Carb Question

[mopar4u]

I struggle when it comes to carbs but i'm slowly learning. What I encountered last night really threw me for a loop.

I converted an old tillotson diaphragm carb on my 75 ski doo tnt 340 f/c and put a mikuni slide carb and fuel pump on there. 1 carb. I've been getting help from my dealer on this. The problem with this is finding the correct jetting. I think the carb is a 32 or 30 mm. I started with a 330 main and 30 pilot. Sled started 2nd pull. After the sled warms up it never idles down even though I have my piston in the carb set low enough. I will set my idle at around 2,000 rpm, after I drive the sled around it idles at about 3,000 rpm and won't drop. I thought I may be running too rich. So I put a 300 main jet in. The sled fired, ran for about 1 minute and stopped. I couldn't get it started. I turned the idle screw way in (more rpm) and finally got it fired, while it was running fuel was coming out of the small ?overflow? hose that comes out of the top of the carb. Why? I thought I was sending less fuel to the motor by putting a 300 main in? So I put the 330 main back in, turned the idle down, boom, fired up and ran again. What happened when I put the 300 in? Am I close to getting the jetting properly calibrated for this motor? Should I go to a larger number main jet, 340, 350?

I found that if i turn the air screw all the way in (with the 330 main in) it helps with the idleing down problem. Does this mean I want to go a 25 pilot or a 35 pilot?

thanks for any advice
Jason

[mopar4u]

no carb experts?

[94ZR580]

I'm no expert. Is this a round slide, or flat slide carb? The "overflow" tubes are the float bowl vent lines, so make sure there is no fuel in them that may interfere with proper venting. If the idle speeds up, that is often the sign of a lean condition. If it is a round slide carb I believe it has a fuel screw. In is leaner, out is richer. If the screw is before the slide it is an air screw, if it is after the slide it is a fuel screw. I may have that backwards, so please confirm this with someone who is certain of the proper operation of those adjustment screws. Good luck.

[mopar4u]

round slide

[Formulaman]

First of all did the dealer tell you what jets to use? This is a starting point. If you have fuel coming out the little vent line then you have a float problem, probably just dirt. Changing the main jet won't really affect the idle so thats not your problem, it sounds like you have either an air leak leak or the dealer is out to lunch on the pilot jet. The reason turning the mixture screw in helped because it controls air, you were richening the mixture going in, if there is no leaks (Manifold gasket, cracks, carb not seated properly) try a bigger pilot. Also make sure that the throttle cable isn't binding and hanging up the slide. One thing at a time!