Mikuni Needle Valve (Fuel Inlet) Sizes

[Ernie Phillips]

My CMF Berkshire with VM28 Mikuni appeared to "run out of fuel" at Diamond Don's after extended wide open throttle. I never had a problem riding around the house or even racing this bike in CC.  But at DD in all the mud, I really flogged the thing.  Between motos, I cleaned the carb, even removing the needle valve thinking I had some trash in it causing the problem.  Moto 2 still had problem.

Last night while searching the internet for 28mm Mikuni, I came across one for sale that appeared to be identical to what I had.  Interestingly, the seller mentioned that the needle valve was a 1.5 (for systems with fuel pump). Hmmm, I wonder ...  I pulled my carb apart and sure enough 1.5!  According to Mikuni rep, gravity feed should be 2.5.  (Mikuni # system is in mm, so 1.5 means 1.5mm hole in needle valve.)   So, when swapping parts, especially grab-bag specials, be sure and check the details.  And, don't just ride around the house, flog it.  A quick run thru the gears will not indicate fuel flow problems  (Man, I love my Bings [8D])  


Ernie P.
Chattanooga, TN

[OUCWBOY]

Ernie,
I prefer a Bing over the Mikuni. Always have. I have a 32mm Bing on a 125 and it is great.
I just read the Factory recommended hop up for the F21M Kawasaki 238cc Green Streak and they (Kawasaki) recommends using a 32mm Amal. Go figure??

Donny Smith
Paragould, AR

[Lew Mayer]

A 32!! on a 125? Any bottom end there Donny? I'll bet it rolls on top after ya get her there. Sounds a tad peaky though. You are Da Man.[8D]

Lew Mayer

[brian kirby]

Ernie, that is exactly what the bike was doing when I first rode it last fall as a 125. It would run great for about a lap then would starve for fuel. I thought it was debris in the carb, but it seems it was the needle valve size.

Brian

[Ernie Phillips]

Yes, too small needle valve seems to be the problem, but something we still have to track down is that Christopher's 125 has a VM28 that I got from the same guy --- same 1.5 valve?  We don't seem to have any problems on this bike but it is mostly CC, where you are on/off throttle more.  But wait, I do remember some problems last year at Barber where he tried to melt the engine ... dang! We still have more work to do.

Lew, Glad you are feeling good enough (or is it the drugs) to participate.

Ernie P.
Chattanooga, TN

[brian kirby]

Actually, it felt like Christopher's bike was doing the same thing when I rode it a few laps in practice at Barber but at the time I wrote it off as a bad Motoplat.

Brian

[Merlin]

..........if your timing was too far advanced you would/should have had a crater in the dome, most likely you have a fuel delivery problem from the tank to the carb allowing the bowl to dry up, a petcock or tank vent problem??........

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Edited by - Merlin on 10/28/2007 1:42:50 PM

 Hmmm??

Quote: Thomas Jefferson, We are all born ignorant, some work to remain that way.
 Quote:Merlin, "it is impossible to teach those that wish not to be taught".

[Ernie Phillips]

Merlin,  You are correct - again!  Question, would drilling out the existing 1.5mm needle valve to 2.5mm be acceptable?  I ordered a 2.5 but would like to have the Baby Berkie ready as a backup for this weekends event.

Ernie P.
Chattanooga, TN

[Merlin]

Ernie, if you can find the proper reamer to finish the hole in a lathe as opposed to leaving it "drilled", (you may want to consider opening the hole up to 3.0/3.5 mm) you should be in business.

Quote: Thomas Jefferson, We are all born ignorant, some work to remain that way.
 Quote:Merlin, "it is impossible to teach those that wish not to be taught".

[Ernie Phillips]

Okay, thanks.  I was worried about leaving a "ragged" hole just by drilling.  I'll try and locate some type of reamer to do the job correctly.

Ernie P.
Chattanooga, TN