MOPED ARMY

mikey, i can give you a +1 on this theory, based on some experimenting i've been doing over the last few weeks trying to dial in fueling.

although my use case is slightly different than your high strung pipe example, i've been battling a similar issue--wot fueling at low-mid rpms vs wot fueling at peak power, and which carb adjustments factor in to each.

i'm on a mikuni vm20, and i've got two needle profiles--4j13 which came installed, and 4l6. i also just ordered a 4j11 off fleabay which will come in friday.

the other very handy tool at my disposal is an adjustable main jet. i often make small real-time adjustments while riding.

i've had a few "aha" moments that reinforce your ideas:

1) it may sound like a noob observation, but i was not always aware that the jet needle remains inside and partially shrouds the needle jet even at WOT.

2) not all mikuni jet needles are the same length. the 4j11 that's coming in friday is almost a full cm shorter than the other two. based on my eyeballing this will actually allow the jet needle tip to completely clear the needle jet orifice (if it doesn't it will be damned close).

3) while tuning top end WOT, past a certain point opening the adjustable main jet has no effect on afr.

4) while changing jet needle profile and/or position, top end WOT afr changes requiring main jet adjustment.

5) as long as the main jet isn't ridiculously undersized, it has no effect on mid throttle fuel.

so i do agree that the "first order" understanding of jet needle=mid throttle / main jet = WOT is a bit more nuanced and gray: there is a good deal of overlap of the jet needle into the WOT area of operation.

in my case the port velocity of a slightly oversized carb falls very low with a quick WOT stab at low rpms, and my afr goes lean. based on my testing and the discussion of this thread i think that has 100% to do with jet needle/needle jet clearance since at that operating point mass flow rate is not sufficient to cause the main jet restriction to come into play. as rpms climb and mass rates increase the main jet will limit max fuel flow.

put another way, at this point my hypothesis is that the main jet represents a max fuel flow limiter, while the interaction of jet needle/needle jet is all other off-idle operating points (like the hole shot WOT snap).

ken

yeah to be honest i'm reading all these words and trying to understand exactly what you are saying but its just not getting through my thick head for some reason.

the one thing i can say: the atomizer and air bleed circuit is proportional to speed, thats why the 'air bleed' is so popular on 2T bikes and the reason for the different atomisers.

the progressivity of the fueling curve is controlled by the air bleed, when the carb is WOT but low rpm, the vacuum signal is weak and pressure in the carb is high which draws from the air bleed.

when the air flow through the carb increases the pressure in the carb drops and the air bleed basically gets overcome by the pressure drop across the carb, thereby enrichening the fuel delivery at high rpms.

the air bleed basically dilutes the fuel in the emulsion/draw tube based on the holes in the atomizer. four strokes that have a very linear fuel delivery curve can get away with low speed fuel metering orifice - like on a PZ keihn or a lawnmower carb, whereas once your fuel curve becomes progressive like on an agressive/piped 2t thats where' the air bleed comes in.

maybe i'm not getting what you guys are talking about, maybe you are over my head? I dunno. I still don't really understand what the different holes in the atomizer mean and why they are where they are. And how all that relates to the needle.

Oh and the Dellorto manual really isn't super helpful with any of this. For example, they say:

"From three-quarter throttle to full throttle the amount of fuel depends solely on the size of the main jet."

But then the very next line is, paraphrasing:

"Therefore to vary the full throttle circuit delivery, you should change the throttle slide cutaway, the tapered needle, the atomiser-needlejet size and type, and the main jet."

I'll have to take a look when I get home, but it does seem that there is some air bypass on the AU atomizer - in section 3.6.1 of the manual, they say that "air from the inlet... flows into the round extension formed by the upper outer end of the atomizer and by the inner part of the nozzle, ...then mixes with fuel metered through the main jet and coming from the atomizer."

Just to clarify the jargon here, the "inlet" is the small hole that feeds the idle circuit with air, and it seems like the "nozzle" is the doodad that sticks up in the venturi that shields the tip of the atomizer, consistent with the parts description on Treatland. Oh, and the bushing even has a hole that looks like it matches up with the air inlet for the idle circuit!

This actually makes some sense, and might even explain things a bit! If there is indeed a little room around the OUTSIDE of the atomizer and inside of the nozzle/bushing for air to bypass, then at low RPM WOT when there's little or no vacuum in the throat of the carb, the bushing/nozzle's protrusion into the throat should cause a pressure drop at the atomizer outlet and draw fuel through the inside of the atomizer, as well as air through the idle circuit inlet to the annular area around the outside of the atomizer/inside of the bushing. This is consistent with something Wobbly mentioned on KiwiBiker, that cutting down the nozzle so it protrudes less resulted in a big performance improvement (jetting was set by the rules, except for main jet - this trick ended up getting banned).

Looks like in terms of WOT operation, they are very similar, with a similar shroud around atomiser outlet. Idle circuit seems a little fancier on the Mikunis tho.

I posted the same postulations on the KiwiBiker thread, so I'll post updates if any of the the two stroke superheroes chime in!

No one tunes for off the idle rpm like I do because I like do because no one likes low end to mid mid power right off the bat like me (because Most like spending a lot of money on their mopeds and making every aftermarket company rich)

I like giving cars a run for their money with my reed induction kits on mopeds,

Many of these junk four-cylinder cars they have nowadays, I most times beat them on the take off with a moped up to 50 mph, I hog whole lane so they don’t try to pass me,

The last time a plastic Corvette a few years ago before there were aluminum body (but corvettes are plastic again) passed me on a double yellow in summit county,

A county sheriff pulled him over going 14 miles over the speed limit, The gold chain dude guy got stopped and I reported that he was riding my ass only 20 feet away for at least 1 mi,

He got three tickets and I wish he got more for he had more like five driving violations prior! The entire time including distracting driving with cell phone in his face he was a knucklehead!

Okay, I at least have a little more understanding now: just measured, atomizer OD (for AU) is 3.97mm or so. The ID of the bushing looks to be around 4.8mm. This gap is clearly fed air by one of the two small inlets at the mouth of the carb (the one on the right in the pic), confirmed by spraying carb cleaner and watching it emerge from around the outer diameter of the atomizer. Does not seem to go anywhere else or feed any other part of the carb. The other hole feeds the idle and starter passages.

Yup, 3000lb car w/ 200hp is just about the same as a 20hp, 100lb moped with 200lb rider. Most "fast" mopeds are 10hp or less, so...