Who out there has a 28mm Mikuni???

[EZRider76]

No problem, homie!

I agree with your buddy you should do a plug chop to confirm however, but i bet with the 28mm you will drop a size or so. I:I

Oh, plug chopping for sure! I just want to get good engine response thru 0% - WOT. Or at least get it to the point where I'm rich enough at WOT throttle that it spits and sputters a bit until it cleans itself off and then WOT is nothing but power....not that I will ever be at WOT that often when trail riding, but nice to know it is there ya know.

I'm sure too, if I sat down, I could probably calculate out the CFMs each carb delivers at WOT, if I knew the negative pressure the piston creates versus bore size and reed strength, accounting for variables of moisture (Fuel/Oil Mix and environmental/humidity) content, and allowing for independent ranges for air filter cfm's with or without lid. Not that I really want to, but I'm thinking I could model something very simple that would allow that. Do I want to...no. LOL...this is a hobby darnit all.

 

[EZRider76]

The cut on the slide is more at idle, right?

yes and no, according to the VM Manual, and I will quote, "When the throttle valve is opened about 1/4 or more, the velocity of air flowing through the needle jet increases and also the negative pressure increases to the point where fuel can be sucked in. When the opening of the throttle valve is between about 1/4 and 3/4, fuel passes through the main jet and, after being metered in the clearance between the needle jet and the jet needle, it is mixed with air that is metered in the air jet and atomization of the fuel is accelerated. **Next Paragraph** The mixture is then injected, after mixing with air flowing through the main bore, to the engine in the optimum air-fuel ratio. During this process of operation, the cutaway of the throttle valve serves to control the negative pressure on the needle jet, thereby regulating the amount of fuel that is injected to the engine. When the throttle valve is opened more than three quarters for high speed operation, fuel is chiefly metered with the main jet.

So, basically, it has to do with forcing the air flow down to the bottom of the bore, creating a vacuum to suck the fuel vapors up thru the jet needle as it passes by. At WFOT, the slide is out of the bore and it is up to the main to allow full fuel vapors out to get sucked up, and hopefully the needle doesn't interfere with that operation. This is why I am focused on that darn needle right now, I'm thinking it might be too short for my carb and mods.

 

[mjgnslngr]

ah man, dont tell me that! does that mean if this needle i got coming doesnt fix this pwk im looking into a slide?

 

[EZRider76]

ah man, dont tell me that! does that mean if this needle i got coming doesnt fix this pwk im looking into a slide?

Shoot, I couldn't tell ya. I understand the theory like it's 10th grade geometry, I understand each component of the carb and what throttle range it works and how it relates to the other components if at all.

I just spend 3 hours, racing up and down my buddies drive way, trying all different settings from mid clip slot to the top of the needle on mains 250, 260 and 270. Each main acts the same way, mid clip I get to half throttle and it starts bogging. Next slot up, I get to 3/4 throttle and it cuts completely at WFO. Top slot, is the same as the previous but at WFO it is so spitty and sputtery...but at least it isn't cutting out.

I put in new plugs, gapped them to within spec, did down and back runs. When it would spit and sputter or cut out, I would pin it for a few seconds, cut the engine and then coast and pull the plug to check it out....saturated. X(

So, I put in my 260 main, 4th clip location on the needle, coast to the end of the drive way. Kick it and let her rip up the drive way into 5th gear at 3/4 throttle, let her wind out for about 7 seconds, cut it and pull the plug....looks brand freaking new. So I quickly swap out to my 270 main, swap to a new plug, run her down the drive way, turn around and then run it back, and this is what I get....remember though, this is only 3/4 throttle...

 

[Awk08]

all these needle and main changes, basically ending up with the same result, leads me to believe you need a leaner needle, or you've got float height problems, have you submerged the floats to look for bubbles from a cracked float ?

remind me again how many turns out on the airscrew does it idle the highest ???

