Trying to resolve pilot jet effect

[KEITH KRAUSE]

A "senior moment"? I have those too!

[wiredgeorge]

Keith, There are VERY VERY few bikes that the jet needle loses some effect at wide open throttle. MOST bikes are adjusted (part of the tuning process) to NOT ALLOW the jet needle to completely come out of the needle jet. This is a safety issue. If you have ever seen the effect of when a jet needle comes out of the needle jet, it can then move laterally a tad and not allow the slides to close. That is why there is a screw stop on Mikuni VM series carbs for limiting the amount the slide can raise to allow the jet needle to stay in the needle jet just a fraction... the way it is set is to open the slide manually and adjust the stop bolt so that the slide stops even with the top of the venturi. It is an important adjustment for safety sake.

Thus, the jet needle is still having a slight effect at wide open throttle. In addition, the needle jet on most bikes will be larger (the orifice) than the main jet. The MAIN JET is the limiter as far as gas flow at wide open throttle.

Most carbs have three circuits... pilot, mid-range (jet needle/needle jet) and main jet. ALL flow air/fuel all the time... the main jet and jet needle share an air jet located on the intake venturi edge. The pilot jet does as well. In addition, the slide cut out or gap between the slide and bottom of the venturi allows air through to add to the air fed by the pilot air jet, also on the venturi edge adjacent to the main air jet. Some carbs (CV type) put air jets in the slide diaphram top as well...

I generally agree that you should probably tune the main jet FIRST if you are tuning but you have to get the bike idling and away first so that usually necessitates a bit of tinkering with the pilot system PRIOR to main jet tuning. The pilot jet is a critical tuning component and should be based not only on the fixed diameter pilot air jet but the slide cut out... bigger air jet and/or bigger slide cut out mean you really need a bigger pilot jet. The earliest GS750 VM26s were a dandy example! Look at the pilot air jet and slide cut out and you will soon figure out why those carb assemblies came from the factory with #25 pilot jets. Those buggers are nasty to tune for idle. later, Suzi spec'd smaller air jets and cut outs and subsequently over a few years ended up with #15 pilot jets which are far easier to tune. Try sticking #15s in the early carbs and it is so lean it won't start, much less idle.

The pilot circuit affects not only idle, but off idle transition. If the bike is too lean in the pilot circuit, you might be able to rev the engine with the bike on its centerstand but when you try and take off, the bike will just fall on its face... Tune the pilot circuit a bit first... get the bike going and get the right main jets and jet needle clip position, then bring it back in and sort out the idle would be the ideal, I think... Anyway, this is one of the more interesting discussions...

[KEITH KRAUSE]

With all respect George, I know how the carbs work.
But I don't understand how anyone can confuse such a simple thing as the fuel flow going through a main jet and then through a jet needle/needle jet space.
The smallest hole in the main circuit will COMPLETELY regulate fuel flow at full throttle. Once the jet needle raises to approx' 3/4 throttle position, the space between the needle and needle jet simply becomes larger than the hole of the main jet. The main will now control flow regardless of how much higher the needle raises. I don't see where that's hard to understand.
I DO agree that the pilot circuit will add a little mixture to what the main circuit is supplying to the cylinder. But it's feeding the carb through a separate passage. At full throttle, the vacuum isn't acting on the pilot circuit as it would with closed/nearly closed throttle plates, but I'm sure there's still fuel some entering the carb throat. There's SOME vacuum present at the pilot circuit.
The jet needle and main jet are "in-line". And as I said, at some point, the jet needle has to give way to the main jet. Completely. That's the whole purpose of the design. That's why there are needles and mains, and not just needles. A tapered needle rising out of a space will create a larger and larger hole until that hole becomes larger than the main jet. Smallest hole regulates.
If anyone can show me how a jet needle will effect the main as in the above condition, let me/us know.
But if this is some silly miniscule amount and is just for the sake of some scientific argument, please don't make jetting any harder than it can be. In the real world, it wouldn't matter.