Idle Mixture Theory, Adjustment Screws, and Bypass Holes

Warning! Hornets nest about to be kicked... 8O

I claim that if the carbs are working properly, the setting of the idle mixture screw should have virtually no impact on the air-fuel ratios at any throttle position other than almost dead closed idle.

If you look very closely into the butterfly end of the carbs, you'll see that there is more than one tiny hole in the roof of the carbs. My Mikunis have a total of four and here's what they look like with the butterfly held open.



The literature refers to hole closest to the engine as the "pilot outlet" and refers to the other three collectively as the "bypass" hole. Note that all four of those holes connect internally to the pilot circuit, but the pilot outlet is the only one metered by the idle mixture screw. The bypass hole(s) connect directly to the pilot air and fuel jets with no metering by the pilot screw.



Why is this so important? Because when the butterfly is closed (like it should be at idle) the only hole exposed to intake vacuum is the pilot outlet hole. The bypass holes don't do anything because they are blocked off by the butterfly and do not see any vacuum.



Hence, at idle with the butterflies closed, the pilot screw can be used to control the fuel ratio. However, as you open the butterfly and transition away from idle, you progressively expose the bypass holes:



First one, then two, and eventually all three of the bypass holes are exposed and supplying fuel. And since the pilot outlet is metered by the mixture screw while these three holes are NOT, the pilot hole will get virtually no fuel. Once the butterfly is cracked open enough to expose the bypass holes to the same vacuum as the pilot outlet, there's no differential pressure to pull fuel past the restriction presented by the pilot screw. Remember... they're all connected to the exact same place upstream.

And since I've already kicked the nest... I also claim that the bypass holes, not the pilot outlet, are responsible for supplying off idle fuel transitioning to the main jet. And since they are not metered by the pilot screw, no adjustment is possible.

If you find that you need to use the idle mixture screws to get your bike to run right at any place other than idle, then you've got problems elsewhere. Clogged bypass holes... clogged emulsion tube air inlets... leaking throttle shaft seals... something else.

Also, if you find that you've got to have the butterflies open far enough to expose the bypass holes (edit: not screws) just to get your bike to idle, then again, you've got problems elsewhere.

OK... I'm ready... Let the stinging begin? :lol:

Burnarr

 

Wow.
Instant sticky.
Great pics.
I hope you are right, because it makes sense to me.

 

Hmm I love a good debate and I see your points but have a couple of questions:

1) If Idle is full closed butterflys how come the idle adjust screw exists at all? Seems to me it would not make any sense to have it in that case. I contend Idle is actually the butterflys open - very slightly but not fully closed - therefore the idle adjust is able to adjust the idle up and down by controlling that gap.

2) If the contribution controlled by the mixture screw is negligible at off idle speeds how come the color of the plugs change when you adjust the mixture screws showing a fuel mix change at those RPMs? While it may be minor I agree, I don't see how we can say negligible when it causes a demonstrable difference in the fuel mix.

Great pics by the way and well stated position. I hope to learn a lot from the discourse that I'm sure will follow....

 

As Rick stated in my thread (compression verification):

The pilot mixture is the supplemental fuel source to the main, off idle.

Without it - the bike will always run lean off idle, with too much in the pilot screw, you run rich. Hence Bill's need for an answer as to why the plugs change color.

I myself must have some really clogged passages, but none of which are CLOGGED fully.

 

I mostly agree with you, with the following points:

The pilot mixture screw controls 1 of the four orifices supplied by the pilot system. When the other 3 come on line they reduce the impact of the screw setting, but they do not eliminate it. Hence opening that screw will effect the mixture off idle, but very little as the throttle goes ahead and opens. I will agree though that once the rpms climb off idle it doesn't do much.

You do bring up a good point though: If the butterfly isn't darn near closed that pilot screw will have very little discernible effect on the mixture. This means that if one or more cylinders isn't pulling it's share of the load the butterflies will be so far open that colortuning is impossible. Also, will be the same problem if out of sync.

So, for those of you colortuning and not seeing the screw have any effect you need to make sure that you are at 1000RPM or less and all 4 are firing.

Oh, also if the plugs don't read right at high rpm and load twiddling the mixture screws is a waste of time. Better to look at the passages, jets and float levels.

 

Yeah, bad choice of words on my part... Sorry about that. I should have said almost dead closed using the idle speed knob to position the butterflies juuuussst the tiniest smidge off dead closed. Just enough contact that I could see a tiny bit of light getting around the butterflies in some areas, but not enough to expose any of the bypass holes.

