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Discussion in 'Airheads' started by Uncle Pollo, Oct 2, 2012.
bought coolant hoses for my carbs. One trip to the big auto store and I was set.
Keep an eye on that coolant hose. It might not stand up to ethanol.
Got the cables in today. Jets, slides, and needles are as mentioned in previous post.
First I had the needles dropped all the way. This would not allow the bike to rev past the pilot jet range at all. I didn't however play with the mix screw in this position.
I moved the needle down a clip- one above middle- and I could rev, but only smoothly when the mix screw was all the way in.
At full throttle the bike isn't happy but it scoots pretty well in the 1/3 -> 2/3 range.
It's very easy to kill with full throttle off idle. I suppose I have a lifetime of CV carb riding to unlearn.
The old rule of thumb is one groove on the needle equals three main jet sizes. And the mix screw on our Mikunis is opposite the Bings as it meters fuel and not air, the air is metered by a separate jet at the mouth of the carb.
Screw all the way in suggests the main is bleeding fuel into the carb even at idle, maybe from the needle height adjustment as well as the large main. You could try backing the idle screw out and just getting the idle circuit tuned. That would at least give you a baseline to start adjusting the mains and needle height. And, of course the variable needle sizes and the slide cut away will affect performance too, but, you can get very close for comfortable riding and then tweak for performance.
Who made that rule? Needle position and main jets aren't adjusting the same thing at all. Your needle regulates mix from idle all the way up to WFO, Whereas your main jet just limits the amount of gas at WFO and just below.
Moving the clip changes the fuel mix by a factor of 5-8%. So maybe in Mikuni terms that's a skosh over two sizes, rules of thumb aren't truly precise and VM jets aren't available in 2.5 increments, only in 5s. Still every move he makes is 8% of 190 at the baseline. 8% of 190 is slightly over 15 and that's three jet sizes. Or it could be 5% and thats 9.5 so it's two jet sizes. Damn rule of thumb variance.
It is true that the main jet determines max fuel flow at WOT but it also determines the baseline that the needle begins with, nothing can change that fact. Then the taper and clip height come into play.
Bottom line, all of the circuits in a Mikuni interact except the "enrichner" which is a totally independent circuit.
Ref: chart 13 page 9 Mikuni tuning manual
+1 I have head the 'one notch equals two needle jet sizes' before. Which is complete BS too but at least I can tell where they are coming from.
The bottom line is that unless you have a too lean main jet. Jet needle adjustment will not effect the main. Sure jets overlap and different combos of jets should be tried if best performance is your goal but . . . .
If it fails I will know rather quickly.
Can Mikuni jetting numbers help me tune a Keihin? If the jets are measured in the same unit, it ought to at least get me in the ballpark right?
I think they use different measurements; one company by diameter and one company by flow. What kind of Keihin? I've had a few CV types and those tune the same no matter what they're on.
Jet needle height adjustment affects the flow, main jet restricts the amount. Bigger mains allow more fuel into the needle jet, smaller mains allow less. Higher needles allow more fuel flow, lower needles allow less. A low needle and a big main or a high needle and a small main work pretty much the same until you reach WOT. Such an easy concept, even a super tuner such as yourself should recognize this.
These are flat-slides. Similar carb to the Mikuni TM that Jenna has. She had more bling than me and it was making me feel inferior Call it 'keeping up with the Jennas" After I get the carbs sorted out I'll start feeling bad about how she has bigger forks than me.
I can recognize how someone might think that until they tried it.
Then you should document your experiences to the contrary and enlighten us.
Would a picture of me holding a jet above a work bench with a micrometer and a shiny new Snap-On 8mm wrench in the background help? Maybe a clean BMW tool kit rag spread out underneath it all? Then it would be gospel! On the idiotnet at least.
I love not knowing anything. I never get in arguments
The smarter you get the dumber you are.
No. Your noted experiences with the 26 variants of the #159 needle jet would work. The documentation from the Mikunis on your personal airhead would work.
Examples of successful installation of Mikunis on customers bikes would work.
Back on track. I'm going to do a good hour test ride today up some mountains and get more precise answers on what clip position and mix screw adjustment do to rideability.
I am hooked on the instant starting I experience now. Feels like I don't even get a full spin of the engine before it fires to life.
Okay. Good forty mile ride up and down some mountainy roads and here are my thoughts:
The whole damn thing is too rich. I have no snap or feeling of real torque. Sure it's smooth, starting is great, and it's certainly adequate for commuting, but I will need to go down on both main and pilot jets.
I'll have to research and see if the slide/needle/needle jet is correct, too.