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Replacing BS34SS with a pair of VMs

  • Thread starter Thread starter bonillan
  • Start date Start date
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bonillan

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I recently purchased a rust bucket of a GS450 for a very low price as a first bike and first project bike. I have a lot of experience in the automotive side rebuilding old Volvos, BMWs and Miatas but this is my first time wrenching on a bike. The fuel tank was and still is until I address it completely rusty inside with loose rust rattling around. I removed it and took off the stock carbs and float bowls which contained bits of rust that I guess it got through the fuel tank. Now I find myself with a dilemma do I bother keeping and rebuilding these old carbs or just get a pair of new VMs? I started adding up costs of all the parts I will need. Gaskets, orings, replacing seized rusty screws and it all adds up quickly not even accounting for the possibility the diaphragms might be shot and cost $150 each on cycle bandit. To me it looks like best case I can spend the same amount rebuilding the old carbs and still end up with old carbs with part availability problems or worst case spend much more than just replacing them.

So I have been eyeing the VM series. Through some research I know that 34mm would be far too large for a 450. 30 or 32 may be a better fit or even 28 but that's where I get stumped I can't find a definitive answer to the size question. I also have no idea where a good jetting starting point would be for this bike. I know it will take a lot of fiddling with different options to get it running right but then again so would refurbishing those old CV carbs. It doesn't help I'm a complete newbie to carburetors, I can fix and tune old complex Bosch Jettronic systems all day long but sit at the workbench stumped looking at carbs :)

Has anyone done the swap and have recommendations?
 
"It doesn't help I'm a complete newbie to carburetors, I can fix and tune old complex Bosch Jettronic systems all day long ...."
Stating that, I think that you'd be better off dealing with the CV's - if your current ones seem hopeless, look for another set. eBay, junkyard,or some member on here.
Have a look at this link to see what needs doing..... Done right the CV's perform nicely for typical rider.
 
I don't believe there is a definitive answer to the size question apart from the general rule that it matters. If you look at this guide for selection VM carbs , Mikuni show that at a nominal 20 PS per cylinder that any size from 22 to 28 will do the job.
http://www.mikuni.com/pdf/vmmanual.pdf Off hand the 400 and 425 were cv units as well so no help there.
You have to do a lot of trial and error to find the sweet spot for you. Even if you found someone who did this conversion what they found acceptable you might not.
I'm with Tom. Best put the effort into getting the stock set up working. If there is a few years of rust and more in there a proper rebuild should sort it. Something you would have to do with a used set of VMs anyway even before you started the experimentation with jets and needles.
 
Thanks for the advice guys. I will continue to tear them down this weekend. My main fear is those diaphragms being shot I cannot justify $113 x 2 on 10 cents worth of rubber. However if they are in good shape I will attempt to refurbish the stock units.
 
Check this link and look for a carb rebuild tutorial...http://members.dslextreme.com/users/bikecliff/ There are a few to choose from, including one dedicated to the 450.

The diaphragm's rarely fail. If they do, there is a very good chance another GSR member will have some to sell you for cheap.

Adapting VM's will be a time toilet. Fix what you got and come out miles ahead.
 
If you decide the stock BS carbs won't do, check out carbs for a GS500. They fit the 450 head, but the stock airbox won't fit. And you'll need the throttle cable from the 500 too.

As far as VM carbs go....I haven't seen anybody fit them to a 450 and document fitment / jetting. Try to find some info for how they work on a Honda CB450.
 
At the moment, VM's are the best $ for $ replacement IMO. 32's would be appropriate. Sudco can i believe supply prejetted reasonably closely. You'd need a split throttle cable too - VM's won't be linked, separate cable to each.
 
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