...... Mikuni 38mm flatslides

[Brook]

...... I am going to try this again,
       [The carbs. are sticking open] ........With the gas tank off and the motor running and turning the throttle. [open or closed] The RPM's stays at that high RPM. and the  SLIDES ARE DOWN.
.. So, my question is, How can that be. there is a great amount of vaccum coming from the back of the carbs...... where is it  coming from??..... the SLIDES ARE DOWN.
                       Note.it runs fine at idle, with a little popping noise
.. At this point, 1-21-10, I have the carbs. off the bike and apart. looking down into the top of the carbs., I can see a little vent on the enter wall on the filter side at the top of the slides,
.. Is this the area, where the vaccum is coming from??   or is the vaccum so strong, coming from the motor, it is pulling from the sides of the slides?
                                                                                                                                                                  I hope this is easier to understand.
                              Thanks,    Jeff

[the fan]

Definitely: overly lean mixture at Idle. Likely causes, clogged pilot jet and /or mixture screw on i or more carbs, too lean pilot jet, too lean mixture screw. Try turning mixture screw on each carb out 1.5 turns. if the total is more than 3.5 out or problem does not completely clear up go up a size on the pilot. check to ensure that mixture screws have O-rings in place.

Probable: Idle adjuster set too low. combined with lean mixture creates very high vacuum, drawing fuel from bowls and feeding problem. Usual sign; slight bog when throttle is cracked from racing idle settling back to high idle.


likely: air leak at carb boot. check with wd-40
         carbs way out of sync If 1 carb is pulling significantly more (or less) vacuum than the rest it can cause the idle to rise in an attempt to balance out the set. Bench Sync.
         Valves set too tight. check clearances

[andyb]

Find the basics before changing jets or making adjustments.

Find air leaks, check for cylinders not firing (floats stuck closed?), loose clamps, clogged inlet screens atop the float seats...

[SlowOldGuy]

Your concern about the high vacuum is confusing.  The engine wants to ingest 1200 cc's of air every 2 revolutions of the crankshaft.  Even though you think the slides are closed, there is a gap on the bottom to allow at least enough airflow for idling (usually referred to as slide cutaway).  Depending on the RPM, that can result in a significant amount of airflow and the vacuum that you are feeling.  If the carbs could cut off ALL airflow/vacuum, the engine would die!

Sounds like a lean condition or a vacuum leak.  There should be an air bleed for the idle circuit (in the stock carb, this is the air pilot jet).  Use your finger to block it off and that should richen up the idle mixture.  See if that affects your idle speed.

DavidR.

[Brook]

..... First things first......... thanks for the help.
.. Before I put the carbs. back on. This is what I am planning on do.
..check for cracks in the boots, .... With a magnifying glass, under a good light.... Chris, reminded me to check the rubber o-ring in the back of the boot. Also, going to spray with WD40 when the motor is running.
.. Making sure all the jets, needles and air, fuel pathways are clear. and the floats are lose and free.

>>>>Question, on other carbs. there is a rubber point on the end of the needle valve assy and a clip, that hooks to the float.... should the F/S's have the same??

.. When I had the carbs. on and running, I adjusted the air mixture screws 1/4 to 1/2 out, going by the owners manual... I will try, up to 3.5 turns out.

..The #'s for the main jets,>130, 135, 140, [those are what I have in hand] the jet needles> second clip, 9chy3, needle jet,> p-4 and pilot jets,> 17.5 That's what is in the carbs. and that is what is listed in owners manual...

.... Like I said earlyer, with the motor running and the throttle sticking, their is a great or I should say, a tremendous amount of vacuum.
It is hard to believe, all the vacuum is coming from the SLIDE CUTAWAY,
>>>>>Question, with the slides down, is the vacuum coming from the cutaway that is pulling from the pilot air jet?? Because, the cutaway, does not go all the way thru the slides
And yes, by putting my finger over the pilot air jet, the RPM's do drop.

.. When I first got the carbs. I could not get them to work right, I decided to take them apart. to see if all the needles and jets were the same. I notice, the holes for the pilot jets, [in the throat of the carb.] was not clear, It look like the factory, did not drill a perfect hole, So, I took a wire and  from the top, poke the wire thru the hole. and of course it made the hole larger.   Could this be a issue ? 

...Sync.the carbs..... I do this the old school way,[ I wish I knew how to transfer pictures a diagrams]    I got this from the archives.
.. Using the two jar with oil and 3/8 clear hose system. ...... and with the motor warm up. and fans going.
>>>>> Question, I do this with the filters off and the lowest rpm's, ...is that best way??
.. I have 1 hose hook to#3 [that is the one, that is pre set] the other hose hook to#2 then #1 than #4. Then I do it again, to recheck... I am not sure, if the old school way, is as good as using the mercury sticks.