I just put my carbs back on my bike and I set the big idle knob to no more than a half turn after it just contacted the linkage and my idle is rock solid at 1100 RPM. All four cylinders are firing and idle is smooth and strong.

I do wonder though... I wonder if I could crank my idle knob all the way off and have the bike still run. The butterflies aren't an absolute perfect seal in the carb bores and even with them aligned as well as possible, there's still some leakage around the edges in spots. I wonder if that leakage combined with what the engine can suck through the pilot air jet would be enough? :?:

If the bypass holes are clogged then the pilot screw adjustment would have an affect at any throttle position.

Also, with no load on the engine, running while sitting still in neutral, I bet you can reach 2000 or 3000 RPM and you're probably not into the throttle far enough to even expose the third bypass hole. In that case, yes, you're still running off the pilot circuit, including some measurable contribution from the pilot hole.

Because of engine loading considerations, RPM isn't a very good descriptive parameter for this discussion though. A better parameter is "throttle plate angle" or "throttle shaft rotation". Why? Because I don't know what RPM my bike will reach with five degrees of throttle shaft rotation under no load conditions, and I also don't know how close yours would be to mine. So the RPMs aren't what's important here. The throttle position beyond which the bypass holes are exposed is the important parameter. The "RPMs that throttle position will cause" is highly dependant on load conditions, engine quality, temperature, gas quality, air density, phase of the moon......... Does that make sense?

Under load while riding is a completely different situation though. If you come back to the garage after a 20 mile bike ride and pull your plugs, only to find that they are all white from running hot and lean... There's no amount of idle mixture screw adjustment that's going to fix that. You've got problems elsewhere. 8O

Burnarr

 

I just edited my original post to "almost" dead closed idle....

 

The reason I chose RPM is that these are constant velocity carburetors. The slide acts like a second throttle. You can see on the bench that with the butterfly wide open there is very little opening to the throat at 0 rpm.

So, the instant you snap the throttle wide open all the fuel still comes from the pilot circuit. The needle jet doesn't really come on until the engine starts pumping enough air to lift the slide.

The snapping open of the throttle tends to make the mixture lean. If the idle is already too lean this can cause the engine to stumble until the needle jet starts to come on. That's why the focus on the pilot mixture screw for stumble coming off idle.

 

I contend that if the vacuum seen by all of those holes is the same, then there will be NO flow out the pilot hole.

Let me put it another way... All four of those holes are connected together at the supply side. The only difference between them is where they are located and the fact that one of them has a restriction between it and it's supply.

If you put the same vacuum on the downstream side of all four of them, physics dictates that the one with the restriction will not be able to steal any flow from the ones that to not have a restriction. There's no force for the fuel to "climb the hill" past the metering restriction.

I guess the question is... "How far must you open the throttle before the innermost bypass hole sees the same vacuum as the pilot hole?"

Burnarr

 

AHH the plot thickens. This is a great discussion.

I agree about the WHITE lean but I know from experience that with a plug chop under load I can adjust my mix to dial it in. Granted it will be a minor contribution but not negligible as it does have an effect.

You can not tune out critically lean with the mix screws - we agree there.

 

Wow, I was going to write almost the exact same thing (but nowhere near as pleasantly) late last night, even typed it all out, and then thought to myself, "why bother"? Let those who choose to "believe" believe what they want, and what comforts them, or makes them feel proud; after all, who cares about science, facts, and reality....that's not meaningful in today's world anyway.

Doug, I'll point this out one last time and then shut up:

a) If you require 6 turns out on your idle screws, then there is something fundamentally wrong, somewhere else, in your carbs or engines or ???? that no amount of manipulation of the idle screws, nor plug chops, are going to detect nor fix.

b) you're going down the wrong path of inquiry and exploration in trying to fix your problem, and that path will make things worse.

The main fuel circuit supplies up to 12 times more fuel under high rpm, high engine load coditions than the pilot circuit provides or even could provide. That a basic fact of math, physics, and geometry. And the amount of air that either the pilot or the main fuel circuits "use" or contribute to the overall volume of air used by the engine (via the "air jets") is so negligible in relation to how much air the engine passes through the throats as to not even being worth counting.

How and where some people develop their theories is beyond me, but as Kurt Vonnegaut used to say, some people's motto seems to be "In Nonsense Is Strength".


To all others, I realize that perfection in the basics of blocking and tackling isn't real sexy, but it's actually how the really successful people operate and produce positive results.

