52 Mk 11 the start

[jasonu]

Carbs are allways sized at the venturie, for a carb to work the venturie and fuel metering section don't have to be at the same place, its just cheeper to make them that way.

Not all carbs of the same size flow the same amount of air, we have checked quite a few with our simple flow bench and found ways to improve them.

Inventiveness is within the spirit of Bucket racing, and this carb mod this has been run past JC and he was OK with it.

Maybe he was more relaxed about it as all the details and theory have been posted on the ESE thread so anyone can copy it if they make the effort.

I will dig up some of the posts...........

With all respect to JC, he does a hell of a job, are you sure it is his call to make? If it was run past the appropriate MNZ official, one who might deal with a protest, and OK'd then that would be that I guess. I agree with inventiveness being a big part of bucketting but I see this as a loop hole that should be closed. I do not think you should be limited to a 24mm stock store brought carb, just one that is equivalent to a single 24mm carburettor, maybe a smaller carb that has been D bored. maybe twin 9mm carbs (good luck getting that to work) for example , as long as the 24mm thing is adherred to. That sort of thing IMO is inventiveness. What I don't want to see is someone putting a great deal of time, work and money(?) into developing a motor around this mod and then finding out it is not allowed. I for one would get a final answer from someone in the position to make the call first. Frankly, if someone wants to run a bigger carb, then build a 100cc motor.

Looking forward to seeing your old posts about this, pix too hopefully.

[TZ350]

With all respect to JC, he does a hell of a job, are you sure it is his call to make?

I for one would get a final answer from someone in the position to make the call first.

Looking forward to seeing your old posts about this, pix too hopefully.

True JC, can't speak for MNZ but I was being upfront with him and you asked.

Good advice, if your one of those at the pointy end then it would be good idea to get an official MNZ opinion.

Yes there are pic's, there are over 1200 pic's on the ESE thread, "Thread Tools" near the top of the page makes it easier to look through them.

I will get onto the finding the Posts now, back soon.

[TZ350]

Looking forward to seeing your old posts about this, pix too hopefully.

There were a number of posts about this, found three.

A carb is sized by measuring the major venturi, common motorcycle carbs have the metering orifice and air control (usually a slide) and a single venture but there are other ways of controlling air flow, fuel metering and fuel atomisation.

Some carbs have mutiple venturis and even venturies within venturis.

So should the Bucket 24mm carb rule also limit us to simple single venturi unmodified 24mm store brought motorcycle carbs and thinking, should a Bucket carb have to follow commercial motorcycle convention and limitations?……………..

Fuel metering and the major venturi of a carb do not have to be in the same place.

For a performance carb it makes sense to have the most restrictive metering bits separate from the highest flow section.

If fuel metering is upstream of a venture smaller than the metering section then fuel atomisation is greatly improved by the fuel droplets bursting as they pass through the high velocity low pressure area of the down stream venture.

Flow tested some carbs today. Thomas made up a manometer, which proved to be very effective at measuring flow through the carburettors.

The current challenge is to get more air into the engine through the 24mm carburetor. A carburetor is basically a simple venture.

Picture-01 of a traditional venture (A) and a low loss flow tube (B) where the maximum pressure recovery (flow) occurs when D2/D1 is between 0.6 to 0.7.

Here are pictures of the actual carbs themselves.

Pic-01 and 02 the 28mm Keihin with a permanently fitted 24mm choke.

Pic-03 and 04 The taper bored power jetted OKO (Keihin Copy) 24mm at the bell mouth 28mm at the engine side.

I think this inventiveness is in the spirit of buckets but if anyone thinks its not within the rules and can explain why please tell me now before I hit the track at Taupo.

Our venturi inserts are as much about improved fuel atomisation and driveability as air flow, all important things in a 2-Stroke.

Click on the Quotes to see the full text and pictures……….

[ac3_snow]

.
I think Chambers is going with a conventional 24 first, then when the bugs in the rest of the new bike are ironed out he might try the 38/24.

But yes, 38/24 feeding both cylinders. We have been successful with a 28/24 combo on the GP125 and now have a 38/24 that shows real promise on the flow rig.

