Re: Idle mixture screw explanation

[theLeopard]

originally I was of the impression that the idle mixture screws on our bs36 carbs were essentially an air bleed, so turning it out meant leaner running, but after a few years of this not seeming to give the desired effect, I eventually read a cv carb tuning guide, at which point I realised that our mixture screws were most likely actually fuel/air mixture screws, meaning what is being feed into the airstream from the mixture screws, is itself a mixture of fuel and air, not just air, which frankly made sense of the effects I was getting when turning the screws.

a lot of blokes fit larger pilot jets and the reasons for this are well established, and I intend on doing the same in the near future.

but, and my query, in the absence of the larger pilot jets being fitted, how far do people go with turning out the mixture screws, and can turning the screws out 5 or more turns richen the afr enough to mask the fact that your pilot jets are too small, even temporarily?

a bloke told me once that after 2 1/2 turns your out of adjustment, but this bloke has told me a few other things that I've found to be a bit dodgy, so I figured id throw it out there, and see what comes back.

Well, technically it is not an "idle mixture screw"...

There are three components used to meter the idle circuit air fuel mixture and then control distribution of the mixture.

1) Fuel Pilot Jet
2) Air (bleed) Pilot Jet
3) Pilot Screw

This is how it works...

The air bleed & fuel jets work together for the metering of the mixture of air & fuel before reaching the pilot screw.

The combination of 155 air bleed jet and a 37.5 fuel pilot jet, which is 1200 stock jetting in the US, creates a specific mixture. So, when someone bumps up the fuel pilot jet to a 40, and still uses the 155 air bleed jet the mixture of air & fuel before the idle screw is now richer. Just the opposite happens if you use a 160 air bleed jet and a 37.5 fuel pilot jet, the mixture before the pilot screw is now leaner.

The change of the pilot fuel jet adds a richer fuel mixture before the pilot screw and the bike generally runs better because it is no longer jetted lean for emissions standards.

The one other thing that is never discussed, because there are very few who actually know & understand the function of the carb pilot circuit. There is a series of three small holes right at the throttle plate, those are called "bypass holes" per Mikuni. What they really are boils down to being the "fixed" idle circuit. Those holes draw the air/fuel mixture at idle and since there is no adjustment, the air bleed & fuel jets must be paired for the best performance and then this is where the screw come into the picture.

Now, the pilot screw...

The common statement thrown around forums is 2.5 turns from seat, but is that really the best for everyone all over the world; no.

The pilot screw allows the enrichment of the carb air/fuel (A/F) mixture volume into the intake port for idle and just off throttle air flow due to increase air speed of the carb throat, kind of like an accelerator pump. By closing the pilot screw you are reducing the amount of fuel being allowed in and by opening more of the A/F is allowed. So, to a point, the more you open the pilot screw the more of the A/F mixture is allowed in, but if the mixture is too lean, because the fuel pilot jet is small or the air bleed jet is big, it is still lean.

Pilot screw; optimum setting & turns...

Obviously the only true way to dial in the pilot screw setting is on a dyno with a four gas meter, but you can get it pretty close in your garage with the blip test.

I can also tell you after rebuilding more than a 700+ carb banks in the last 20 years, not all pilot screw holes are creates equally. How can you tell if yours are, remove your carbs and gently seat all of the pilot screws into the carb throat. Now measure then distance each one is sticking out of the carb and you will see what I am talking about. I have differ as much as .080". What this means is if you start from a seated position and just back out a "number" of turns the amount of pilot screw needle tip controlling flow is different for each carb.

So, I saw Pat post this a while back and this is how I set up all of my carbs and how you should establish your base line as well. I run all pilot screw needle tips into the carb throat slightly, no need to seat them. I then put my fingernail on the needle and begin to back out the screw. When my finger nail falls off the needle it is now flush with the hole of the carb and all four pilot screws are set equally.

From there, depending on the application, riding style atmospheric and altitude I make my adjustments to the pilot screw to control the distribution of the A/F mixture of the air bleed & fuel jets. Most bike applications end up being no more than one turn out from my base line settings and some require the screw to be tighten from the base line due to atmosphere & altitude.

Maximum number of turns before the mixture no longer changes...between 5 & 6 depending on the carb. Fifteen years ago when I was doing dyno work that is the number of turns out I found on the tail pipe probe when the tail pipe reading would not longer change.

So, don't be afraid to turn them out, but if you are way out at four or more, that is a sign the fuel pilot jet is too small or the air bleed is too big...

This differs from automotive carbs which have an "idle mixture screw" which only regulates the "amount of fuel only" distributed for mixture with the air stream of the carb.

Randy - RPM

Adjustments to the pilots screw for atmospheric pressure,
Wouldnt this necessarily require a larger air bleed screw?
& if so, why does the pilot screw...  

[theLeopard]

Ok i got my coffee & been thinking about this so please correct me if im wrong but the pilot screw requires adjustment at atmosphere because wait no im still stumped the volume being pushed through the fuel jet should theoretically remain constant even in outer space.

[the fan]

The fuel is not being pumped, its being pulled.

[theLeopard]

Irrelevant!
Thank you for correcting my error 

[the fan]

knowing how something works is a good start toward knowing why it works.