Mikuni vm34

[75xscafe]

Ok guys, just picked up a 79 xs650.... The guy I got it from bought it from a friend with the usual misinformation.. He said it has a 750 cc big bore kit on it but I don't see a bigger cylinder? Does this make sense? Other than that the guy put on stock carbs and said they needed to get rebuilt to make the bike run. In a plastic box he had 2 mikuni vm 34 carbs he said he took off because a random bike mechanic said they were crap?the bike doesn't run now, but I'm just trying to make sense of it all. How do I know what to look for for the model carbs on the bike, and to see if it is really a 750cc big bore kit?

 

[yamaman]

Dont worry over the capacity, it is whatever it is. The only way to tell is pull it down, there are no external differences. find and read the carb guide, it will tell you all you need to know about the BS38 carbs that came std with your bike. The VMs are a great carb, but need to be set up right for a XS. My advice would be get the 38s up and running and maybe later get the 34s up, or if the 38s come up fine, sell the 34s. Good luck and welcome to the site, cheers

Posted via Mobile

 

[bbillington]

See what jetting the VMs have as that will give an idea of whether they were ever even run on that bike.

 

[XSLeo]

On the engine being a 750, until recently the way you did it was to rmove the sleeves from a stock cylinder, bore out the cylinder to except larger sleeves and use bigger pistons.
Depending on the piston the compression ratio will be higher, up to 10-1.
One thing to check would be a compression test. On a stock engine the compression will be around 145, my 750 engine with 9.1-1 gets around 195. 10-1 will be higher.
That's an indicater but pulling it apart and measuring is the best way.
As Yamaman said, don't worry about it now. Read the carb guide. there is also a VM carb guide, www.amckayltd.com/vm34-36.pdf this has links to the Sudco manual.
Leo