What size is a 24mm carb?

[TZ350]

Someone on the FXR150 site sought a clarification from MNZ ( http://www.fxr150.co.nz/forum/showth...3141#post13141 ) re, "where is a carb measured".

There seems to be some confusion as the posters interpretation of MNZ's reply indicated that MNZ regarded the carb size is measured at the very end of the carb where it is largest.

The only carb manufacturer I know of that does that is Lectron. There is no real industry rule but the convention followed by every other manufacturer I know of, sizes their carb at the venturi just behind the slide where it is smallest.

I have registered on the FXR150 site so I could ask that the original question put to MNZ and their written reply, could be posted.

Anyway what is the size of a 24mm carb or its equivalent?

And where does MNZ's 24mm equivalent rule come into this???

[Motu]

CV carbs is where the confusion is. Slide carbs are measured at the flange or mounting...CV carbs are always bigger, and some (Japs) measure at the flange because it sounds bigger, and everyone wants a bigger carb. Others (EG Bing) measure at the bridge where it is narrowest.

[TZ350]

(Japs) measure at the flange because it sounds bigger, and everyone wants a bigger carb. Others (EG Bing) measure at the bridge where it is narrowest.

It would be great to see some carburettor industry literature on where their carb size is measured.

[Grumph]

CV carbs is where the confusion is. Slide carbs are measured at the flange or mounting...CV carbs are always bigger, and some (Japs) measure at the flange because it sounds bigger, and everyone wants a bigger carb. Others (EG Bing) measure at the bridge where it is narrowest.

Where it's measured is irrelevant IMO - the rule says "equivalent to a single 24mm carburettor" i read that as, in the case of a single carb, somewhere within the confines of the carb body it has a bore of no more than 24mm.

I'd point out that for years Mikuni made slide carbs with oval throats. The sizing was the equivalent area of the oval...But invariably the engine side was a larger bore. VM30's were 33mm on the inlet stub, VM26's were 28mm....Then the VM29 flatslides on GSXR750's had a 29mm offset entry but past that, in fact from the front of the slide, they are 33mm clear bore...

If the rule sinply said - All entry air must pass through a hole no larger than 24mm dia, The intent would be clear as would enforcement.

[Motu]

Find a carb with a stamped size, measure it, find another, measure it. Soon you will have a data base and know everything you need to know.

[Grumph]

Find a carb with a stamped size, measure it, find another, measure it. Soon you will have a data base and know everything you need to know.

It would be nice to think that whoever answered the question at MNZ had that much experience...But I've never found that to be true sadly...

[TZ350]

If that definition is accepted as is, then most of ESE's work on taper bored carbs is illegal.

I was worried about that for a moment. But in our flow bench testing we found a much larger carb with a 24mm venture at the engine end flowed a lot more air than a plain 24mm venture by itself. Our conclusion was that the larger carb bore acted like a flow straightener so there was less disruption at the face of the 24mm venture.

So if I have to, to meet the new MNZ interpretation I would simply get a 24mm OKO and bore it out to 32mm all the way to the last mm or two. The last part would be moulded into an aerofoil leading edge shape with just a smidgen of the original 24mm bore left for the convergent part of the venture.

That way the carb would measure 24mm right at the engine end to satisfy MNZ's interpretation of how to measure a carb and the divergent part of the venture could be formed by the rubber carb mount and inlet tract. Our flow bench tests showed that this 24mm configuration in a 32 carb passed an air volume similar to a std 30mm carb.

Easy Peasy and anyone could do it.



These are pictures of a 28mm carb with a 24mm insert but the idea is basically the same. Simply cut the venture insert off at the end of the carb. The insert measures 24mm there and that makes the carb by definition MNZ legal. And the longer off cut part of the venture insert is glued into the rubber carb mount to form part of the inlet tract.

Actually this whole thing has got me interested in carbs again because you can flow a shit load of air through a large carb with a 24mm venture at the end. We even did this with a carb that was originally 38mm. It ran Ok too but using it brought me into conflict with people who thought that clever thinking and engineering in Buckets was somehow cheating.



