What size is a 24mm carb?

[jasonu]

Now that 100's are allowed to go out to 110cc on re bore, is the restriction still relevant.

A 100cc water cooled motor already has an enormous advantage over air cooled engines, maybe to be fair, the water coolers should be restricted to 24mm and the air cooled engines unrestricted. Now that would be more balanced.

110cc on rebore is only 4cc more than they had so from what I have read that is more to do with ease of obtaining pistons rather than a power advantage.

TZ, Green and probably others have proven you can make 30 or more HP from an air cooled 125 with a 24mm carb. As soon as they figure out how to get them to hold together, and I am sure they will, they will hold the advantage over the 100cc 2t water coolers and likely the 150cc 4t's as well.

It sounds like you have a 125cc 2t...

[FastFred]

Attachment 318936

At 30 rwhp Speedpros unrestricted 100 Blue line betters my 125 in the dyno stakes.

TZ, Green and probably others have proven you can make 30 or more HP from an air cooled 125 with a 24mm carb.

No, they are both good engine builders but it looks like Green has shown that his water cooled 100 can better TZ's air cooled 125.

To be fair it looks like it is the water cooled 100's that should be restricted, not the air cooled 125's.

Just logically following the evidence.

[Grumph]

To be fair it looks like it is the water cooled 100's that should be restricted, not the air cooled 125's. Just logically following the evidence.

The equivalency thing has been fluid for years with no one type holding an advantage for very long - except on the big tracks where until the watercooled 100's get sorted fourstrokes will continue to rule.
Given any new motors being brought out which are bucket legal are 4 strokes, things could change again - or maybe not...

[TALLIS]

I think the restricted carb rule for 125cc a/c should be dropped, the difference in power will be marginal and they still need to be reliable to be competitive. Just a thought.

[TZ350]

.... they still need to be reliable to be competitive ....

Yes and that is probably the greatest restriction on an aircooled 2T engine.

I think the restricted carb rule for 125cc a/c should be dropped.

Me too, as there is no useful evidence that shows its necessary or effective in its purpose.

But if we are going to have it, it would be good to have a consensus about how its interpreted.

The common convention has been that carburetors are sized on their choke size and/or flow/min with slide carburetors measured at the narrowest point in the venture, this is usually just behind the slide. It is the usual thing Mikuni, Delorto, Kehien to name a few do this but it is not a rule and not every manufacturer does that, Lectron is an example. Pumper carbs are commonly sized at the venture, Tillotson is an example.

And CV carbs with their variable venture are measured at the flange. Maybe MNZ were thinking of them when they issued their clarification. Because its not the way most of the current manufactures make slide carbs, so if MNZ's rule clarification was applied as is, it would make a lot of the 24mm carburetors from the major manufacturers illegal.

It would be good to have some sort of consensus about how we want to apply "the equivalent to a single 24mm carburetor rule".

[richban]

The common convention has been that carburetors are sized on their choke size and/or flow/min with slide carburetors measured at the narrowest point in the venture, this is usually just behind the slide. It is the usual thing Mikuni, Delorto, Kehien to name a few but it is not a rule and not every manufacturer does that, Lectron is an example. Pumper carbs are commonly sized at the venture, Tillotson is an example. And CV carbs with their variable venture are measured at the flange.

It would be good to have some sort of consensus about how we want to apply the current 24mm equivalent rule.

All you need to do is ring all the carb manufactures you mentioned and ask them how they measure there carbs. Easy! Then put it in a document and send it to MNZ for them to include into the rules.

Really though asking for consensus is a big fat waste of time. There are to many personal agendas to get past. And this site is no true representation of the racers. If you go searching for all the clarifications for all the rules you are limiting yourself more and more. Build race blow up then build again. At the moment the biggest issue is reliability.

[mr bucketracer]

24mm is 24mm , not that hard realy lol

[kel]

Really though asking for consensus is a big fat waste of time. There are to many personal agendas to get past. And this site is no true representation of the racers. If you go searching for all the clarifications for all the rules you are limiting yourself more and more. Build race blow up then build again. At the moment the biggest issue is reliability.

Summed up beautifully!

[speedpro]

Really though asking for consensus is a big fat waste of time. There are to many personal agendas to get past. And this site is no true representation of the racers. If you go searching for all the clarifications for all the rules you are limiting yourself more and more. Build race blow up then build again. At the moment the biggest issue is reliability.

Absolutely correct. Any bucket engine built to the edge of what's allowed under the rules will be a winner with the correct rider on board. The horsepower is there to be had. Most blowups aren't related to the power but rather the tune or a mechanical limitation of the base engine. FXR rods and TS crank bearings for example. MBs of course have no weak spot and I just blow them up now and again so everyone else doesn't feel too inferior.

[seymour14]

Absolutely correct. Any bucket engine built to the edge of what's allowed under the rules will be a winner with the correct rider on board. The horsepower is there to be had. Most blowups aren't related to the power but rather the tune or a mechanical limitation of the base engine. FXR rods and TS crank bearings for example. MBs of course have no weak spot and I just blow them up now and again so everyone else doesn't feel too inferior.

And KR's are an unknown quantity.

But not for much longer...

[Pumba]

And KR's are an unknown quantity.

But not for much longer...

I thought that had already been blown up twice? (or parked up before it did blow up is maybe more correct)

[seymour14]

I thought that had already been blown up twice? (or parked up before it did blow up is maybe more correct)

Third times the charm...

[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".

If you go searching for all the clarifications for all the rules you are limiting yourself more and more. Build race blow up then build again. At the moment the biggest issue is reliability.

Agreed, and I didn't ask for this clarification, just needed to get a feel for how others saw it. Happy to go along with the consensus if there is one.

[TZ350]

here is the original answer from mnz.

Chapter 24 has been changed to chapter 16 in the latest version of the MoMS. So it becomes 16.2 now. That rule does not specify whether it's at the venturi or the bore size but the usual convention when specifying carburetor size is to measure the inner bore diameter on the exit side of the carburetor.

I have understood the convention a little differently but have not been able to find anything current on the net that shows how credible carburetor sources see it.

I cut my motorcycle racing teeth in the days when Amal carb specs said "Choke Bores Measured Behind Throttle Valves" and this is the convention I grew up with.




On the face of it the MNS clarification of measuring the carb at the exit side (because a carb is larger there) looks like it makes the Mikuni Kiehin and other common manufactures 24mm carburetors illegal for us to use, I am sure that wasn't the intention so there must be something I have missed or maybe "exit side" is MNZ's way of saying "behind throttle valve????.

In the end, the interpretation of the rule is whatever MNZ wants to say it is, and that is Ok..... Kiwi-azz, thanks for posting their reply.

[kel]

In the end, the interpretation of the rule is whatever MNZ wants to say it is, and that is Ok

The correct answer from MNZ is exactly as per the first part of the reply i.e. "That rule does not specify whether it's at the venturi or the bore size" end of story. Besides "usual convention" doesn't fit with buckets .