Tuning WOT at Low RPM

Mikey Antonakakis /

First, go take a read here, this is absolutely not my own original idea, I take zero credit, but I think it is important to share:

Wobbly On Needles

Maybe everyone already knows this, but it is something I have been struggling with for a long time, and could never find a solid answer:

For a high-strung 2-stroke with a peaky power curve (frankly most mopeds don't fit in here, I'm talking 180deg or more exhaust timing, 25deg or more blowdown), it's obvious that fuel requirements are vastly different on and off the pipe. You can easily make 2x or 3x more torque on the pipe, meaning you're trapping 2x or 3x more air/fuel mix in the cylinder. In my mind, for the longest time, it never made sense that a single carb circuit (main jet) could fully account for this change in required fuel.

This idea that needle tip vs atomizer diameter controls WOT at low speed and main jet controls WOT at high speed was the first I'd heard of speed-dependent jetting. Any tutorial or carb manual I've seen only mentions the impact of different circuits based on throttle position.

I can't say I fully understand the dynamics that cause this differentiation of main jet and needle annular area between low speed and high speed at WOT (my only guess is that at low speed, the atomizer is full of fuel, so needle orifice is the limiting factor, but at high speed, it gets sucked dry, so main jet limits the flow?), but I certainly trust Wobbly and Frits.

The PHBG manual itself *almost* addresses the subject, in the section where it shows how to calculate/check that your needle opening isn't too small (comparing needle annular area to main jet cross sectional area). For most mopeds this is definitely not a problem. With atomizers around 2.6mm, needle tips at 1.4mm, and main jets usually around 1mm or less, there's a few hundred percent more needle area than main jet area. So it's more of a matter of the opposite, I think: making sure you don't have too much needle flow area that if you jet for WOT at high speed, you get way too rich at low speed.

In fact, it's hinted at in the very first post of that enormous KiwiBiker thread: take out the main jet altogether, and it will still run at WOT, at least at lower RPM. I didn't try that, but I made a big change, and tried a W11 needle vs. W9 (1.8mm tip vs 1.4), and went much richer on the main jet (69 to 80). It was a huge improvement at low RPM WOT: it leaned out and got a lot more heat in the pipe, and made a whole lot more power. A little too rich when it revved out. Overall much better than the previous setup.

I will keep experimenting (hopefully with actual data for comparison), but in the meantime, thoughts? Does everyone know this already? If so, I think it should end up in the Wiki.(edited)

Re: Tuning WOT at Low RPM

Working with some very small numbers but it is possible to calculate the annulus area for the needle (with your throttle indexed so you can gauge position) and you can calculate the perfect needle taper angles for your set up. May end up profiling your own custom needle if you have patience.

Re: Tuning WOT at Low RPM

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

Re: Tuning WOT at Low RPM

Mikey Antonakakis /

> ken gilbert Wrote:

> -------------------------------------------------------

> 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

I think we are on the same page there. Today I pulled out the main jet, no go. Clutch slips around 6k and right now I can't rev past 12.5k (I think due to carb tuning - should be able to get at least 14k). Even at "just" 6k, it was flooding out the engine.

So then I popped in a 77 main and went one notch leaner on the needle, second notch from the top i.e. second leanest (vs. 80 main yesterday, which worked well). Not as good as yesterday. Went back to third clip position, better. Tried the richest clip setting, still worked quite well but EGT was dropping. Tried second leanest clip position again, not as good. Overall with the 77 main jet, third notch seemed best. Don't think the main jet is right - most of the time, rolling back on the throttle from WOT to about 3/4 while fully revved out got me a couple hundred more RPM. Today's testing was done on a 1-mile long ~5% uphill grade.

So I think in my case, the main jet is so relatively small compared to the needle orifice that it is acting as a flow restriction at relatively low rpm. Moving the needle makes a very noticeable difference at WOT and low rpm, but it's definitely not the only factor.

Re: Tuning WOT at Low RPM

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.

Re: Tuning WOT at Low RPM

Mikey Antonakakis /

Graham, my AU atomizer for my PHBG has no holes for air bleed.

I guess the tl;dr is that the idea is low RPM WOT is controlled by needle tip diameter vs atomizer diameter (annular area) and as engine speed increases, at some points the main jet takes over. I don't really get why/how this should be the case with the non-air-bleed style AU atomizer, but there seems to be something to it.

The little bit of testing I've done so far shows it's not 100% influence of either needle annular area or main jet circular area, but changing the needle position/size with the same main jet makes a big difference at lower RPM WOT condition even though that annular area is like 5 times bigger than the main jet on my setup... I dunno.

I also need to check that I'm not foaming my float bowl, because if so, anything I've tested/seen so far may be a moot point... I have a rubber spigot but it's acting similar to my Peugeot when I had that problem. Same RPM, around 12k, it just straight up cuts out like I hit the kill switch, then comes back on full power when the revs drop a bit.

Re: Tuning WOT at Low RPM

Mikey Antonakakis /

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).

atomizer.PNG

Re: Tuning WOT at Low RPM

yeah i've spent more time in the world of VM's and PE's and stuff and to be honest PHBG's still kindof confuse me at this level.

the VM super tuning manual is really well written and my knowledge of how these carbs work is basicaly just copy-paste from that, haha.

