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Message started by walleye4us on 10/09/14 at 13:33:29

Title: jets
Post by walleye4us on 10/09/14 at 13:33:29

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

Title: Re: jets
Post by Serowbot on 10/09/14 at 14:37:55

No difference in our carbs...

Title: Re: jets
Post by Boogieman on 10/09/14 at 15:32:31

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.

Title: Re: jets
Post by walleye4us on 10/09/14 at 17:15:40

So the jet without the side holes would run leaner than the 1 with holes. I have installed #55 with side holes and mixture screw is half turn out and will run with mixture screw all the way in, so I believe its running too rich. It runs way better than 2013 factory setup, but I'm looking for perfection in performance LOL. Intake and exhaust r stock and white spacer is half of original. Bike ran absolutely terrible (back firing and surging at all speeds) before I followed guidelines from this forum, thank you to all.

Title: Re: jets
Post by Dave on 10/10/14 at 04:21:25

6C7A77777E627E2F6E681B0 wrote:

So the jet without the side holes would run leaner than the 1 with holes.

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.
http://i57.tinypic.com/1h41uv.jpg

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.
http://i62.tinypic.com/2vvvr4m.jpg

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.

Title: Re: jets
Post by walleye4us on 10/10/14 at 17:39:51

Thanks Dave 4 all the pics and explaining of what goes on. So in my case with the #55 jet installed and stock intake and muffler, the bike is running rich as I can turn the idle screw all the way in and it still runs even better than 1/4 turn out, correct?

Title: Re: jets
Post by Dave on 10/11/14 at 05:22:09

30262B2B223E22733234470 wrote:

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.

Title: Re: jets
Post by walleye4us on 10/12/14 at 16:21:15

The stock pilot was a 52.5 and the bike ran terrible ( surging at all speeds up to about half throttle ) that's with the white spacer at half original size, like sittin behind a 30 cal machine gun, really. The bike runs extremely well with the #55 pilot when set at 1/4 turn out or less. Could there b something else buggin this machine at 1600 km. I may open exhaust and free up intake and see how it performs. Thanks again

Title: Re: jets
Post by Dave on 10/12/14 at 17:03:35

7C6A67676E726E3F7E780B0 wrote:

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.

Title: Re: jets
Post by walleye4us on 10/12/14 at 17:26:04

In Canada here AH, #4 washers r like tryin 2 find hens teeth. Checked Home depot stateside and no go. Goin check 4 a ace hardware in U.S. with fingers crossed than switch spacer thickness like U say. Thanks Again

Title: Re: jets
Post by Dave on 10/12/14 at 18:09:56

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

Title: Re: jets
Post by walleye4us on 10/13/14 at 05:00:58

Thanks, but I cant send a pm yet, only after 10 posts. Ill find some somewhere.

Title: Re: jets
Post by Dave on 10/13/14 at 05:54:45

15030E0E071B07561711620 wrote:

Thanks, but I cant send a pm yet, only after 10 posts. Ill find some somewhere.

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

Title: Re: jets
Post by Dave on 10/13/14 at 06:17:57

Here is a chart that shows where the various jets control the air/fuel mixture. You can see that the Pilot Jet Circuit mostly affects the idle mixture and very quickly loses influence as the throttle is opened. Folks are putting too big of a Pilot Jet in order to held reduce backfires - this makes the idle fuel mix to rich - but does help reduce the backfires by providing more fuel when the throttle is closed and the other jets are closed by the slide dropping all the way down and closing off the flow from the needle jet and main jet.

If you are having a lean condition just off idle to about half throttle, you need to raise the needle by reducing the size of the spacer on top of the needle.