turning vm 32s into 34s

[two-stroke-brit]

is it possible/practical to bore a vm32 carb out to a 34.
or would you break into any of the passages etc?.
thanks mark

[tz375]

According to other people a 28 can go out to 29.5 and I think that 34s can be bored to 35.5, so a 32 can probably be bored to 33.3.

My notes seem to be telling me that a slide for a VM32 is 35.4mm diameter, so if we allowed say 1mm each side to avoid air leakage past the slide, that would allow it to be bored to 33.4mm. You can't go and just bore it concentric. They have to be offset bored so it takes just a skim off the floor and takes the rest off the top. The slide may need to be shortened to clear the bore.

In the case of a bored 34, supposedly they flow better than a 36 but I have never read anything about how well they jet after machining. Jusdt try to avoid spending more on machining that you would pay for a replacement set of carbs.

[ja-moo]

Put in UFOs that will just about equalize the difference.....

[two-stroke-brit]

thanks for the pointers .
just kicking ideas around in ma heed.
cheers mark

[tgrogan]

Be certain that you are taking about 'VM's. The stock 750 carbs are not really VMs, but simply stock carbs. They do not work as well as real VMs. You would do best to start with real 34mm VMs and decide what to do after that. Boring is an option, but you have to limit it to what will retain idling capability if you intend to use it on the street. Probably not worth it for the small gain. If you can find 3 of them, a better approach might be 35mm TZ350 carbs.

[tz375]

Terry, That's an interesting post. You mention that the VM32 carbs are not real VM carbs and do not work as well as real VM carbs. Other than the fact that they are OEM and maybe slightly different in some way, in what way are they different and how does that impact their performance? Are some of the airways or fuel passages different, in the way that say RD Yamaha carbs are different to those in generic VM carbs, or something else?

BTW, my TZ350 came with 34mm carbs and as AFIK, they are teh same as any other VM34SS carbs. What are the differences we should be looking for here?

We are on the same page about not spending more on them that a set of replacement 34s

[tgrogan]

I have about 20 stock 32mm carbs from T500s and GT750s. I also have several sets of VM carbs that were bought 'directly' from Mikuni. The major telling point is the difference in the main jets. The stock carbs use round jets and true VMs use hex jets. My reason for saying that there is a huge performance difference is from the experience of using them - not from any specific measurement. It only takes a quick look into them to see some differences in the finish and shapes. To me the telling difference is in the metal used - the stock ones are dull and VMs are bright. There seems to be a weight difference too, but not enough to be a deal breaker.

WRT the TZ carbs, I have to go with some of the information passed on to me from the previous owners. Don't know if you remember a lady by the name of Wendy Epstein who raced superbike and TZs way back in the 70s, but that is who I got the carbs from. (Later she rued the time she switched to Lectrons.) She reportedly got them from Freddy Spencer who did come from the same part of the country. They said that they were bored out 35mm carbs, but they could have been originally 34mm. I do know that they must be made of magnisium or some such material since they are way lighter than any similar real VMs. If you took the slides out they would almost float. ;>) They are not capable of idling in any way. I'ld have to measure to be sure, but I believe that they are 37.5mm now. I do know that they made my T500 outrun just about anything in a straight line - from 3rd gear on, including GS750/1000s and all but one Kaw 750 2 stroke.

If you would like I can box them up and send them to you for you to examine. They are just wasting away in my parts stash.

[tz375]

I'd be delighted to examine them.

I think that the round versus hex jetting issue relates to the needle jets. On a VM34, they are 159 series and take a large hex main jet. Stock/OEM VM32 use a 188 series which uses round mains IIRC.

Funny thing about OEM carbs is that they are often the same as aftermarket supply even though they are stamped differently. For example different models of TZ have different numbers stamped on them and yet they are identical. Other OEM carbs are very model specific. Material is usually different though as you mention. OEM were often Mazac (sp?) a zinc based pot metal, where aftermarket were usually aluminum for the general market and magnesium for the factory race teams with big budgets.

VM34 can usually be bored to 35.5mm

[tz375]

Terry was good enough to send me a few different carbs to compare.

Thanks Terry.

Stock OEM crabs are made out of Mazac (sp?) a zinc based material and they are really heavy.

Although they look very similar to CM series lightweight aluminum carbs, they are a little different. For example they use a common/shared air bleed rather than two separate circuits and that slightly complicates jetting.

Air and fuel passages are all different as is bowl venting and overflow.

I'm sure it's quite easy to just change out jets and needles to correct for small changes such as a pipe, but they are not a performance carb. If you want more performance I'd buy a set of VM34 or larger carbs and jet them to suit. I personally wouldn't spend too much time trying to make OEM carbs work.

Several guys on this board use OEM zinc carbs with different jets and I often wonder how much better they would work with "real" carbs.

[husson73]

Not mazac but zamac
http://en.wikipedia.org/wiki/Zamak" onclick="window.open(this.href);return false;

[tz375]

I grew up with motorcycles in the UK and that was where I heard the name MAZAC which is what AMAL used. I didn't know where the name came from. Thanks for that interesting link.


I just remembered that Mikuni manufactured carbs under a license agreement with Amal, so it's anyone's guess what material they made them out of let alone what it was called.

[two-stroke-brit]

[quote="tz375"]I grew up with motorcycles in the UK and that was where I heard the name MAZAC which is what AMAL used. I didn't know where the name came from. Thanks for that interesting link.


+1 on that.