Re: Tuning WOT at Low RPM

Mikey Antonakakis /

> Graham Motzing Wrote:

> -------------------------------------------------------

> yeah i've spent more time in the world of VM's and PE's and stuff and to

> be honest PHBG's still kindof confuse me at this level.

>

> the VM super tuning manual is really well written and my knowledge of

> how these carbs work is basicaly just copy-paste from that, haha.

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!

Re: Tuning WOT at Low RPM

Terbo Speghetti /

Stellar info from a brilliant dude here. Do you have a dyno or any plans for one Mikey?

Re: Tuning WOT at Low RPM

Overpriced Parts /

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!

Re: Tuning WOT at Low RPM

Mikey Antonakakis /

> Terbo Speghetti Wrote:

> -------------------------------------------------------

> Stellar info from a brilliant dude here. Do you have a dyno or any plans

> for one Mikey?

I hope you mean Wobbly and not me!

I do not have a dyno (yet), but I do have access to a GoPro with accelerometer data, which is better than nothing. Also there are some nice hills/mountains here which make for real easy binary judgments - i.e. did I make it up the hill or not?

I do want to build a dyno - keeping my eye out for spin bikes on local CL. The flywheels are 40-50lb, usually about 20" diameter, so I don't have to spin them too ridiculously fast to get a decent inertia. Just missed a couple nice cheap ones this weekend.

If you need some more nerding out, go check out my General build thread! Did some data crunching from the GoPro in there.

Re: Tuning WOT at Low RPM

Mikey Antonakakis /

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.

IMG_20190916_1655422.jpg
IMG_20190916_1658472.jpg

Re: Tuning WOT at Low RPM

Brandon Weiss (Detective brandon to you) /

> Overpriced Parts Wrote:

> -------------------------------------------------------

> 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

> n 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

> e 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

> e driving violations prior! The entire time including distracting

> driving with cell phone in his face he was a knucklehead!

Pretty sure even the slowest production cars can out-accelerate even the fastest mopeds, save for maybe dual variated race bikes with scooter cylinders.

Re: Tuning WOT at Low RPM

Mikey Antonakakis /

> Brandon Weiss (Detective brandon to you) Wrote:

> -------------------------------------------------------

> > Overpriced Parts Wrote:

>

> > -------------------------------------------------------

>

> > 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

>

> > n 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

>

> > e 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

>

> > e driving violations prior! The entire time including distracting

>

> > driving with cell phone in his face he was a knucklehead!

>

> Pretty sure even the slowest production cars can out-accelerate even the

> fastest mopeds, save for maybe dual variated race bikes with scooter

> cylinders.

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...

Re: Tuning WOT at Low RPM

Overpriced Parts /

Of course most cars nowadays can beat a moped,

What I'm talking about is a normal take off then all of a sudden the driver just thinks think he can just go around a moped crossing/going over the double yellow with oncoming traffic just a bit away with oncoming traffic just a bit away and exceeding the speed limit,

Being a bully trying to force me to move over in curb/imaginary 3 foot lane (that's full of sticks, acorns, buckeyes and rocks just to wipe out)

I won't do that for anybody !

This happens several times a year after a stop at a stop sign or after one of the few traffic lights I have near my area or just out of the clear blue with no warning!

No one wants to be behind a moped for 3-4 mi so they give it a bit more gas after take off but then I'm already well in in the power in first gear getting ready to hit second gear at 35-40mph then it's lawbreaking time!

Then they do anything to get in front of me usually passing me around 60mph or faster in 35-45mph zone, so when they get pulled over they deserve it,

If I'm stuck behind somebody doing the speed limit in my van or any vehicle I except it.

I don't pass someone causing a dangerous situation just to gain one car length,

Most people go a few miles over the speed limit anyway,

But today with these super cars and SUVs out here (heck even the newer Honda, dodge/Chrysler minivans have 300 HP) a few mph over the speed limit is not good enough!

Most are going 50-55 mph in a 25-35mi speed zones like nothin especially on open roads that police SUV 's cant hide on,

Most people don't realize when you pass vehicle you're not supposed to exceed the speed limit per state law no matter who's in front of you doing the speed limit or near it,

For me the best asset is having fast low end takeoff on these small bikes,

Re: Tuning WOT at Low RPM

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Re: Tuning WOT at Low RPM

mikey I remember reading something else about the atomizer theory of operation wrt the main jet... I think it was in the thread you linked.

at rest, engine off, the "chamber" will be filled with gas up to the float level.

at idle, the air jet will allow pre butterfly air into the chamber which froths and breaks up the liquid improving vaporization. at this point mass flow is very low so rate of fuel delivery is limited by jet needle/ needle jet clearance. the level will be slightly lower than the engine off case.

at sustained (ie peak power), that chamber will have a much lower level of gas since the main jet is doing the limiting, and that's "upstream" of the chamber.

what may also be happening is during quick throttle transitions the chamber gas works as an accelerator pump, since those two levels are different. ie you have effectively enriched high rate throttle deltas with the volume of the atomizer chamber. after a few seconds the chamber level drops and the main jet sets ultimate afr.

I do think there's some degree of temporality to the switch over and this theory made sense to me.

Ken

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