Main jet and afr?

[Jeffd]

To the dyno operators out there.  What effect does going down one size on main jet have on afr at WOT?  ie if at WOT with a 190 you were at 13.2 what would you be at with 185 at WOT.  I know there are other factors but ball park it.  seems I used to read that 12.8 to 13.2 was the harley sweet spot at WOT but now it seems like the afr sweet spot is getting higher. thanks jeff 

[Jeffd]

Nobody tunes carbs anymore? LOL.  It seems like if main only effects WOT then tuners would have a pretty good base for knowing that if a 190 main was good for 13.2 afr a 185 would be around 13.4 afr, 195 around 13 etc. (these are hypotheticals and random numbers).

[ederdelyi]

It's tough to quantify, just too many variables that can affect how main jet size relates to AFR. If you get down to the nitty gritty, you may find that the granularity of the std jet sizes available won't get you where you want to go. I still have my jet drills for those "just a little more should do it" situations. I used to dink around with figuring how much fuel could be passed through a given jet size ... too old and lazy to play that game anymore :>)

[Sonny S.]

Nobody tunes carbs anymore? LOL.  It seems like if main only effects WOT then tuners would have a pretty good base for knowing that if a 190 main was good for 13.2 afr a 185 would be around 13.4 afr, 195 around 13 etc. (these are hypotheticals and random numbers).

Yes,  but you addresed your question to dyno operators so I kept quiet 
you know I'm teasin ya  .... this was my edit

The 185 should put you in the 13.5+/- range.....and that is fine unless you plan on holding WFO for long times in hot temps.
However, changing your main will also change your cruise AFR...yup it will.
I know that you are already running a lean needle so unless you have an AFR meter or know what your cruise AFR is, you will have to check plugs to make sure cruise is good.

[ederdelyi]

>>However, changing your main will also change your cruise AFR...<<

As will changing the pilot jet or the low speed adjustment. The three basic circuits interact a lot more than most realize. Fine tuning a carb takes a lot more than just changing jets and needles ... I used to get big bucks for reworking Holleys and other carbs for street/strip applications. Lucky for the HD world that the carbs used don't use things like power valves and auxiliary venturis ... I sure like dialing in the AFR with a keyboard versus a drill :>)

[fuzznut5197]

This is all interesting. The CV's main jet interaction on cruise, is that due to the emulsion tube? You have fixed air bleeds, and put in a bigger jet, now your air and fuel "mix" being metered by the needle will be richer?

[Sonny S.]

>>>The three basic circuits interact a lot more than most realize.<<<

[Sonny S.]

This is all interesting. The CV's main jet interaction on cruise, is that due to the emulsion tube? You have fixed air bleeds, and put in a bigger jet, now your air and fuel "mix" being metered by the needle will be richer?

Yes
The needle doesn't limit the amount of fuel that comes out of the emulsion tube by restricing fuel. It limits the amout of air, thus limiting fuel. A bigger main jet will allow more fuel regardless of if the needle is 1/4 out of the tube or all the way out.

[fuzznut5197]

Ahhh, I see.  So in my case, a Mik with no emulsion tube and metering pure fuel, there should be no main jet interaction?

[Sonny S.]

My Mik. knowledge is limited but I believe you are correct. That is why tuning a Mik. is different than a CV. The CV you do the main first, Mik. main last I believe. The Mik. needle & tube is truly a "  jet needle & needle jet ".

[FLTRI]

How's this?
Since the needle never is completely out of the main jet/emulsion tube's influence, the bigger the main jet the more fuel during all throttle positions, except idle.
So when you need more WOT fuel you will also be enrichening the part throttle running.
As Ed mentioned even the pilot jet and idle mixture screw affects overall AFR, albeit not as much as the main jet or needle.
Bob

[ederdelyi]

My take on this:

The emulsion tube, the purpose of which is to introduce air into the mixture through holes to help atomize the fuel and correct excessive richness at higher engine speeds. Also can be a  perforated tube which extends from an air bleed in the top of the air horn down into the main well. In this case it admits air from the air bleed into the main well to emulsify the fuel in the main well. Improves idle response and stability when the engine is hot and prevents fuel percolation and general hot-starting problems. Also improves response in the main metering circuit during part throttle conditions. Also somtimes called called Main-well tube.

