View Full Version : Tune CVs or go to VMs?

01-04-2014, 9:22 PM
Hey guys,
The bike is a 78 XS500. I'm going for a sort of scrambler/tracker build with it. I only had one good day of riding on it and had a blast, but noticed a few things.

1. When it gets warm the idle races to about 3500, out of control.
2. Throttle response is slow.
3. Petcocks only ran the bike when in the prime position.

So far I'm planning:

1. Basic non-vacuum petcocks (on the way). I like to be in charge of things like fuel flow and throttle and the stockers aren't functioning properly.

2. Chopping the mufflers down - they start off about 3 feet long behind the headers, I'm going to leave about a foot with some sort of baffle and endcap.

3. Get carbs squared away:

Option 1: keep stock airbox and CV carbs
Needs: Jets for flowy exhaust, probably diaphragms @ ~$120

Option 2: ditch stock airbox and CV carbs
Needs: VMs and velocity stacks (with filters) @ ~$250

I'm not going to race this thing, just want to have fun without headaches. I don't have a whole lot of time or money and don't want to waste either.

Anyone have any advice or opinions?


01-04-2014, 9:42 PM
I got my bike running with the cv's and hated em they suck,dumped em and went for a joewiseguy setup and a vm36mm tuned to his recommendation but i haven't run it but everything i've read says the vm is better,78 so you still have bs38 correct?

01-05-2014, 7:40 PM
Thanks for the input. I'm assuming they're the BS38's like you said, but it's a couple hours away now and I can't check.

Still on the fence. I feel kind of bad giving up on the stock CV carbs, but after diaphragms and jet kits, I'd only be $100 or so away from new VM's.

Anyone make their CV's function ok on slightly flowier exhaust?

ETA: Grabbed a pair of VM34's with the throttle cable for $100 on the bay. They came off a snowmobile and have an oiler hose, but I'm guessing they'll work aside from that.

Now I'll need a pile of jets.

Anyone want to guess where I should start?

01-09-2014, 4:01 AM
just be careful i believe motorbike and snowmobile carbs are different but i cant remember why

01-26-2014, 5:44 PM
just be careful i believe motorbike and snowmobile carbs are different but i cant remember why

i believe the venturi's are too large or something like that. i forget now but its the same reason i bought the other vm's, maybe they are designed for a 2 stroke. let us know if you get them too work though.

01-27-2014, 7:15 PM
Thanks for the input guys. I grabbed them after seeing this thread:


Here's to hoping they know what they're talking about :cheersmate:

I'll update this thread if I come across any better info or when I give it a go.

Thanks again