VM28 Jets On VM26 Carb?

[HawkTwoFifty]

So I have a Mikuni VM26 30mm carb and have been gathering jetting suggestions here for my Hawk 250. The recommended jets that happen to be recommended so far have a vm28 in their part number. I have been confused about this but ordered them anyways thinking that the vm26 and vm28 are similar carbs as far as jetting.

Can please anyone tell me if I ordered the correct jetting for my vm26 carb when the jetting bags I received say VM28?

For my VM26 carb, I ordered jets that are labeled on the packs as "VM28/486. When I ordered them I was not aware of this discrepancy as I am new to this carb (26, 28, 30mm etc) and ordered without much thought. But I am now wondering if the jets which say "VM28" on their packaging, are actually intended for my VM26 carb.

Sigh... I am such a newbie that its killing me! Thanks!

[Megadan]

Yes, its right. These clone carbs run a vm28/486 pilot jet, and n100.604 main jets.

Considering these are the part numbers I have in my carb, and what I told you to order...

[Ariel Red Hunter]

Originally Posted by HawkTwoFifty

So I have a Mikuni VM26 30mm carb and have been gathering jetting suggestions here for my Hawk 250. The recommended jets that happen to be recommended so far have a vm28 in their part number. I have been confused about this but ordered them anyways thinking that the vm26 and vm28 are similar carbs as far as jetting.

Can please anyone tell me if I ordered the correct jetting for my vm26 carb when the jetting bags I received say VM28?

For my VM26 carb, I ordered jets that are labeled on the packs as "VM28/486. When I ordered them I was not aware of this discrepancy as I am new to this carb (26, 28, 30mm etc) and ordered without much thought. But I am now wondering if the jets which say "VM28" on their packaging, are actually intended for my VM26 carb.

Sigh... I am such a newbie that its killing me! Thanks!

No, that is a good question, without an obvious answer. These jets work in a whole family of Mikuni carburetors. When you have a much smaller, or much larger carb, they are either too big or too small....ARH

[HawkTwoFifty]

Thanks guys for the help. Would I have been better off getting the VM28 over the VM26? I see that there are carbs for sale that say Mikuni VM26 and are descirbed as 26mm, and VM28 as 28mm carbs. Now my VM26 (which says mikuni on it but sells for lower than these other VM carbs) says mine is 30mm!

So is my VM26 carb (https://www.amazon.com/MIKUNI-VM26-C...ds=vm26+mikuni) a 30mm, and would I have been better off getting the VM28 at this link: https://www.amazon.com/Mikuni-Round-...uni+carburator

Also, what size is the stock carb? The stock carb is a "PZ30", right? If so, I cant find any info on the PZ30 to compare.

The more I look into this, the more questions I have!! Thanks!

[Megadan]

The VM26 clone you have has a 30mm throttle bore, which is the same as the stock PZ30. If you sit the two side by side you will see no distinguishable differences. The reason we swap to the "VM26" is entirely because of ease of getting all of the jets for it. Otherwise, there is no real benefit over the PZ30.

[bogieboy]

thats the lovely discrepancy of carb measurung of an oblong bore.... what Mikuni measures as a 26mm (because the actual area of the venturi would equal that of a 26mm circle) the chinese manufacturers call a 30mm carb, because the height of the carb opening when all the way open, and the bore past the venturi are 30 mm....

[HawkTwoFifty]

Originally Posted by Megadan

The VM26 clone you have has a 30mm throttle bore, which is the same as the stock PZ30. If you sit the two side by side you will see no distinguishable differences. The reason we swap to the "VM26" is entirely because of ease of getting all of the jets for it. Otherwise, there is no real benefit over the PZ30.

Thanks again Megadan for your reply and info, YOU ROCK!

Thanks again... for the dozenth time!

[HawkTwoFifty]

Originally Posted by bogieboy

thats the lovely discrepancy of carb measurung of an oblong bore.... what Mikuni measures as a 26mm (because the actual area of the venturi would equal that of a 26mm circle) the chinese manufacturers call a 30mm carb, because the height of the carb opening when all the way open, and the bore past the venturi are 30 mm....

Very interesting!! THANKS Bogieboy for your reply!

[HawkTwoFifty]

Originally Posted by bogieboy

thats the lovely discrepancy of carb measurung of an oblong bore.... what Mikuni measures as a 26mm (because the actual area of the venturi would equal that of a 26mm circle) the chinese manufacturers call a 30mm carb, because the height of the carb opening when all the way open, and the bore past the venturi are 30 mm....

Reading your reply a second time, I just thought about something... would I have more performance and top end speed having spent the extra money to get the more expensive "VM28 carb at https://www.amazon.com/Mikuni-Round-...uni+carburator for $80 over my VM26 carb at https://www.amazon.com/MIKUNI-VM26-C...ds=vm26+mikuni for $35?

I will happily spend more to get more top speed and performance. Would this VM28 in the link above be an advantage to my VM26?

Thanks!!!!!!!

[Ariel Red Hunter]

Originally Posted by HawkTwoFifty

Very interesting!! THANKS Bogieboy for your reply!

Motor cycle carburetors have always been rated by the venturi size. That's why it is called a VM-26. 26 mm venturi, on the diagonal. They could have used area (pi x [1/2 radius squared], but they went the easy way. The English called this "choke size" we call it throat size, or venturi...ARH

[Megadan]

To answer the question about a larger carb size being of benefit. In theory a 28mm venturi would be about a2 big as I would go given the displacement of the engine. However, given the stock head restrictions thanks to the ports, and low rpm range of the motor, it wouldn't be of much benefit. Just my opinion. There has been cg250 engines putting out 20hp using a 26mm venturi.

[bogieboy]

i agree with Megadan... theres not going to be enough airflow thru the head to support the added capability of the 28mm, this also presents bigger issues, in that there will be a larger vaccum drop when you crack the carb open, causing stumbling and poor low speed throttle response.... just think how a race car idles, and how they perform at low throttle inputs... not too great..... but wide open is where they want the performance, so they size the carb to flow the best wide open.... while sacrificing low speed driveability....

[Megadan]

Originally Posted by bogieboy

i agree with Megadan... theres not going to be enough airflow thru the head to support the added capability of the 28mm, this also presents bigger issues, in that there will be a larger vaccum drop when you crack the carb open, causing stumbling and poor low speed throttle response.... just think how a race car idles, and how they perform at low throttle inputs... not too great..... but wide open is where they want the performance, so they size the carb to flow the best wide open.... while sacrificing low speed driveability....

A 28mm venturi, or 28mm smooth bore carb wouldn't give up much on the low end, and really help on the top end of the rev range. That is about as large as I would go if low RPM power still matters somewhat. A 30mm is about the largest I would recommend going, and it will lose most of its low RPM throttle response, and without some head work be a complete waste of time.

I do plan on running a 30mm smooth bore after head porting, but I already expect the resulting loss of low end. Currently I have a PWK clone with a power jet, but I may also get my hands on a Mikuni VM30-83. if the flat slide behaves in a way I don't like. My backup plan is to drop back to a 28mm OKO/PWK carb. I kind of have a small collection of them now. All I need is a 34mm to round out my set.