......A good or bad idea ? 
While I have the carbs. setting on the bench,.... two things I am thinking of doing.
Or I should say, one. I have already done the other.
.. I do not like the way, the springs setup, on the shaft throttle. for the tention for lowering the slides.
So, this is what I did, in between #3 and #4 carbs. I drill a hole in the shaft throttle, Then took a bar from a set of front brake caliper's, that holds the brake pads..... and drilled three small holes in the bar. [for spring adjustments.]. Then put the bar in the hole in the shaft. ..... Then I made a little elbow with a hole drilled, that fits under the bolt for the #3 float bowel. Then hook a three inch spring to the elbow and bar. 
. I do not know if this will make a difference,... If not, I can put it back, the way it was.

.. I have ask, this before.... What is the difference, between the Mikuni F/S and the Keihin's.
And the answer has been, the ball bearing that the slides work on.

..........This is what I am thinking of doing....
...With the carbs. on the bench, with the slides in ...and the throat facing up, ... take a magic marker and make a circle on the slides, following the circumference of the barrel. Then take the slides out, and cut off the bottom part of the slides..... on the out side of the marked line. leavening a 1/2 in the middle of the slide alone.......That should be the only metal on metal......And above the untouch 1/2 in the middle, just polish.
.... The reason for doing this is, .. less friction,..and  less of the slides that the vacuum can suck on.
By removing the part, below the marked line, .. There should not be any difference, when the slides are closed. ......just a idea!!                                                   
                                                  Thanks,   Jeff

       
 
         

 

[SlowOldGuy]

.. When I had the carbs. on and running, I adjusted the air mixture screws 1/4 to 1/2 out, going by the owners manual... I will try, up to 3.5 turns out.

I'm sure the flatslides have different settings than the stock FJ carbs.  The 3.5 turns is more of a stock adjustment.

.... Like I said earlyer, with the motor running and the throttle sticking, their is a great or I should say, a tremendous amount of vacuum.
It is hard to believe, all the vacuum is coming from the SLIDE CUTAWAY,

Again, there's 1200 cc's of vacuum being pulled through the carbs, it's gotta get there somehow.  The more the slide tries to stop it, the greater the vacuum force.  Don't confuse vacuum with airflow, they vary in inverse proportion.

  And yes, by putting my finger over the pilot air jet, the RPM's do drop.

I think you found the problem

.. When I first got the carbs. I could not get them to work right, I decided to take them apart. to see if all the needles and jets were the same. I notice, the holes for the pilot jets, [in the throat of the carb.] was not clear, It look like the factory, did not drill a perfect hole, So, I took a wire and  from the top, poke the wire thru the hole. and of course it made the hole larger.   Could this be a issue ? 

Uh... YES!!!!!  Did you do this to all the carbs?  You re-drilled a passage in the carbs, of course it's going to be an issue.  Sounds like a good way to ruin a set of carbs.

>>>>> Question, I do this with the filters off and the lowest rpm's, ...is that best way??

Filters should be on for best sync.  Check it at several different RPMs.

..........This is what I am thinking of doing....
...With the carbs. on the bench, with the slides in ...and the throat facing up, ... take a magic marker and make a circle on the slides, following the circumference of the barrel. Then take the slides out, and cut off the bottom part of the slides..... on the out side of the marked line. leavening a 1/2 in the middle of the slide alone.......That should be the only metal on metal......And above the untouch 1/2 in the middle, just polish.
.... The reason for doing this is, .. less friction,..and  less of the slides that the vacuum can suck on.
By removing the part, below the marked line, .. There should not be any difference, when the slides are closed. ......just a idea!!                                                   

I can't picture in my mind exactly what you're trying to describe here, but it also sounds like an incredibly efficient technique to turn an expensive set of carbs into JUNK.  I don't think friction is the problem.    

DavidR.

[Brook]

.... First of all,.. my last post, did not turn out the way that I was expecting it to.
.. I was halfa sleep.
...... The air mixture screw..I was going to try, the 3 turns out, Just to see what the difference would be.
..... The pilot air jets.. Maybe, by dropping down in size, will control the vaccum.
......The hole [in the carb.] above the pilot jet,... I did not use a drill, just a peace of wire. I do not think, the wire would have made the hole bigger, than it was suppose to be.
..... synchronizing the carbs... The two jar and 3/8 tubing system, seem to work well... Next time, I will have the filters on..........And at. at different idle speeds.
..... I really screw that up..... Trying to describe what I was trying to do with the slides.
Try to picture this, looking into the back of the carbs..and the carbs. are all to gather, ready to be put on the bike.... and the slides are down or closed. There will not be any openings,... all you will see is the slide.
  That's what you will see, After cutting the bottom corners off.

