Mikuni questions

[tomale]

I finally got aroung to changing out that Bing. I found another manifold and altered it so that I could mount the mikuni. That worked fine. I got it to start and it seems to idle pretty well..... better than the Bing. Snapping the trottle seems to work pretty well. I adjusted the air mixture screw starting at 1 1/2 turns from fully closed. like the book said. I first turned the screw in a half a turn but it ran worse and then I turned it out. The bike began to run faster and continued to do so until I got to 2 1/12 turns out. I think this means that the pilot jet is too rich. and I need to go down one step. The carb came with a # 35 but I put a #40 in it and figured that if I needed to I could go down. I must be close but I am not sure. would it be better to go down one and then readjust the air mixture or leave it alone. Also I noticed that it seems to load up pretty easy. I had left the float valve alone figuring that it had been adjusted corrctly at the factory. I took if off and found that I had been adjusted according to the book but why then the loading. I decided to adjust the float so that it shuts a little sooner and try that. I did the old hose trick and adjusted it so that it would shut off a few MM sooner that stock. I will try it and see what happens. The Bing always gave me trouble with the float valve not closing and everytime I rode I shut the valve off before I shut the bike down. That way there was still gas in the carb but it would not over flow. I figured that I would not have to be so carful with the mikuni. Float valve seens to be working fine. but if the valve was adjusted right why the flooding?

Thom Green,I own and ride a 76 250 MC5 MX which I bought new.

[john durrill]

Thom,
 re-ajust the idle speed then adjust the mixture and see what happens. if you have the right pilot jet in it it will stumble when you turn in and when you turn it out. max out is 2 1nd 1/2. and i would change the pilot jet to get it some where in the range of 1 to 1 & 3/4 out . what slide cut away do you have? what air jet are you runing?
 above 1/8 open  not only the slide cutaway but the needle and needle jet begin to take control.
 I have seldom recieved from a supplier a prejeted carb that would work with out some jet changes.
 post all your carb info and maybe someone on the board can help that has something simmilar.
 John .

 

[tomale]

John,
As of the first posting I was running,
280 main
40 pilot
2.5 slide
6DH4 needle with it on the second notch from the top
The air mixture screw was at 2&1/2 turns.

I made some changes and this is what I changed. I put in a 370 main, 35 pilot, and readjusted the air mixture. It now is at just about 2 turns.I pulled the plug and it was pretty wet. Also the bike smoked more that usual. I think that it is still too rich of a mixture over all. It did start really easy. It seemed to me that adjusting the air mixture screw was rather sluggish. I could hear the change but it was slow coming and it did not make that much of a differance. Also I noticed that I could turn the screw all the way in and the motor did not die. It ran rough but it did not die. Adjusting it out did make it idle better and it did run alittle faster. The thottle response was alot better than with the old settings.  I did have to adjust the throttle stop down so that the motor would idle at a comfortable rate. One other thing that should be considered is that I am running a custom pipe that was made for that bike. It was great having friends in the biz.
Also, The temp. when I made the changes was 78 degrees and it was clear and dry. The humidity was probally pretty low. We seldom have a problem with that unless it is just about to rain really hard.  This may become a problem as the winter sets in.
I know that the main is too high but I wanted to leave it alone until I got the pilot in the ball park.
question, will the main jet effect the pilot and the slow system if the main is too large? Will I end up changing out the pilot again once I get the right Main jet in place?The cool thing is that I am learning alot. I use to just leave the carb alone and hope for the best. That will never be enough from now on. Thanks for all the help!

Thom Green,I own and ride a 76 250 MC5 MX which I bought new.

[tomale]

Sorry,
Correction.... I did not put in a 370 but rather a 270 main. This is still too large but is closer I think. I would rather it be too rich than too lean.

Thom Green,I own and ride a 76 250 MC5 MX which I bought new.

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Thom what needle jet are you running I just fine tuned a 360 Bul which should breath somewhat similarly.As follows 40 pilot (this is a flattracker so the pilot is one richer than I think is optimum.) 6f76 needle with a P-5 needle jet in middle positon ,again this is slightly richer than I'd jet an MXer.3.0 slide filed slightly to lean it just a tad.I siezed it on a half mile with a 270 main but that is about right for a short track.This is on a 36 R/S.1.5 turns on you air jet is right, try to find the right pilot for that setting.Do plug chops on you main and look for a light brown color.Bultaco's need to be somewhat richer and flat track is a different environment but this should get you close.I tried a P-8 nj with a 6DP1 but the above combo is much better in the mid range.Frank

 

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Correction typo 6FJ6 needle.

 

[tomale]

I do not yet understand the needle jet and the jet needle. what is with the combination number and letters and what do they mean. what is small and what is larger. how do the letters effect all of this. does it have something to do with the taper?
Also how do you get the needle jet out. I got the carb off the main out and the slide off but I do not know how to get it out. Is it screwed in from the slide side? do I need a special tool to get it out?
Do you think that the pilot is about right but because the needle and needle jet are too large it is still too rich? Again, thanks for the help

Thom Green,I own and ride a 76 250 MC5 MX which I bought new.

[Rocket]

Thom
Sudco has a site that may explain about Mikuni's.  Maybe that will help.
http://www.sudco.com/
Rocket

 

[john durrill]

Thom def. get the mikuni tuneing book from Sudco. it shows all you will need to know about jets and sizes and whats available.
Most mikuni needle jets come out the top through the carb bore. the brass pin in the side keeps the jet from turning.
 look at the rear of the carb , where the air enters from the air box. at the bottom should be a hole with a jet in it. its the air jet and controls how much air is fed to the needle jet emulsifier. see what size you have. they run from .5 to 2 in sizes. .5 is rich and 2 is lean. its used to fine tune the needle and needle jet combo. if you have a .5 you might want to take it out and see how the bike runs. if you think your very rich in the middle it should show you which way to go. it sounds like a 30 pilot will help . thats given no air leaks or mechanical problems are causing what looks like a rich condition. make sure your air filer is not over oiled.you would see oil built up in the air box below the filter after the bike has set overnight  if thats the case. that can give you a bum reading on your jetting.
John D.




Edited by - john durrill on 09/29/2003  09:56:20 AM

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The needle jet is what the main screws into.Push it out from the bottom toward top.The needle jets are lettered and numbered logically the advice about the Sudco book is dead on.It's cheap and real careful reading will make you an expert.Pro Flo has them for the same price as from Sudco.The relationship of needle to needle jet is critical.Needles differ by taper the main factor is the size of the needle jet and how it relates to the needle.They are listed by general ranges richer to leaner in the Sudco book.

 

[tomale]

Ok, I got it out. Thanks! It says that it is a Q 2 So I can not go any leaner than that. Going to a 30 pilot may be the way to go. The only other thing I know to do to get it to lean out at idle besides a smaller pilot is to go to a 3.0 slide. Not sure I want to do that just yet. I am going down tomorrow to get some more jets.... I did do some testing to see if I am getting a leak some where else. I had changed the right side crank seal earlier this year. It was not leaking but it had never been changed, so I did it. The left I was told would show up by excesive smoke out of the exaust. I started the bike and sprayed carb cleaner around the intake manifold and the bottom of the barrel no change in engine.... Do I need to sort out the jetting on the on the bottom half of the throttle before I start changing main jets? Oh, I am careful not to over oil my filter. I am using "NO Toil" and I like it really well. Sure is nice to clean up with. I would think that it would really have to be dirty to make this much of a change and I just cleaned it. It has lest than 2 hrs on the bike since the last cleaning.
Thanks for all the help!

Thom Green,I own and ride a 76 250 MC5 MX which I bought new.