Mikuni carb jetting

[John B]

I have a 1972 CCW 340 . It was equipted with a Walbro carb. I have retofit a mikuni carb off a '98 Jag. It will run poorly. I have changed the main jet , higher and lower, it will idle as soon as I pull the throtle it bogs down.If I hold my hand over the carb air intake controlling the air flow it will run OK from low to high throttle. At the high end I can remove my hand and it will run.I think the problem is in the pilot circuit.Either a smaller or larger pilot jet.Does any one have any suggestions Thanks, John

[Matt]

If it ran better by reducing the air flow, it means you don't have enough fuel, and need more to balance the mix. To confirm, you can start it, let it idle until it dies, then check your spark plugs. If bone dry, not enough fuel, if wet, too much fuel.

[John B]

Thanks for your reply. I should have said that the plugs are very wet after running. And this makes no sense as the engine runs better when air flow is restricted.The motor has been completely rebuilt with new condensors external coils and main bearing seals.If I raise the slide in the carb. with my finger the motor runs great through the entire acceleration. I have sprayed starter fluid around all intake connections to verify no leaks. As I said before I 've gone up and down with main jets, it seems to run better and will idle with the 105 main jet. Thats why I am wondering if changing pilot jet will help with the low and mid range. I'm just not sure the direction to go in. Thanks again John

[400brian]

How big is the carb off the Jag? A 34mm is probably as big as you should go.

[John B]

Yes it is a 34mm. I have taken it apart and cleaned it , float slides moving fine and main and pilot jets are clean . It almost seens that it would run better with a generic silencer, but I believe that will not be solving the real problem of fuel air mixture. I know they sell jet kits with a wide range of sizes, but I hate to spend that kind of money for dozens of neeles, and jets that I will ever need. Thanks John

[lwb140]

id start by jetting it close to what a 340/22 motor in a cyclone would have i assume that your 72 motor is a piston port but the case reed jetting i would think would be a place to start .that info is in the tech manual which the site has links to just my thoughts wade

[ski]

Keep in mind (as a general rule) that the pilot jet determines off idle performance, as well as slide cutaway, mid range is needle shape and clip position, main jet is wide open throttle. Again this is a general rule. If you need more speciifcs Google Mikuni carb tuning and read. Also to properly jet.... an air density gauge along with plug readings is the more accurate means to accomplish perfect jetting (for the conditions). A stock motor shouldn't be as tempremental as a modified engine. My $.02.

[400brian]

In my experience with a 340/5, 340/22 specs will be WAY too rich.

I'd guess a pilot of 25, and a main around 200-220. I'd work down to those numbers to be safe.

Remember, free advice is worth what you paid for it.

[John B]

Thanks for all the help,I will try a 25 pilot jet and see what happens. I have the week off and plan on doing some tuning

[John B]

I had a chance to run the engine thursday. I had been running it with the clutch off.I put the clutch back on and it it ran better, could be balance or the added resistance. Next step will be to put it back in the snowmobile and run it . Its a 1973 500 this was not the original motor, but my guess is it was changed out a long time ago as it is a 1972 340 engine.