... All I am trying to do is. possible make the Mikuni F/S work better. It's just a thought...... I know it could become a $800.00 pile of junk....... Just trying to get other idea's

.... My FJ, does not like the way that it does, by me thinking inside of the box!!!!
                          Thanks,     Jeff

[Harvy]

.... First of all,.. my last post, did not turn out the way that I was expecting it to.
.. I was halfa sleep.
...... The air mixture screw..I was going to try, the 3 turns out, Just to see what the difference would be.
..... The pilot air jets.. Maybe, by dropping down in size, will control the vaccum.
......The hole [in the carb.] above the pilot jet,... I did not use a drill, just a peace of wire. I do not think, the wire would have made the hole bigger, than it was suppose to be.
..... synchronizing the carbs... The two jar and 3/8 tubing system, seem to work well... Next time, I will have the filters on..........And at. at different idle speeds.
..... I really screw that up..... Trying to describe what I was trying to do with the slides.
Try to picture this, looking into the back of the carbs..and the carbs. are all to gather, ready to be put on the bike.... and the slides are down or closed. There will not be any openings,... all you will see is the slide.
  That's what you will see, After cutting the bottom corners off.

... All I am trying to do is. possible make the Mikuni F/S work better. It's just a thought...... I know it could become a $800.00 pile of junk....... Just trying to get other idea's

.... My FJ, does not like the way that it does, by me thinking inside of the box!!!!
                          Thanks,     Jeff

Jeff......if the jets that you cleaned out with a piece of wire are made of a softer metal than the piece of wire, then you have almost certainly enlarged the jet hole.

Cutting the slides will reduce their weight...... I don't know if that's going to do anything other than to maybe create more problems than it solves.

I know I'm not helping, except maybe adding to the list of things not to do.     
Cheers
Harvy

[Brook]

...... No Harvy,... I used the wire only in the hole in the carb. , not the jets.
.. Yes, cutting the slides would reduce the weight, ..... But, I was looking at reducing the metal on metal = less friction. .... That means, with less friction, the slides would possibly slide easier..... and maybe, be in the same ball park, with the Keihin's.
                                    Jeff

[SlowOldGuy]

But haven't you said that it idles up even when the slides are at their powest point?  If so, then the slide position is not the problem.  If there was a slide friction problem, then Mikuni would have already solved it.  Besides, friction is NOT a function of contact area, it's a function of the coefficient of friction and the force normal to the area of contact.

DavidR.

[Brook]

..... SlowOld Guy....  "The Mikuni would have already solve it" ..Then why does the Keihin's  have the ball bearing or rollers on there F/S??

.... About.. the idles up when the slides are down..... When I start the motor, [and it start's right up]... and  do not touch the throttle, idle RPM's stays the same.     

..... If the motor is creating 1200cc's of vacuum, on every 2 revs. of the crackshaft......that's allot vacuum coming from the motor, and pulling on the slides... most or all of the time.
....And that's what makes me feel,.... that less of the slides touching the enter walls, would cause the slides to move smoother.

The only reason, why I think it may not work is, creating to much space in between the two and allowing the vaccum to get by the slides.

.. I think it's safe to say, ...... that our motors are considered to be a big air pump.
And I am not sure what causes the amount of vacuum,  But, I was told, that more vacuum, the better!!

.... The bottom line is, I am going to put the carbs. back together and on the motor, drop the size of the pilot air jet's, [That may be the reason, why there is so much vacuum] and sync.....and see what happen's.
                       Thanks again, Jeff
... Does anyone have opinion, on the fan that I installed on my FJ???

[Brook]

...... To SlowOld Guy..... One thing about writing on the internet,.. you do not know how it's going to be accepted, on the other end..... especially, when I am doing the writing.     

..when I replied to,...  Mikuni would have already solved it... If it sounded like I was being rude..... I apology.
                   Jeff 

[ccsct203]

ok... first I did not read every post here... ADD i guess... I have the 36mm Mikunis' on my bike and I had the same issue. I am running 125 mains and I have 122.5 and 127.5 to try. the 125 run great! 130 and up were to rich. I also have wiseco 10.5 to 1 and a 1200cc cyl. on the bike.
I'm running megacycle cams made for midrange torque with all the hardware too with dyna 2000 ignition system.
The pilots only need to be 1/8-1/4 out on my bike.
The issue I had was a fault of my own when my bike was reassembled... I forgot the o-rings between the head and the  rubber manifolds.
After using WD-40 sparingly to spray to find the "leaks" i found the issue in seconds.
Just a thought.
You have to be careful with sprocket size too. I ran a 38 rear and i believe a 18 front the bike get bogged down because I'm trying to open the mechanical  carbs where you once had carbs opening by vacuum.