Husaburg now owns the mighty 38.

Because reversion takes place at significant cross section changes, using a venture insert in a larger carb could effectively shorten the inlet tract to the length of the venture insert itself, this could be very useful.

[TZ350]

Where it's measured is irrelevant IMO - the rule says "equivalent to a single 24mm carburettor" i read that as, in the case of a single carb, somewhere within the confines of the carb body it has a bore of no more than 24mm.

It is how I see it too.

[jasonu]

Someone on the FXR150 site sought a clarification from MNZ ( http://www.fxr150.co.nz/forum/showth...3141#post13141 ) re, "where is a carb measured".

There seems to be some confusion as the posters interpretation of MNZ's reply indicated that MNZ regarded the carb size is measured at the very end of the carb where it is largest.

The only carb manufacturer I know of that does that is Lectron. There is no real industry rule but the convention followed by every other manufacturer I know of, sizes their carb at the venturi just behind the slide where it is smallest.

I have registered on the FXR150 site so I could ask that the original question put to MNZ and their written reply, could be posted.

Anyway what is the size of a 24mm carb or its equivalent?

And where does MNZ's 24mm equivalent rule come into this???

Mate as you should already know MNZ does not give one fuck about Buckets (except for the large amounts of cash the class generates). They are only interested in 'real bike' racing. The MNZ will not reply with any useful information or clarification.

[richban]

Where it's measured is irrelevant IMO - the rule says "equivalent to a single 24mm carburettor" i read that as, in the case of a single carb, somewhere within the confines of the carb body it has a bore of no more than 24mm.

I second that..........

[cotswold]

Easy Peasy and anyone could do it.



These are pictures of a 28mm carb with a 24mm insert but the idea is basically the same. Simply cut the venture insert off at the end of the carb. The insert measures 24mm there and that makes the carb by definition MNZ legal. And the longer off cut part of the venture insert is glued into the rubber carb mount to form part of the inlet tract.

Actually this whole thing has got me interested in carbs again because you can flow a shit load of air through a large carb with a 24mm venture at the end. We even did this with a carb that was originally 38mm. It ran Ok too but using it brought me into conflict with people who thought that clever thinking and engineering in Buckets was somehow cheating.


Some of the lads in the UK's freetech 50 series use a 28pwk with a venturi as they reckon it speeds up the air flow, damned if I can find the post now but if I do I'll put it here

[Grumph]

Mate as you should already know MNZ does not give one fuck about Buckets (except for the large amounts of cash the class generates). They are only interested in 'real bike' racing. The MNZ will not reply with any useful information or clarification.

Very true - but in this case they have responded. And their answer is contrary to how the rule has been viewed and interpreted for some years...
Either they withdraw the answer or clarify it.

[seymour14]

Might be time they give us a minimum length that the 24mm diameter must be before it gets to the inlet manifold. That would solve all conjecture.

This is now scary shit to me, as the carb on our GPR125 is a straight 24mm...

[richban]

Might be time they give us a minimum length that the 24mm diameter must be before it gets to the inlet manifold. That would solve all conjecture.

This is now scary shit to me, as the carb on our GPR125 is a straight 24mm...

Lets just say its 24mm held for 2mm. Job done. Don't be scared its only a carb. Lets say we have a PWK style carb. After hours of flow beach work and then hours of dyno work the best combination of fueling v flow will be found for a particular engine.

Not many people will be doing that work will they? And if its rob, he will publish the work for all to see. So not really a big deal me thinks.

Lectron will make you a 24mm carb if you want. They are a carb manufacture! Maybe a 24mm HV carb from them would the ticket. Could be 28mm at the spigot 24mm at the slide and big on bell mouth. Completely legal as they are a carb manufacture

[AllanB]

I can't believe this is not clearly defined in the rules - mind you no being so clear leave it open to modifying as above which is also good for development.

Me - I agree - provided there is a point within the carbs physical inlet makeup that measures 24 mm it's 'legal'.