 

the holes don't have to be on the engine side of the butterfly valve to see vacuum as the air comes to the opening it picks up speed and looses pressure
so they may be passing fuel before the valve gets to them
the hole closest to the valve would have the most effect on the mixture since it has the lowest pressure but just because the first hole has a restriction doesn't mean it passes no fuel , it just passes less

 

That's what I suspect. :lol:

My Mikunis are surgically clean inside.
I've got new seals and O-rings everywhere.
My float levels are right on spec.
My pilot screws are 2 1/2 turns out from fully seated.
And my idle speed knob is one RCH into contacting the #3 linkage.

My carbs work flawlessly. Go figure. :wink:

 

All four of those holes are in a smooth non-diametrically changing part of the carb bore.

I can guarantee you that with the throttle opening at 90 degrees they will all be seeing the exact same vacuum to within a level of accuracy that neither you or I possibly care about.

The only thing that can cause a pressure differential in that area of the carb is the butterfly plate. The slider venturi is well before this area.

 

I have new seals o rings etc everywhere as well, even silicon greased them.

The bodies got soaked in dip, but I don't think I much through the ports.

So that is probably my issue. That's why I think the seafoam in my gas started giving me slightly more positive results.

 

Hey I'll freely admit I don't KNOW the right answer - I have been asking this question for a while myself and trying to learn.

What I do know is I can effect the plug coloration under load so how come?

This is a great discussion, some ideas will be proven wrong some right and some folks will never be convinced one way or the other.

BUT none the less I think it's a discussion worth having.

 

agreed.

 

The Mikuni's are totally different from Hitachi's.

The Hitachi Carbs are Cast differently and Drilled differently.
The Mikuni's draw Pilot and Main AIR from External Ports on the Outside Ring of the Intake Venturi.
The Hitachi Carbs draw Pilot and Main AIR from Internally drilled Ports (Some under a limiting cover.)
Some of those holes you show on the Mikuni Carbs do not exist on the Hitachi's.

The discussion of Supplemental RICHNESS and the CONSTANT Supply of Fuel through the Pilot Mixture Screw Ports is limited to Hitachi Carbs.

The Supplemental Richness of the Pilot Mixture Settings determines the GROSS Air~Fuel Mixture and whether or not you are Rich or Lean.

I cannot say from looking at the photographs if those Mikuni Carbs are the so-called "Racing Carbs" or the ones that were used on "California" Bikes which have an additional vacuum and intake Ports to allow auxiliary Exhaust Recirculation to reduce emissions.

 

Haha!! No... Those pics are from your run-of-the mill stock Pennsylvania grade Mikunis. No Cali racing carbs in my stable. :lol: If you're questioning the existence of the bypass hole(s), then just take a look at the 550 service manual. The holes are clearly shown and labeled on the cutaway in the carb section.

As for the Hitachi's being so totally different, I disagree. The basic castings of the Hitachi's are clearly different, but the operational concepts look absolutely identical to me.

That's not quite right...

The Mikunis do NOT draw pilot air from the venturi ring. The Mikuni pilot air jet is up under the slider piston diaphragm just like the Hitachi's. In fact, the only difference between the two is that the Mikuni's have the main air jet out there on the venturi ring while the Hitachi's put both the main air jet and the pilot air jet up under the slider diaphragm. That side of the diaphragm is at atmospheric, so there shouldn't be any functional difference though.

I don't have a set of Hitachi's here for good photos of the are in question, but again, I believe they operate exactly the same. From the pic heavy Hitachi thread (http://www.xjbikes.com/Forums/viewtopic/t=3649.html) here's a cutaway diagram of the Hitachi's. Note the location of the Hitachi's bypass hole on the diagram and also note that the bypass hole is upstream of the pilot screw just like the Mikunis:



From the same thread, here's a shot down the outlet of the Hitachi's:



Hard to tell with all the dirt specs in there and the butterflies closed, but I think I can see the pilot outlet and just a tiny sliver of a bypass hole that isn't quite completely covered by the almost completely closed butterfly.

Looks functionally identical to me... You got a better pic of the pilot area on a clean Hitachi so we can tell for sure?

Burnarr

 

That's easy... Because all carbs are open loop compromises.

The human race is yet to create a carb that can supply the exact perfect correct mixture for all varying operating conditions. You can tune great for one RPM and one set of loading conditions, but you cannot tune perfect across the entire range from 1K to 10K RPM with conditions from no load idle to big highway hill with two heavy riders sitting on the saddle. And that's not even considering all the variable ambient conditions like temp, humidity and 02 density.

So you do the best you can for what you expect to see most of the time. If you want better, use a computer controlled closed loop fuel injection system. We've got no oxygen sensor, no incoming air temp sensor, no manifold pressure sensor, no finely controlled pulse width modulated fuel supply, and no computer to close the loop and put it all together.

It's an open loop compromise. One that works surprisingly well, but not perfect!!