 

[EZRider76]

all these needle and main changes, basically ending up with the same result, leads me to believe you need a leaner needle, or you've got float height problems, have you submerged the floats to look for bubbles from a cracked float ?

remind me again how many turns out on the airscrew does it idle the highest ???

That float height problems actually jogged my memory. I have adjusted the float flapper a couple times. I am beyond my 17mm and tonight when I trailered the Blaster, I came back a few minutes later and it was leaking fuel out the float bowl vent tube still. I measured the height EXACTLY like the VM Manual said too...except to get the height off the rib in the carb housing, I took a small flat piece of plastic that would lay across and then I measure with my caliper to that, and added the thickness. I measured right at the point where my flapper is rubbed by the floats. If I remember correctly, I'm at 18.3mm as of right now, but it still seems to be over flowing and it was on level ground. X(

Anyhow, if I go to a leaner needle than I'm already running, and considering my best results for WOT is at the leanest slot on the needle that I'm currently running, won't that cause over heating and possible catastrophic meltdown at cruising speeds?

I have checked for cracked floats, they are fine. Actually, when I change out the mains, I leave some fuel in the bowl and they bob around like styrofoam in water.

My air screw setting, since I have sealed up my exhaust leaks and that put me back down to a 32.5 pilot, is at 2-3/4 (ish) turns out. That is including the initial 1-1/2 turns out, so a little over 1 more full turn.

 

[Awk08]

so, if i understand correctly, you've had the float heaight correct, and now it's off again ??

on these stock carbs, you measure from down in where the gasket would sit, not off the rim

if float height isnt shutting off the fuel, you may have a bad float valve/rubber tip, allowing fuel in no matter what setting ?

i would suspect, the next leaner needle would have very similar flow at it's lowest clip setting, as your current one at it's highest setting ?
or at least they should overlap like that ?

that 2-3/4 pilot setting is borderline needing a smaller pilot, in my book, i like them to high idle at 2 or less turns out

 

[EZRider76]

so, if i understand correctly, you've had the float heaight correct, and now it's off again ??

on these stock carbs, you measure from down in where the gasket would sit, not off the rim

if float height isnt shutting off the fuel, you may have a bad float valve/rubber tip, allowing fuel in no matter what setting ?

i would suspect, the next leaner needle would have very similar flow at it's lowest clip setting, as your current one at it's highest setting ?
or at least they should overlap like that ?

that 2-3/4 pilot setting is borderline needing a smaller pilot, in my book, i like them to high idle at 2 or less turns out

wow...so you only like the high idle at 2 turns out? That is only half a turn from the starting 1-1/2 turns...that's pretty particular.

as for the needle and seat and floats. I have the floats beyond maximum measurement and it is still leaking...not sure why. I have polished both the needle and seat with The toothpaste method. When I attach a hose to the fuel inlet, tip the carb upside down so gravity holds the flapper against the needle, I cannot blow thru the hose, the needle is sealing off the seat so nothing gets thru. now I will add, there is no rubber neither the needle or seat...but it is sealing to air, so it should be sealing to fuel/oil mix.

maybe I am just more confused than I think I am and don't really understand it the way I thought I did....so Damn frustrating....to the point now I won't give up til I kick this pig and win the war.

 

[EZRider76]

Oh, if I am reading the illustration correctly in the vm manual, there is a small .25mm rib that is in the gasket sealing area, and that is here it shows to measure too. Either way, .25 is nothing to being 1.4 over spec.

 

[Awk08]

1.5 turns is just the starting point
under 1 turn for high idle = smaller pilot
over 2 turns out = larger pilot

2-3/4 turns out suggests a leaner pilot

 

[EZRider76]

1.5 turns is just the starting point
under 1 turn for high idle = smaller pilot
over 2 turns out = larger pilot

2-3/4 turns out suggests a leaner pilot

So, after reading this and thinking about it on my way to work, I decided to check out the VM Manual again. You're thinking that I'm running to rich of a pilot and that might be affecting my WOT? I see how that could be, but why would it make my 1/4-3/4 throttle lean then? That would point at the needle or the needle jet then.

