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VM carb rebuild guide now available!

  • Thread starter Thread starter ptm
  • Start date Start date
P

ptm

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Well, I guess it's finally ready to 'publish'. I started this last winter when I rebuilt the carbs on my '78 GS750, and I've been working on it off and on since. The '77-'79 VM carbs are enough different from the later ones that I thought a guide just for them would be usefull. I also wanted to get in some information for initial setup and tuning. There is a lot more I'd like to do with it - especially more pictures - but I figure it would be good to get out what I got so far before this winter's rebuilding season gets into full swing.

Thank you to everyone on the forum who has ever asked or answered a question about VM carbs. I read hundreds of threads that I've tried to distill down into this document.

Please let me know if you see any mistakes. I'll try to fix glaring errors quickly. Less serious ones will have to wait until Version 2.0.

Here's the link:
http://home.sprintmail.com/~pmusser/sitebuildercontent/sitebuilderfiles/vm_carb_rebuild.pdf

Also I've collected a number of links to Web sites with information about carb theory, rebuilding and tuning that you may find interesting reading during those long winter nights.

http://www.motocross.com/motoprof/moto/mcycle/carb101/carb101.html
http://www.dansmc.com/carbs.htm
http://www.dansmc.com/carbs2.htm
http://oldmanhonda.com/MC/Rcarbs.html
http://www.mikuni.com/pdf/vmmanual.pdf
http://www.mikuni.com/fs-tuning_guide.html
http://www.mikuni.com/fs-performance_guide.html
http://www.sudco.com/ see the diagrams section

Good luck, and have fun!
 
Most excellent work! As someone mentioned, forward to Bwringer for archiving.

~Adam
 
Awesome. I also vote to make sure Brian gets a copy for his web site. And for it to be added to the GSR garage section.
 
Great article, sure wish that was available when I did my carbs and it should be added to the GS Garage section. There's still lots of GS's out there with the VM carbs. One thing you may mention is to make sure the choke orifice in the float bowl allows fuel into the choke passage. If you've had a rusty tank or other fine debris that has gotten past the filter it can quite often plug that tiny jet in the bottom and be the cause of hard starting or only starting on some cylinders. This can be tested by setting an empty float bowl upright and putting some fuel in the float side and see of it seeps slowing into the choke passage or not.
Float.jpg


I also found a quick fix that now seems to be permanent for fixing the leaky rubber coated fuel tubes. Simply put a small chamfer on the carb body where the tube fits, put a small, good quality o-ring against the shoulder of the tube and reassembly the bodies onto the rack. As you tighten the carbs onto the rack the o-ring gets compressed into the chamfer and against the shoulder making a seal. Mine have been like that a year now and are still bone dry. Sorry about the quality of the editted photos but I was in rush this morning and thanks again for a great article. It looks like you really put a lot of time and love into it.
Body.jpg
 
Sandy said:
One thing you may mention is to make sure the choke orifice in the float bowl allows fuel into the choke passage. If you've had a rusty tank or other fine debris that has gotten past the filter it can quite often plug that tiny jet in the bottom and be the cause of hard starting or only starting on some cylinders. This can be tested by setting an empty float bowl upright and putting some fuel in the float side and see of it seeps slowing into the choke passage or not.

I also found a quick fix that now seems to be permanent for fixing the leaky rubber coated fuel tubes. Simply put a small chamfer on the carb body where the tube fits, put a small, good quality o-ring against the shoulder of the tube and reassembly the bodies onto the rack. As you tighten the carbs onto the rack the o-ring gets compressed into the chamfer and against the shoulder making a seal. Mine have been like that a year now and are still bone dry.
Click to expand...

Thanks for the additional tips. I'm starting to compile a list of things to add, expand or clarify - I'll be sure to put these in the next version.

I've asked Frank to put a copy in the Garage so it will be more accessible.
 
Hoomgar said:
You guys amaze me. Thanks Sandy for the fuel tube/o-ring tip :)
Click to expand...
You're welcome. As it's said "Necessity is the mother of invention". I was just getting ready to ride in the spring and the f#%*ing carbs started leaking just after I got everything dialed in. Phoned around and everybody wanted about $75.00US and a 4 - 6 week wait to build me some tubes, and they wanted me to send away mine as a sample. NO WAY!!! So I sat there for a while staring a my pile of parts when I must have been struck by a bolt of lightning. Found some o-rings at a buddies transmission shop and an hour and a half later I was back in business. Cost - zero.
 
Like I said in our PM's, great idea and thanks for taking the time to help others! :D
 
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