ESE's works engine tuner

[katinas]

Interesting cylinder from Rotax Max EVO 2017 engine. Other photos about KR 250 and looks like cylinder trans A B ports interflow to one window.

[husaberg]

Interesting cylinder from Rotax Max EVO 2017 engine. Other photos about KR 250 and looks like cylinder trans A B ports interflow to one window.

Nice Kr Drawing here a a few Crankcase pics

[katinas]

Early KR 250 version from Brno GP with rear cylinder exhaust around the engine.
And two cuts in fairing for air on Mang's KR at Imola 81'.

[lodgernz]

I've often wondered how accurate is the long-accepted mantra surrounding carburettors.
Forever, we have been told that the main jet only controls the last 1/4 of the throttle opening, as shown in this widely-published diagram and others similar.


To satisfy my own curiosity, I stripped the 28mm genuine Keihin PWK that has been perfectly fuelling my 90 bucket racer for years.
I measured the needle jet very accurately at 2.60mm, and the needle diameter at 1mm increments from the point at which it is highest in the needle jet, i.e. at full throttle, to the throttle closed point.

Then I calculated the annular area between the needle and the jet at each 1mm from fully closed to fully open.
The main jet is a 140, so it has a CSA of 1.4776mm2.
The pilot jet is a 40, so its CSA is 0.0993mm2.

It seems to me that no matter how big the annular area is, the main jet, being the feed for the needle jet, must limit the fuel delivery to its own flow capability.

My calculations show that, when the throttle is open 13mm, not quite half throttle, the needle/jet annular area is 1.5080mm2.
Bearing mind that the main jet CSA is slightly less than this at 1.4776mm2, my conclusion is that the main jet actually controls fuelling from at least half throttle, despite the needle/jet annular area dramatically increasing with further throttle opening.
If I add in the pilot jet CSA to the calculations, the areas change but the 13mm limitation remains.

This doesn't seem to make sense. Is there some other secret fuel circuit that I'm missing, or is venturi action somehow massively variable throughout the opening?
I note that the needle jet tube has tiny holes in it, but these are in an area that is linked to the atmosphere in the bellmouth, so don't appear to be a fuel source.
Clearly the amount of fuel drawn through the jets by venturi action will increase as the air flow speed through the throat increases, but that doesn't explain why the needle/jet area continues to increase past the point where the main jet has apparent control.

Please discuss...

[wobbly]

If the emulsion tube has holes in it, then its a 4T carb.
These holes bend the fuel curve shape depending upon their height and diameter.
A 2T carb uses a shroud in the venturi , and this operates ,along with the main air corrector to change the fuel curve shape - away from simply getting richer as the airspeed rises.
What very few realize is that the fuel delivery at WOT - low rpm , is set by the tube/needle annulus area.
In any 2T tuned correctly such as the 125 karts ,I can remove the main completely,and on the dyno it will pull WOT from 7000 to around 12,000 ,then go rich.
If the needle/tube combination isnt correct ie too rich down low ,as well as a needle end too small ,it will hardly pull any load at all - let alone WOT.
I have recently done a test of varying the shroud height (2mm lower ), as this was a trick we used for years in Rotax classes where the jetting ( apart from the main ) is a tech item.
Lowering the shroud made those engines light up big time, but someone in Tech discovered what we were doing and got it banned.
But in the TM test I did ,a lower shroud just gave way higher egt as the rpm rose.
Rejetting the main to get the egt back, gave identical power,sadly.

[lodgernz]

I'm confused. If the needle and needle jet annulus controls the flow from idle to WOT, what is the main jet for? And why is it not limiting the flow at the needle, since it is smaller in area?

[wobbly]

The combination of the annulus area and the flow restriction of the main , control the total fuel delivered only at higher rpm.
This is shown when using different combinations of needle end diameter and emulsion diameter.
Depending upon the WOT annulus the main can vary from a 132 all the way out to 185 and this will give the same peak egt.
The trick is to find an annulus area that gives the best low rpm WOT response - then choose a main that delivers the final peak egt required.
Of course I am talking 2T carbs here , the situation is very different without the venturi shroud.

[ken seeber]

Isn't it a simple matter of the Cd of the round hole in the main jet vs a very thin annular section of the needle within the needle jet?

[Frits Overmars]

I'm confused. If the needle and needle jet annulus controls the flow from idle to WOT, what is the main jet for? And why is it not limiting the flow at the needle, since it is smaller in area?

The main jet controls fuel flow; the needle/needle jet gap may control the larger-volume fuel-air emulsion flow, depending on the type of carb.
A secondary aspect, as indicated by Ken Seeber, is the difference in boundary layers. The main jet just has an internal boundary layer that subtracts from the flow area. The needle jet has an internal boundary layer and the needle has an external boundary layer, that may or may not overlap, depending on whether the needle is centered in the needle jet.
Sideways movement of the needle in its jet causes a variation in effective flow area. That is why some carbs have a small spring pushing the needle sideways, so it is always in the same position in relation to the jet.
In the drawing below the needle could still move a little further to the right until it leans against the jet body, which would increase the effective flow area, or it could move further to the left, decreasing that area at first, and then decreasing it again.

[husaberg]

If the emulsion tube has holes in it, then its a 4T carb.
These holes bend the fuel curve shape depending upon their height and diameter.

I dont agree with that Wob,I have seen a few factory carbs from Disc valve bikes that have had emulsion tubes on Mikunis
Ie (B) type for primary bleed needle jets.

Pretty sure its in the Bell book as well?

This site is worth a look for intakes and etc
http://kocherenterprises.com/wp-cont...and_intake.pdf
Alot looks sudco though

I seen someone was making needle jet blocks for Kehins PWK's the other day too
I have not seen these generally available like on a Mikuni it has replaceable needle jets

https://gaming.youtube.com/watch?v=FCyW6Ads2kA
holey shit though the price...
https://www.mxsouth.com/stic-fuel-sy...ring-block.htm
MX South Price:$495.00 USD

https://thumpertalk.com/forums/topic...ew-2006-cr250/
Interesting stuff with Mikuni needles and some differences in the PJ on the PWKs

[Muhr]

If the emulsion tube has holes in it, then its a 4T carb.

Rotax classes where the jetting ( apart from the main )

Is not the rotax classes you referring to running FN 266?

[Flettner]

any old carburetors😆

[Frits Overmars]

any old carburetors😆

You could melt 'm and cast your own injector bodies.

[wobbly]

And I agree with you as well.
There have been a very small number of factory jet setups that use the bleed type emulsion tubes on a 2T.
One model of Yamaha RD had this , and it is common practice to ditch that in favor of the next years jets that used the shroud , as
they respond " normally " to tuning , the bleed system was always near on impossible to get spot on.
But the vast majority of 2T setups use the shroud - the old story of the exception proves the rule.

The history of the Rotax is that they used the FN 4 stroke tube in the early model engine ( with a shroud as well ) then when the EVO kit was introduced the FN 266 was
dumped in favor of a " normal " DP 267 with a shroud - same setup as a KZ engines carb..
During the engines life various shrouds were introduced , and we mixed them up to get the best results ,but as I said Mr Tech finally got onto us and specific shroud heights are now fully specified.

[Flettner]

HaHa, a view inside that rubbish bin.😆