Hmmm...I'm glad I'm going away for the weekend, I need a freaking break from this carb! X(

 

[EZRider76]

1.5 turns is just the starting point
under 1 turn for high idle = smaller pilot
over 2 turns out = larger pilot

2-3/4 turns out suggests a leaner pilot

Ok, good weekend off and now I'm ready to dig back into this pig. So, the green above, is that saying it is suggesting I RUN a leaner pilot or I'm RUNNING a pilot that is currently too lean? Because if I have to go to a larger pilot, that would me a richer pilot, would it not?

 

[mjgnslngr]

Ok, good weekend off and now I'm ready to dig back into this pig. So, the green above, is that saying it is suggesting I RUN a leaner pilot or I'm RUNNING a pilot that is currently too lean? Because if I have to go to a larger pilot, that would me a richer pilot, would it not?

you got it! one size leaner pilot and see where that puts ya!!!

 

[EZRider76]

you got it! one size leaner pilot and see where that puts ya!!!

So, for pilot #'s, does the larger the number mean the leaner the pilot?

 

[mjgnslngr]

no, its the same as mains, smaller the number, leaner the jet. imo, i think 2 3/4 turns is right on the edge of needing to jet the pilot down. is it idling ok? any bog Right off idle?

 

[EZRider76]

no, its the same as mains, smaller the number, leaner the jet.

but I'm 2-3/4 turns out on my air screw, which could suggest a larger pilot depending on how tight you want to control the pilot setting specs. So I would want to go to the next size pilot, meaning a richer pilot.

 

[mjgnslngr]

but I'm 2-3/4 turns out on my air screw, which could suggest a larger pilot depending on how tight you want to control the pilot setting specs. So I would want to go to the next size pilot, meaning a richer pilot.

i had a 35 pilot on my stocker and it was boggy just off idle. i switched back to the 32.5 to lean it and found thats what the carb wanted, no bog. when the air screw is turned counter clockwise, your allowing more air in to even up the fuel that the pilot is producing. if it was 1 turn out, id say a richer jet would be needed.
this is from your sig...
THE PILOT/SLOW CIRCUIT The pilot/slow circuit has two major components that affect the mixture; they are the pilot/slow jet and the air mixture screw. The pilot/slow jet has a fixed diameter hole that controls the amount of fuel being delivered threw the slow circuit. The mixture screw is an adjustment screw to control the amount of air being mixed with the fuel from the slow jet.

 

[EZRider76]

i had a 35 pilot on my stocker and it was boggy just off idle. i switched back to the 32.5 to lean it and found thats what the carb wanted, no bog. when the air screw is turned counter clockwise, your allowing more air in to even up the fuel that the pilot is producing. if it was 1 turn out, id say a richer jet would be needed.
this is from your sig...
THE PILOT/SLOW CIRCUIT The pilot/slow circuit has two major components that affect the mixture; they are the pilot/slow jet and the air mixture screw. The pilot/slow jet has a fixed diameter hole that controls the amount of fuel being delivered threw the slow circuit. The mixture screw is an adjustment screw to control the amount of air being mixed with the fuel from the slow jet.

Yeah, I get that, and trust me, I've been reading that damn VM Manual over and over. Something just isn't clicking.

 

[mjgnslngr]

bigger pilot jet (larger number) more fuel. air screw turned out counter clockwise means more air let in to compensate for the amount of fuel. when you unscrew the air screw, youre letting in Air to mix with the fuel. more air means its too much fuel.

 

[EZRider76]

bigger pilot jet (larger number) more fuel. air screw turned out counter clockwise means more air let in to compensate for the amount of fuel. when you unscrew the air screw, youre letting in Air to mix with the fuel. more air means its too much fuel.

Yup, I get it...I have gotten it....