One of the problems I can see, is low signal strength at the main jet. Our 28/24 run conventualy uses a 130 main. Keihin 38’s on 125 MX’s run 180-190 mains std.

So I expect that on a 38/24, the main will need to be even bigger. Because of the reduced signal from lower airflow passing through the 38mm metering section at full throttle due to the more restrictive 24mm venturi section of the carb down stream.

My guess is 200+ for the main on our 125's using a 38/24 venturi carb...................

Also we figure, if you can't drop a 24mm ball brg through a carb with a circular throat then the cross sectional throat area is equivalent to or less than a carb of 24mm and so satisfies the 125 2-Stroke carb rule.

The beauty of the 38/24 combo seems to be that the airflow in the 38 section gets straightened up before entering the 24mm venturi allowing a higher flow rate in the venturi area.

The details and pictures are in earlier posts. From memory, if the venturi is 60-70% of the pipe (carb) diameter then there is a 94-98% pressure recovery (air flow) down stream for Reynolds (gas velocity) numbers less than 0.5.

So you can see that its possible to make a 38/24mm combo carb that flows like a conventional 34mm. But wether it works on the track we wont know untill we get one of the new bikes going.

Remember you heard about this idea of 38/24 and 28/24 carbs from Team ESE first..............

Ohh yea, I found this one on page 147 of the ESE thread. Very interesting, the way you describe it makes alot of sense, I now retract most of my previous statement after reading your posts.
So how many people are currently running this 24mm venturi system with an 'oversized' carb?

[TZ350]

So how many people are currently running this 24mm venturi system with an 'oversized' carb?

Up here, only me at the moment that I know of, and there is someone in CHCH with one I think.

So there is no unfair advantage, anyone can follow the links like you did and find out everything they need to know to make one for themselves.

Anyone can have one of these carbs if they make the effort.

[jasonu]

Up here, only me at the moment that I know of, and there is someone in CHCH with one I think.

So Rick is running a straight 28mm and therefore illegal carb.

I am still unsure of exactly what you have done. Does your insert go right through the carb and does it measure no more than 24mm (or equivalent) where the slide shuts off the fuel/air delivery? If you buy Mik or Keihin carb that is specified 24mm you will find that measurement at that same place ie in the middle of the venturi where the slide is.

[TZ350]

So Rick is running a straight 28mm and therefore illegal carb.

I am still unsure of exactly what you have done. Does your insert go right through the carb

I don't know about Ricks setup, its new and I havn't seen it yet, didn't know he had a 28 untill you pointed it out. The info about how to improve these carbs has been around on the ESE thread for a year or so, he could have made 28/24 like mine ...........

Here are pictures of the actual carbs themselves.

Pic-01 and 02 the 28mm Keihin with a permanently fitted 24mm choke.

Internaly the high velocity atomisation choke area looks like the shaded part in the first picture and the tail of it can be seen sticking out of the carb in the second picture.

[jasonu]

No not Rick...........



Internaly the high velocity atomisation choke area looks like the shaded part in the first picture and the tail of it can be seen sticking out of the carb in the second picture.

The whole thing looks grey to me. I think the rule should be rewritten and include the point where the measurement will be taken and that point should be where the slide cuts through the venturi. ie if you were to take a felt pen and trace out the opening onto the closed slide, that area should be the 24mm or equivalent.
I know you said in some cases the venturi is not always in the same place as the slide, that is more of an automotive thing where there is a butterfly instead of a slide and neither of us can recall seeing a 2 stroke carb with the slide in a different location to the venturi except for kart carbs which have butterfly valves.

[TZ350]

So Ricks bike is illegal due to the 'normal' 28mm carb???

I don't know about Ricks setup, its new and I havn't seen it yet, didn't know he had a 28 untill you pointed it out. The info about how to make an improved carb has been around on the ESE thread for a year or so, he could have made 28/24 like mine ...........

I think the rule should be rewritten and include the point where the measurement will be taken and that point should be where the slide cuts through the venturi. ie if you were to take a felt pen and trace out the opening onto the closed slide, that area should be the 24mm or equivalent.

It would have to be some sort of good definition like that.......

