Why does my 1975 arctic cat lynx 250 twin snowmobile backfire after its good and warm I have no power at all?

it has a new 32 mm mikuni carb it calls for a 30 mm any ideas why its doing this wgat jets should i change and richer or leaner please help me with this problem|||I'd say its too lean. Check the plugs. If they are rusty or red color, that means too lean of a mixture. What you want is a nice tan color. If they are black and sooty that means the mixture is too rich. Adjusting the airfuel mixture is more of a screw setting and not a jet change. If it runs good without breaking up at top end then don't touch the jets. You need to adjust the airfuel mixture screw. Bogging at low end means you need to adjust the slide needle.|||check your exhaust you went to a larger carb and richened your fuel mixture ( it should be 40 to 1 ) and clogged your exhaust|||Eric, field strip both Mikuni carbs. No power and backfiring is a dead giveaway that the motor is running lean. Carefully remove the pilot jets and be absolutely they are clean and clear make sure the holes are clean on the extension barrel of the pilot jets. There is a tiny number stamped on the top. It will say: 25, 30, 35, or as high as 40. By now you will have removed the slide valves. Check to see at the very top if one of the clips are missing that secure the jet needle to The top of the slide valve. Note the groove that the clip is in. To make your system richer raise the needle in the center and snap the clip lower toward the tapered end. Remove the main jets on the bottom of the float bowls. Make sure they are clean.


Make sure bith floats are light in weight and emty. pull the veedle valves out and spray through them with Berrymen carb cleaner. Make sure a float hanger is not set too low as there won't be enough avalable fuel in the float bowl to supply the main jets or the pilot jet. Douche every hole of the carbuerator. Now for the final test make sure your diaphragm fuel pump is squirting a liberal amount of fuel (without being hooked to the carbs) by pulling the recoil starter. If you are getting a week shot the fuel pump may need rebuilding with a new diaphragm kit. Make sure your impulse hoses aren't split or cracked. The impulse hoses provide vacuum and pressure to the fuel pump to make it work properly. While you're it be prepared to re-hose everything.





If all else fails do a compression check. You should see a minimum of 125 lbs. pressure. If not you may have a bad piston or rings. Remember, proper compression designates how much impulse pressure gets to the fuel pump. As I rember these motors have chrome cylinder liners and you can't re-bore them. I hppe you don't need new cylinder juggs.