The Kehin CV and the Mik both will interact with cruise (needle) and WOT (main) as both use  a scheme to alleviate the symptom that all carbs naturally have to go rich at WOT. THe CV uses the Emulsion tube, the Mik uses the air bleed/well concept. This is where tuning the emusion tube or air bleed comes into play if the motor is going lean or rich under WOT high RPM conditions but the AFR is close to where you want it at WOT lower RPM.

The main jet sets the max amount of fuel that the carb can draw through the main circuit when the needle is completely out of the picture, at lower throttle positions it controls the amount of fuel passed by the main jet by altering the metering signal to the main jet. FWIW, I set main jet and WOT AFR first and then concentrate on cruise/part throttle and idle circuits for both the CV and the Miks. And then there are my least favorite, the  SnS carbs ... but I won't go there :>)

[Sonny S.]

>>>How's this?
Since the needle never is completely out of the main jet/emulsion tube's influence, the bigger the main jet the more fuel during all throttle positions, except idle.
So when you need more WOT fuel you will also be enrichening the part throttle running.<<<

you are correct.
Am I contradicting myself somewhere ?  I get confused easy, and don't always explain things well.
Good news is I think I often understand what I think I'm trying to say.   

[ederdelyi]

>>Am I contradicting myself somewhere ?<<

Not that I can see.

>>Good news is I think I often understand what I think I'm trying to say. <<

See, you're ahead of some of us! Sometimes I don't know what I don't know, and I'm not smart enough to know it :>)

[Sonny S.]

>>> Sometimes I don't know what I don't know, and I'm not smart enough to know it :>)<<<

I think I'm like that too.... but I just don't know   

[Sonny S.]

I just realized that the " How's this " in Bob's post wasn't really a question   ....... see what I mean by saying I get confused easy ?

[fuzznut5197]

Ed, are you sure about the Mikuni, specifically the HSRs? It has a needle jet, a main jet extender, and the main jet. The is nowhere for air to enter that stack, even though the HSR has an air bleed on the horn.

[ederdelyi]

Fuzz,
There actually is a main air bleed on the HSRs, it is usually blank in stock trim. it's there for "fine tuning" if needed. The needle is usually sufficient to control the signal well enough to maintain a fairly flat AFR. Main and needle still interact, changing the main will still have an effect on the off idle to 3/4 throttle positions and the overlap of the 3 circuits is substantial. Biggest mileage killer on the Miks is the accel nozzle size and pump adjustment.

[fuzznut5197]

Thanks Ed.
Yeah I changed the "40 gallons per minute" nozzle they put in stock. 

[ederdelyi]

To be more precise on the Mik main system, for these types of carbs there are two basic main sytems used.

"Primary main" is the type with a solid needle jet (tube) and a jet needle. No air is introduced into the fuel stream in the needle jet and all mixing takes place in the venturi. This is what the stock HSR 42/45/48 uses. The needle still controls the signal (and fuel flow) over it's normal range of influence but all fuel must pass through the main jet and thus, the main has an effect on the needle jet fuel flow.

"Bleed main" is the type with a hole or holes in the needle jet (tube) and a jet needle. Air is introduced into the needle jet to help mixing the fuel and get better droplet size before the final mix in the venturi. This type of main can also be used to compensate for the mix going rich or lean at higher RPM and loads.
The jet needle still controls the signal and fuel flow over it's normal range and all fuel must flow through the main as in the "primary main" setup.
With the proper needle jet type and enabling the "blank" main air bleed in the HSR carbs they can be converted over to  "bleed main" operation. Rare that this needs to be done. Most times the 48 just needs a fuel pump and a different float needle assy (smaller) for high HP motors.

[fuzznut5197]

Thanks Ed, more stuff I didn't know! You are an endless fountain of information.