The Dutch specify a (24mm?) diameter ball, if it doesn't drop through its legal, the smart boys machine the bore oval for extra area.

I know you said in some cases the venturi is not always in the same place as the slide, that is more of an automotive thing where there is a butterfly instead of a slide and neither of us can recall seeing a 2 stroke carb with the slide in a different location to the venturi except for kart carbs which have butterfly valves.

Does a Bucket carb have to follow convention....... is that what Buckiting is about, following convention.

neither of us can recall seeing a 2 stroke carb with the slide in a different location to the venturi except for kart carbs which have butterfly valves.

I think the Karts specify a max venturi diameter and as a matter of conveniance the fuel discharge nozzels are placed there and the butterfly is in a larger diameter area.

I will see If I can find a picture.

[TZ350]

I think the rule should be rewritten and include the point where the measurement will be taken and that point should be where the slide cuts through the venturi. ie if you were to take a felt pen and trace out the opening onto the closed slide, that area should be the 24mm or equivalent.

The rules say, "restricted to carburation equivalent to a single 24mm carburettor" so there is no restriction on make or style, the carburettor could be anything, even some new fangled approch that you cobbled together for yourself from old taps and the kitchen sink and something else from the laundry.......

So I propose a definition.

A light is shined through the open carb and the smallest cross sectional area seen must not exceed a 24mm Dia equivalent.

I know its not conventional but is there anything wrong with that for a definition that ensures compliance with the rules.......

[Rick 52]

The 28mm carb would not fit with the time I have to play with so this is the carb I am running with ,And it goes great today was tricky in the wet but lots of good power with magic feeling ..Very pleased with my legal bucket thank you ..

[TZ350]

I hope that tape measure has been calibrated recently...........

[rwh]

So I propose a definition.

A light is shined through the open carb and the smallest cross sectional area seen must not exceed a 24mm Dia equivalent.

You'd get away with a really big carb if it was curved ...

But it seems reasonable to me (as someone with no involvement whatsoever in racing) to simply say "all the air must at some point go through an orifice of max 452.4 square mm, at or below atmospheric pressure"

Then you can have all the tricky designs you like - with the slight problem that it might be difficult to measure.

Oh, and I guess you could do some tricky stuff by having the restriction on the intake to a really big airbox ...

edit: or by having a super/turbocharger downstream of the restriction

Richard

[TZ350]

You'd get away with a really big carb if it was curved ...

Good point, I hadn't thought of that.......

Oh, and I guess you could do some tricky stuff by having the restriction on the intake to a really big airbox ...

Checkout the plenum idea on the ESE thread.......

Small carb, big plenum, motor sucks directly from the plenum.

[RMS eng]

The rules say, "restricted to carburation equivalent to a single 24mm carburettor" so there is no restriction on make or style, the carburettor could be anything, even some new fangled approch that you cobbled together for yourself from old taps and the kitchen sink and something else from the laundry.......

So I propose a definition.

A light is shined through the open carb and the smallest cross sectional area seen must not exceed a 24mm Dia equivalent.

I know its not conventional but is there anything wrong with that for a definition that ensures compliance with the rules.......

24mm carb for a 125 two stroke needs to be measured 24mm over the area where the slide runs,which is where the air fuel mix is controlled which makes it a 24mm carb,running a 28-30mm carb with a 24mm spacer at one end is still a 28-30mm carb,unless you press in some kind of 24mm sleeve where the slide shuts off air coming into the carb.a 28mm carb is 35% bigger in area than a 24mm carb and putting a small 24mm restricter at the inlet end of a 28mm carb won't do much to stop this carb working like a STD 28mm carb at most of the rev range on a 125 2 stroke.if any one wants to use a kart type carb good on them but it must be the eqvivalent to a 24mm motorcycle carb,eg KT100 carb for karting must measrure venturi 24.13mm throttle bore 25.65mm[bell book].when Jason ,M green and myself made up these rule we did our home work,i still have the books we got the info from,and the 24mm carb rule was set,as i new a 1993 TZR 125 with a 28mm carb made 24,5hp at 9700rpm and 15hp at 8000rpm stock.