Hooked up my 21mm Mikuni set-up on the Polini 4 petal and working on tuning it.
My question is where is everyone buying their needles? Are dellorto needles compatible? Tried all of the settings and nothing is working out.
Also, the needle has 614CM2 or EJ4CM2 stamped on it. Hard to tell which it is. Is this a genuine Mikuni needle? I
Could anyone take a pic of a Dellorto needle next to a ruler? Or tell me how far until the tapers start from the top. Might try and use one and shim if necessary.
http://www.mopedarmy.com/wiki/How_To_Tune_PHBG_Carburetors
there's a chart with measurement on that page if you scroll down. Also, there are many charts of mikuni needles in Two-Stroke Performance Tuning by Bell, pages 98-108ish.
What part of your throttle range are you trying to affect by changing the needle or needle position?
Thanks. I'll check those out. With the needle set to the richest setting it's still too lean. Thought it was strange, but it ran best on that setting only getting worse if I leaned it out by moving the needle lower (clip higher).
Guess I need to find a less cut slide for richer mixture as well.
John, the diagram helped, but the Dellorto chart doesn't list the measurement from the top of the needle to the where the taper starts. The Mikuni chart in Bell's book does. Any idea if the length is close enough to use Dellorto needles in the Mikuni?
You still haven't answered the question of what part of your throttle range you're trying to tune by messing with the needle.
Why would you want to use a dellorto needle in your mikuni? Your local motorcycle shop should have mikuni VM carb needles in stock.
If you're running lean at WOT, the answer is not changing your needle, it's changing your main jet. Main jet is the only carb part affecting mixture at WOT.
i mean, these are 4 stroke carbs, and have 4 stoke needles, the needles dont allow for enough mixture to keep the engine cool at higher RPMs
so you got your main jetting perfect and only your partial throttle is fucking up?
from about 75%-100% throttle its almost entirely on your main jet, with your needle only being like a bit of the equation. at wide open throttle you almost rely entirely on the mainjet.. i'd go up a jet size.
That's a big no on 4-stroke carbs. The Mikuni VM is a great carb and perfect for two stroke use. Works on four stroke too. Carbs made specifically for 4 stroke use will have things like...sayy...an accelerator pump...which makes em not work on a 2 stroke so much.
Can't keep it running pass 1/16-1/8 throttle. If it somehow does than it hesitates a lot around 1/8th before it picks up to 15mph until it starts bogging again.
Already bought 30 main jets and 3 pilots 15, 17.5 and 20 trying every possible combination. I'm pretty confident the needle is way too thick to allow enough gas to pass by.
Maybe I missed something, but did you say it was running OK at WOT and does it idle fine?
Mikkis basically have three ranges of tuning, most people never have to change slid cut-aways and almost never needles except for extremely fine tuning for elevations or a particular track condition.
The LS jet regulates idle to about 1/8. It should have some adjustment if jetted correctly with the air screw about 3/4 to 1 1/2 turns out (if it's the type that monitors fuel and not air) This is sometimes one of the differences between so-called 2 & 4 stroke carbs, aside from an accelerator pump.
From about 1/4 to 3/4 it' the needle clip position (and possibly the taper). Basically all of the needles have the same largest diameter for a given model and it's all about the taper diameter/length, or even multi-slope length. Note: the cut-away of the slide can also effect the 1/8 to 1/4 if it's really off, which doesn't really happen too much.
From 3/4 to WOT it all about the MJ, nothing more.
Check out my Spec Sheet when I was running a 70cc w/4-reed 20mm VM. I think I had about a 160MJ and 20LS, std needle & slide in the #4 position(?) That should get you in the ballpark. If not, it may be starving.
What I would recommend is to start by pulling the fuel line off, and if the flow is in question, pull any fuel filter(s) out and ensure that you're getting a full flow to the carb. Many of the older bikes have restricted fuel flow from sediment/aging/swollen petcock seals, or weren't designed to flow enough fuel for a larger carb. This happened on my Extreme Urban. The petcock sealing disk had nearly closed up. I had to drill the rivets out and thoroughly clean it, reshape the sealing/directional disk and tap it for some 4-40 screws to re-assemble it. After that, it was flowing fine and helped to resolve some early jetting problems.
It dies before I can even open it to 1/2 throttle. Idle is perfect and can stay running at idle all day long. The main jet I installed is only a 135. Guess I'll go bigger, but don't see how it will affect fuel flow from 1/16th to 1/2 throttle.
Also, fuel flow to the carb is not an issue. Checked all the basics. While tuning it ran better and better as I raised the needle (richening the mixture). At that setting I was able to pull off the line, but not brake 15mph. Therefore, I can only conclude that my problem lies with the needle. I'll recheck some things and give it another shot with a bigger MJ. Understand that Mikuni's are usually good as is, but maybe in my case I need to switch the slide and/or needle. I'll keep you posted.
yo dudersnakez, what kind of air filter do you have on there? that changes mikunis by like a jet # of 40.
A K&N knock off. Do you think I should be up in the 160's or higher. I bought jets 115 - 140.
It's hard to say, what model mikuni do you have anyway? What days are you free, i could cruise down and take a look
Thanks, but a friend is flying in for the weekend that I haven't seen in years. Gonna order some more jets, an extra pilot, main jets, a needle or two, and a couple slides. If I can't get it going within the next two weeks I'll greatly appreciate some help. Keep you posted.
I have the Sudco / Mikki catalog and was looking into needles and slides and they are pricey. The slides were about $30 and needles were around $20. I get my jets local for $5. Just saying, it's not like dialing in the carb is cheap so be sure that there's nothing else wrong with it.
Do you have any way to hang it on another bike to see how it works? I still can't see why it doesn't work better if you're in the ball park, 140 MJ and a 20 LS should be pretty close.
Thanks. Might try it on another bike. I'll keep you posted.
Want to post in this forum? We'd love to have you join the discussion, but first: