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Mikuni VM26 (and some tips on carb tuning in general)

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Re: Mikuni VM26 (and some tips on carb tuning in general)

Postby pjrey » 17 May 2012 12:51

hey jose, thanks for your reply

ok.. put a new plug in... warmed up the bike (drove 2 miles)... then drove 1/2 mile with the throttle wide open, 100%... with it wide open, hit the kill switch, pulled the clutch and stopped.. popped the plug out.. white (i could take a picture, there is a black smudge on the right side.. very small, otherwise it is totally white!)
i then put a fouled plug in, did the same test, and it was WHITE...

but that is what is interesting.. the fouled plug came from my bike.. i drove 30 miles, with the throttle wide open.. when i got home, i took it out, and noticed the plug was fouled.. i thought maybe i lowered the clip needle too far... (had it all the way at the bottom clip setting)... so i moved it up in the middle again...

been getting white ever since (EVEN after i moved the clip to the last clip setting again!)
so i dont get it....

anyhow, is 1/2 mile long enough test for it to correctly show up on the plug? should i have held it wide open longer?

also, if i whack the throttle wide open from idle, it will die.. every time... im talking, whacking! fast!.. dies. i have played with the air screw, needle clip settings, nothing seems to work.. got this from motoscoot (stage 3) gutted exhaust, 26mm mikuni, 17/39 sprockets

someone help me please!
thanks!
pj
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Re: Mikuni VM26 (and some tips on carb tuning in general)

Postby Amahoser » 17 May 2012 14:44

You need a larger main jet. Take the float bowl off and pull the main jet and the pilot jet and see what the numbers are. I am not sure what jet you will need but for my bike with a yx160 and stock muffler a 180 to 185 main jet seems about right (in the process of rejetting myself... have a 185 in there and it is a bit rich, a 175 is too lean so I am going to a 180 today to see how it does). Idle is a bit rich at a 20 size pilot jet so I am going to try a 17.5 jet next.

No amount of fiddling with the needle is going to remedy needing a larger main jet.
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Re: Mikuni VM26 (and some tips on carb tuning in general)

Postby pjrey » 17 May 2012 15:29

thanks for replying!
haven't opened up the carb yet.. figured i would wait for the jets to arrive from motoscoot.. but cant i just remove the nut on the bottom of the carb for changing the main jet?

valve clearance would have nothing to do with this correct? i checked and i have .005 in and .008 out.. shouldnt those stock numbers change due to the fact i have the big bore kit?

thanks again for all the help guys!

p
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Re: Mikuni VM26 (and some tips on carb tuning in general)

Postby Amahoser » 17 May 2012 17:24

Nope, Valve clearance wouldn't have anything to do with it.

You can use that large nut to remove the main jet but in order to remove the pilot jet, you will need to remove the float bowl. No big deal, just 4 screws.

BTW, half a mile is too much for a plug chop. I just go down the block a bit for my plug chops. You are also not being to kind to your motor doing that. If you are running so lean that your plugs are white, I would take great care and not run the bike too much or too hard until you can put richer jets in.

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Re: Mikuni VM26 (and some tips on carb tuning in general)

Postby pjrey » 17 May 2012 17:31

thanks again for posting!

the weird thing is the other day i drove to my town and back, 15 miles one way... and when i got home, i pulled my brand new plugs out.. and low and behold they were dark dark brown... too dark... what gives!??

how can that be? i ran the whole trip, 30 miles, with the throttle WIDE open! all the way...
and the plug was darrrrkkk...

so, during all my tests now, i decided to put that old plug in.. and even using an old fouled plug, it came out totally white! it ate all the 'foul' right off!

so, my question is, how did i get a dark plug once.. and have never ever ever been able to re-create that now...?

the mystery continues!

p
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Re: Mikuni VM26 (and some tips on carb tuning in general)

Postby Amahoser » 17 May 2012 17:51

Not sure, but the point of using a new plug and only running that plug at WOT is to eliminate any other throttle states. I would use a new plug and a plug chop as the only way to determine main jet selection... unless you have a wide band air/fuel gueage as I do! :) makes the process much, much easier!

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Re: Mikuni VM26 (and some tips on carb tuning in general)

Postby pjrey » 17 May 2012 19:07

ohhhh, i want one!
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Re: Mikuni VM26 (and some tips on carb tuning in general)

Postby pjrey » 17 May 2012 20:46

motosoot said they had sent me the jets on april 30th, monday... called a twice and was told they should be arriving anyday.. .never came... called again, same story.. called today, the 17th, and he said he sent them out just yesterday..? hmm.. anyhow.. new jets are on the way!
but just in case these jets never show up.. where can i get a jet kit for the mikuni vm26?
i would like a couple pilots as well (i dont think andy is sending any of those, just main jet and a new carb-to-manifold thingy.. (current one is cracking.. maybe getting air through the cracks.. could be a cause of alot of this..)

so, again.. where can i find jets?

thanks!
p
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Re: Mikuni VM26 (and some tips on carb tuning in general)

Postby Amahoser » 17 May 2012 21:55

Thats standard with Moto-Scoot... for whatever reason they NEVER ship on time, even when they have it in stock. I wish they would get their act together. I cancelled an order after 2 weeks of waiting and promises. You can try http://tboltusa.com/ or http://www.carbparts.com/ or http://www.firepowerminis.net/

The carb boot is cracked? There's your problem and why you are running lean. In the interim, maybe seal it with some silicone and wrap it with tape or something.... If air is getting in the carb boot that will easily make it run lean.

