mikuni bs 40 carb with #55 pilot jet. Wants the difference in 1 jet with  side holes and 1 without

I thought I remembered having to switch to a pilot jet without bleed holes because I couldn't lean out the mixture enough. hmm. wait! wats it say at the bottom of my post?

Ahh. Ok so prolly what happened was the pilot with bleed holes was a 55 and too big anyway.   Right.    Carry on gentlemen, carry on.

walleye4us wrote on 10/09/14 at 17:15:40:


No.  Our carb is set up so that the idle mixture screw regulates the flow of fuel to the idle circuit.  The fuel flow from the float bowl goes up through the small hole in the pilot jet that regulates the flow....and the size of that small hole controls the rate at which fuel flows.  Then as the fuel leaves the pilot jet it joins with a passage that has air flowing in it, and that hole comes from the inlet side of the carb.  In that passage the air and fuel mix, and then they flow up to the top of the carb into a channel that has two outlet holes.....one is unrestricted and the fuel flows into the air stream, and the other one is restricted and adjusted by the fuel mixture screw.  When you adjust the idle mix screw you are regulating how much flow goes through that second hole to regulate the amount of fuel mixing with the air coming past the throttle butterfly.....and once you have opened the mixture screw 3 turns you have reached the maximum flow and turning the screw out any farther will not provide any additional fuel flow.  Now back to the pilot jet - you can see in the diagram that the jet has no bleed holes on the side....as it is a "blind hole" with no additional fuel or air being able to reach the holes in the side of the pilot jet.  


Some carbs like the Mikuni TM round slide carbs use the pilot jet in a different way.  Intead of using the idle mixture screw to control the flow of fuel - it is used to control the flow rate of the air that is mixing with the fuel from the pilot jet.  It is easy to tell which system is being used - as the carbs like the stock Savage carb that regulate the idle "fuel flow" have the mixture adjusting screws on the cylinder side of the carb - while the idle "air flow" carbs have the mixture screw on the air cleaner side of the carb.  In the "air flow" mixture adjustment the pilot jets have to have the bleed holes in the side of the pilot jet - as that is where the air is mixed with the fuel from the pilot jet.  In the following drawing you can see the air passage that is at the air cleaner side of the carb goes to the "air screw", and from there the passage goes to the side of the pilot jet, and the air is introduced into the bleed holes of the pilot jet.
 

You can used the jets with air bleed holes in our stock carbs....as the carb body has nothing to introduce more air or fuel through the bleed holes....the holes do nothing and have no effect.  You cannot use the jets without holes on a carb that is the "air bleed" type.

walleye4us wrote on 10/10/14 at 17:39:51:


Yep......Mikuni claims the correct jet is the one that provides 1.5 turns for the proper setting.  I think you can still have good result on a street bike it you have to turn out to a bit more than 2.  My bike ran best with a #50 pilot, #150 main, and 2 washers on the needle.

walleye4us wrote on 10/12/14 at 16:21:15:


Buy some #4 metal washers at the hardware store.  Three of those washers works for most folks - my bike still surged.  I removed one washer and tried 2.....problem solved and I was able to drop back from a 52.5 pilot to a 50.  The idle mix screw tells you which pilot jet is correct, then you can work on the needle setting and main jet size.

Send me a PM with your address....I will mail you some.

walleye4us wrote on 10/13/14 at 05:00:58:


You most likely can click on my name/ID on the left side of any post I have made, and find my email address.