Basic carb question

[Tommo]

I came across a set of VM32's for pretty short money and was wondering if they'd be fine to run on an RD motor.
Right now they're on an old 2T snowmobile motor.
If they'll work, what do I do about the intake boot?

[m in sc]

i mean, you can but the jet tube needle and all the bras need to be changed out. they make bigger intakes but if its a relatively stock bike theres zero benefit. 

[IR8D8R]

UPP polyurethane manifolds.
https://uppracing.com/index.php?main_page=index&cPath=14_15.

Price all the parts. If carbs are set up for a fuel pump you need gravity inlet valves. Maybe 3 sets of main jets, pilots, needles, and emulsion tubes. Plus (custom?) cables, air filters, gasket sets. Then figure out the choke (which is probably a cable too). Plus whatever the carbs cost you. Adds up to the price of 1 flat slide TM32 carburetor. At least... ($175 at Summit). Then you're starting from scratch to tune them. I had the same idea with some snowmobile VM-34s until a friend told me the same thing. Check the price of everything you need before trying it and compare to brand new TMs. You'll still need the UPP manifolds with TM32 (40mm spigot) unless you get carbs with spigots you can turn down to `36 mm or use the VM/TM28-418 with 33 mm spigot ($132 at Summit). Stock VM28 spigot is 35 mm.
 Stock rubber manifolds seem to be more resilient than the urethane. At least until the rubber hardens. Bad things happen if they crack.

IR8D8R

[m in sc]

to that point..w big carbs, support them and the manifolds stay happy. 

[Tommo]

Hm. I only ask because I found that VM28's seem to be hard to come by, and these are on an old '71 340cc snowmobile, a little googling shows one being started with a cord like an old lawnmower. A hundred bucks gets me the whole motor with the carbs and that's it, I don't know if this is more hassle than it's worth or not.
I figured, 2cyl 2T at 340 cc's, jetting must be close?
Or should I hold out for a set of 28's.

[m in sc]

totally different.  I mean, I've done it but I had to change everything. as said, the needle and seat seat diameter might be too small. 28sarwnt hard to come by, where are the original carbs?

[quocle603]

You need some stock carbs? I went to a motorcycle swap meet and there was a guy there with a tote full of stock vm28 carbs for rd350. I got a set of stock r5 carbs for 20 bucks from him. They are out there!

I should have a set or two if you need one.

[Tommo]

totally different.  I mean, I've done it but I had to change everything. as said, the needle and seat seat diameter might be too small. 28sarwnt hard to come by, where are the original carbs?

I'm putting together this bike from parts, because budget racer.
I got (foolishly) an RD motor sans carbs, which looked to have been stored underground somewhere and was only good for the six speed transmission and heads. An RD roller frame with decent wheels and forks, and a non running R5 that came with a pile of junk and a single VM28 and something like a 34 that wasn't worth anything because the slide was stuck. But the inside of the tank looks like new, so...
I'm giving myself six months to put this together, probably optimistic there, whether or not I make it doesn't matter. I have another bike I race but I was looking to get into a smaller displacement category next season.

[m in sc]

gotcha.  34s are goung to be too much on a stock motor.  they just are. if you can find another r5 carb, use those. they are already Dave f' modded, stock . I assume you're going to run premix as well so no need for rd carbs w the oil injection port. my 0.02

[Tommo]

Yeah, premix.  The exact premix I'm going to figure out literally last thing.  It's been about twelve years since I had an RD, and relearning all this is harder than I thought.

[quocle603]

Yeah, premix.  The exact premix I'm going to figure out literally last thing.  It's been about twelve years since I had an RD, and relearning all this is harder than I thought.

R5 carbs are your best bet. They don't come with the oil port on the carb. People will say 32:1 but you can get away with 40:1 with today's oil though. You got this!!!

[m in sc]

yup ^ . I set up mine to run at 40:1 on a very, very modified street bike and its been fine.  fwiw, summit racing sells new vm28s. economy cycle does as well but a bit pricey BUT also set up already. but r5 carb is the way to go imho.  note: ds7 carbs are 26mm so keep an eye out you don't buy one by accident