Good Luck!
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Re: Mikuni VM26 (and some tips on carb tuning in general)

Postby Amahoser » 17 May 2012 22:03

http://www.dynotunenitrous.com/store/sc ... roduct=342

Thats the wideband I got.. good price for a wideband. Makes tuning a snap!

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Re: Mikuni VM26 (and some tips on carb tuning in general)

Postby pjrey » 17 May 2012 22:11

thanks again jose
it is not cracked through.. i am almost certain... just aesthetic ... for now.. and will go through im sure.. (sounds like andy knows this.. _that's why he is sending out a replacement...)


we will see!
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Re: Mikuni VM26 (and some tips on carb tuning in general)

Postby Amahoser » 17 May 2012 22:30

To be sure, get some carb cleaner and spray it on the boot while the bike is running. If the idle increases or decreases (cant remember which) then the boot is leaking air into the engine. Old trick that works well.

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Re: Mikuni VM26 (and some tips on carb tuning in general)

Postby pjrey » 17 May 2012 22:38

it would increase if you there were holes in it and you are spraying pure alcohol on it...
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Re: Mikuni VM26 (and some tips on carb tuning in general)

Postby pjrey » 18 May 2012 11:49

hmm, but look at these pictures i took of my plugs that i took while i was out in california (the reason i took these photos was because i was having the same issues out there that i am here, bucking and stuttering...)
i was at 80 ft sea level, here in MN i am now at 400 foot sea level.. shouldnt matter much..

but why was the color spot on out there? and now when i do a chop block or whatever, it is totally white...?
probably running more mid-throttle huh?
but that is when it bucks the worst!
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Re: Mikuni VM26 (and some tips on carb tuning in general)

Postby pjrey » 18 May 2012 20:12

ok you guys, new jets came today, and new carb-to head rubber manifold (old one was cracking)

the jets that came were three main jets... 155, 165, and 175..
i opened up the carb tonight and found that i currently have a 190 main, and a 22.5 pilot

what givess???
i have the clip on the lowest postion (clip is on the last clip setting, 5th from the top)
and my plugs are still white...

drove to town again, 30 miles total... WOT the whole way.. came back, the plug looked a litttttttle brown.. still pretty white..
just took it for a two mile cruise, throttle positions all over.. i rode WOT in 3rd gear and at the highest RPMs.. it started bucking, and cutting out... then in fouth it is ok, because the RPMs never get that high...

came back, looked at the plug (same old plug, didnt put a new one in) and now it is WHITE again...
alllll white...

i need some help guys

running 91 octane gas

p
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Re: Mikuni VM26 (and some tips on carb tuning in general)

Postby Amahoser » 19 May 2012 21:42

I dunno bud...

You said you have a stage three kit. The 190 main should be way rich for that. Something else is going on. I would clean that carb to make sure nothing is blocked. Then I would confirm no leaks at the intake manifold (carb boot and the 90 degree elbow). Although I highly doubt it, check your exhaust for leaks. The bucking may be something else, I would check all electrical connections including the kickstand and clutch safety switches (I would bypass them, my kickstand switch failed after a month). Definitely check your electrical ground connection as well.

Also, check that you fuel valve flows free. Do you have a vent for the fuel or is the line blocked?

Another thing to check is your float level.Take a clear vinyl tube that just fits the nipple under your float bowl. Fit it to the nipple. Bend the tube so it it pointing up beside the float bown (your bike needs to be level for this to work). Open your fuel valve. Besides that nipple is the float bowl drain screw. Loosen it until gas flows freely. Gas should flow up the tube. A proper float level is when the gas stops flowing right at the partition line between the float bowl and the carb body. If the gas is lower than the partion line, that can cause your engine to be lean. If the level is off, you will need to remove the bowl, remove the actual float and bend the tab that the float valve is attached to to change the level. There is an actual float level spec in millimeters but I don't know what it is. The partition line method seems to work on the majority of carbs.


