets and 360 main in non power jets on the same motor. You will want to know what size the powerjet is and what needle as well to know your jetting. In 1980 the richened the carbs way up, then found out it was way too much and came back down to a 290 main jet with other changes.

I have a chart in my 79 service manual I'll try to take a pic and post that tonight.

Here are the jetting settings I have found for the various sleds

factory 1979 Sting and TK

Main Jet : 280
Pilot Jet : 35
Power Jet : 125
Needle : 6DH4 - e-Clip was in the 3rd groove (middle)

80 Sting factory before update (book says 360 main, but all I have seen had 350 main in them). The main jet is a lot fatter than the non-CDI Motors.

Main Jet : 350
Pilot Jet : 45
Power Jet : 155
Needle : 6DH4

In 1980, there was a "low end performance kit" which was put out because of the underhood temps on the 80's were a bit high. Below is the carb jetting from that kit. There were other changes with this kit, but this was the jetting specific ones.

Main Jet : 290
Pilot Jet : 40
Power Jet : 155
Needle : 6DH21 - e-Clip in the 2nd groove from the top

[scorp11]

Sorry for the quality, but wanted to get it to you. This is the jetting chart for the 1979 Sting / TK from the 1979 service manual



If it was me, I'd pull the bowl and find out what your powerjet and main jet currently are. Just to be sure. Main jet and powerjet are both important.

[jason4402003]

I have had my 77 sting up to 7000+ ft..Where we ride we start at 3000 and ride up to 7000.. this makes it tuff to have spot on jetting..the rule i have used is drop two jet size on the main.. the problem that i ran into was the slide cut out... down low i was fine but the higher i rode it it was gasping for air on the bottom end ...i tried different slides with larger cut outs and had a lean condition.. what i did is take a stock slide and lightly grind it with a dremel till you get a nice idle and easy start up.. example,, if you have a 2.0 slide a 2.5 is to much at 6000ft but 2.25 is the sweet spot. cayuna's get finicky 4000ft and up .another thing you should do is take your vent tubes and extend them into your air box this help equalize the air pressure at the bell and the vent..its like a hill billy high altitude compensation.. and one more thing ..get a stiffer primary clutch spring or install a 102c that is more tunable if you don't have access to power thrust parts. it takes time to get a scorpion setup for mountain riding so make small adjustments.. when you hit the rite setup people wont believe how well your sled runs when your chasing down rmk's and summit's....well on the trail [lol] have lot's fun.........................Jason

[sdsnocop]

Thanks all for the advise.