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Re: Mikuni VM26 (and some tips on carb tuning in general)

Postby pjrey » 20 May 2012 19:17

anyone else have the stage 3 kit on? what size jets are you using/clip position (agian, i am at 400 feet)

electrical? ill check.. but it is interesting that i am able to change the bucking from 1/4 throttle to 1/2 to 3/4 by adjusting the air screw....
again, full throttle and there is no bucking... but plugs are not fouled...

should i try bypassing the fuel filter? i will try your carb level test you mention when i get a minute...

i think i will try putting the clip in the middle, and testing out the main jet 175 and see what happens...

thanks again for all your help!
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Re: Mikuni VM26 (and some tips on carb tuning in general)

Postby pjrey » 21 May 2012 10:50

ok, i tried the test.. hope i did it correctly...
had the petcock off.. took off the bottom plug on the carb.. put a tube on the lowest port... and turned on the petcock.. nothing...
i even tipped the tube downward.. nothing..
ive attached a video...
http://youtu.be/XHO7jC3tMkw

is this the problem then??

thanks!
p
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Re: Mikuni VM26 (and some tips on carb tuning in general)

Postby pjrey » 21 May 2012 13:19

ok, i tried the main jet #175
took away the bucking and sputtering.. but lost lots of power/speed.. and also now when i full throttle it, i actually feel it struggling.. if i let off the throttle a hair, i go faster than if i go full WOT... so.. too lean a jet...

you guys, i don't know what else to try here...
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Re: Mikuni VM26 (and some tips on carb tuning in general)

Postby Amahoser » 22 May 2012 01:02

pjrey wrote:ok, i tried the test.. hope i did it correctly...
had the petcock off.. took off the bottom plug on the carb.. put a tube on the lowest port... and turned on the petcock.. nothing...
i even tipped the tube downward.. nothing..
ive attached a video...
http://youtu.be/XHO7jC3tMkw

is this the problem then??

thanks!
p


My apologies....
Thats what I get for not actually checking the MY VM26 before giving you those instructions. There SHOULD have been a drain screw that you loosen in order for the gas to flow out.... The VM26 does not have that drain screw. I guess the big 17mm nut at the bottom prevents room for the drain screw. Every other carb I have worked on has it. So again, my apologies. You'll have to do it the old fashioned way. In that super tuning manual that you got, it gives specs for float height. Follow their instructions on setting the float level. Float level is critical, if it is set too high, gas will leak out of that lower hose and you will run rich. If low, your motor will lean out.

I just looked at the super tuning manual, Float Height for VM26 (page 5 of the manual) says 15-17mm. Measure (with carb upside down and the float bowl removed) from the base of the carb to the top of the float.

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Re: Mikuni VM26 (and some tips on carb tuning in general)

Postby Amahoser » 22 May 2012 01:14

For my yx160, 175 was too lean and 190 was too rich. I would assume that your stage 3 should be close (same hp according to moto-scoot). Right now, I am running a 180 with a 17.5 pilot and the needle in the middle position. Seems pretty close. I Get a slightly rich idle, mid range never getting too lean and WOT a bit rich.

FYI, I NEVER (even running lean with the 175) experienced the mid throttle bucking you described. Mid throttle has been very smooth throughout my testing. I did have a bit of bucking on a full throttle run with a 175 (too lean) that went away with the 180.

Thats why I wonder if something else is going on....

Where do you live? If you PROMISE to return my carb right away, I can send you my carb to test... Maybe your carb is defective... Maybe electrical or some other engine problem...

Who installed your stage III??

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Re: Mikuni VM26 (and some tips on carb tuning in general)

Postby Amahoser » 22 May 2012 01:25

Do you still have the needle at the low extreme? Move it to the middle... if Your bucking was solved by the 175 jet MAYBE you fixed mid throttle by leaning the top end, which would lean the mixture throughout the throttle range. Stabbing in the dark here... Also, moving the needle in and out of the carb might have damaged it. Pull it out and confirm that it is free of burs and scratches. Pull the e-clip and roll the needle on a flat surface (your coke mirror should work good for this!) and confirm that it is not bent or deformed. At this point, try everything!!

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Re: Mikuni VM26 (and some tips on carb tuning in general)

Postby MilwaukeeMadass » 22 May 2012 07:17

Amahoser wrote:If you PROMISE to return my carb right away, I can send you my carb to test


:shock: Offering to do that is like letting someone borrow your kidney.

You sir, are a true gentleman and MadAssassin. :handgestures-salute:
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Re: Mikuni VM26 (and some tips on carb tuning in general)

Postby pjrey » 22 May 2012 14:52

wow... thank you! that is very kind of you jose!
i would LOVE to try that...

the 175 was tooo lean.. at WOT it kept dipping at 100./// struggling, i would lower it to 3/4 and it would rev up better...
and also, by looking at the plug readings, it appears as though 190 is too lean...
i have check for leaks.. (visually, have yet to try the starter fluid trick... will do that soon)

what speed should i be getting? 500feet, 39t rear, 17t front... stage 3, gutted exhaust, mikuni vm26mm i am 195lbs 6'3"

i am getting mayyybee 60
( was getting faster, or so i thought, by my spedo was not calibrated correctly...)i used to get 65mph, now i can barely get 60
is that normal?
my RPMs in 4th at 55 do not sound that high at all
i have an old .49 cc.. and i get cruising in 4th, it really revs! (and i can go 48 mpg on a .49 cc.. should be able to do better on a 142 than what i am getting i would think..)

i will PM you jose
again, thanks so much for all your time! you have been a real help!

p
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Re: Mikuni VM26 (and some tips on carb tuning in general)

Postby pjrey » 22 May 2012 14:54

andy installed my stage 3
needle is true and straight...
no scratches anywhere.. i am very careful with this thing

i am in Minnesota
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