VM Carb tuning

rvRoyce · Posted: April 12, 2007, 8:09 am Post subject: VM Carb tuning

After finally getting the bike to run well enough to ride with an electronic ignition (turns out that the advance springs were stuffed) I am moving on to the tuning of the carbs.
The bike came with a new set of 34mm roundslide carbs .
Can anyone give me a step by step guide to tuning these?

Also the old carbs had crossmembers holding them together. The new ones are just clamped on and not joined to each other. Is this right?

royfisk · Posted: April 12, 2007, 3:46 pm Post subject: Re: VM Carb tuning

yes thats right you need a special cable for the vms that splits into 2 cables which will need to synchronised. As for jetting I too am looking for a starting point for a used set that I have. need to know needle# and height starting point, main jet and idle circuit. 650 central wants $25,00 for the info even if your buying the parts from them. If anyone can shed some light to the subject that would save a few $$ on jets. my engine is stock with 34s and K&N filters

5twins · Full Member Joined: Aug 05, 2006 Posts: 1616

deleted

rvRoyce · Posted: April 13, 2007, 3:50 am Post subject: Re: VM Carb tuning

royfisk I have the split cable but the previous owner installed a new grip with 2 seperate cables. Works OK but I might try and install the split cable.
Thanks 5twins. Very usefull stuff.

royfisk · Posted: April 13, 2007, 8:25 am Post subject: Re: VM Carb tuning

either way works, the earlier bikes came from the factory with two cables. Of course they are different ends on the cables. Both synchronise simularly. I guess its time to dissmantel mine and see where I am at now and go from there.

rvRoyce · Posted: April 13, 2007, 8:53 am Post subject: Re: VM Carb tuning

After reading some of the posts 5twins suggested I checked to see where the carbs had been purchased. You can guess that it was from Sportingforless.
I thought I was safe here in Australia. Looks like a strip down to check the jets, slide and needle

Cafe_Shop · Posted: April 13, 2007, 9:53 am Post subject: Re: VM Carb tuning

But can you blame them for going to sporting for less since it appears that 650 Central has dropped off the face of the planet? I've decided to hold off on my carb acquisition until there is another face on the scene.

royfisk · Posted: April 13, 2007, 10:07 am Post subject: Re: VM Carb tuning

If we can get information on a base starting area with part#s such as 650 central has we could probably get mike to make up jet kits as a package. Which then it would not matter whom we purchased the carbs from

Jack · Full Member Joined: Mar 15, 2005 Posts: 411

Michael Morse is a good guy and will bend over backwards to help you,how would you like be pestered with jetting questions and not make a sale. I think the 25.00 should be dropped if you purchase a proper jetting kit though. You might wanna try carb parts warehouse out of Ohio......Jack

royfisk · Posted: April 14, 2007, 10:14 am Post subject: Re: VM Carb tuning

Jack your right; I just think that he would be better off selling a base line jet kit, to whatever he thought would work on a stock motor with k&N filters. Then there is a starting point. from that point on would be tunning information. I wouldnt even put in installation directions. Just the jets and appropriate needles. Then from there its $25.00 for tunning info. Heck at that point he already has a sale. I would do it on 34s,36s, and 38s.

RainLand · Posted: May 22, 2007, 4:03 pm Post subject: Re: VM Carb tuning

After all this pain and suffering, I've decided to tune the VMs to a
motor that"s running before applying it to my project motor. I
guess I'm about to own a bunch of jets.

xs1961 · Posted: May 22, 2007, 4:24 pm Post subject: Re: VM Carb tuning

My three ,650,750,840 are all running 34mm's& have been for many years, i've kept a list of the jets & setting I'm using if that helps

INXS650 · 650Rider Supporter Joined: Mar 22, 2007 Posts: 645

Not to sound like a moron, but how do you tell which mains you have installed? There is a number stamped on mine which is 200. If this is the size, I need to cut it way back right? To something like a 145 or 150? Stock bike with the 36mm round slides.

xsjohn · Posted: May 22, 2007, 4:36 pm Post subject: Re: VM Carb tuning

INXS650......Sorry was thinking about stock 34's......

John Underwood

INXS650 · 650Rider Supporter Joined: Mar 22, 2007 Posts: 645

So is the number on the bottom the jet size?

xsjohn · Posted: May 22, 2007, 5:17 pm Post subject: Re: VM Carb tuning

exactly......
john

INXS650 · 650Rider Supporter Joined: Mar 22, 2007 Posts: 645

Thanks John. I guess I need to go way smaller. That is part of the problem getting a bike that someone else has "fixed". You don't know what you got until you start poking around. The only way I knew was that the carb started leaking, and had to take it apart to clean. Must've been one of those from sporting for less deals.

INXS650 · 650Rider Supporter Joined: Mar 22, 2007 Posts: 645

Another question for the peanut gallery. Do I need to change the needles when I go to a smaller jet?

xsjohn · Posted: May 22, 2007, 5:45 pm Post subject: Re: VM Carb tuning

Wouldn't unless they were change also...Doubt they were changed....Most people don't mess with changing them....They think big jet go faster..duh..

John

INXS650 · 650Rider Supporter Joined: Mar 22, 2007 Posts: 645

I am assuming the motor can only use so much fuel. It's kinda like putting a Holley 850 double pumper on a stock 6 cyl. It'll work, but not too good.

xsjohn · Posted: May 22, 2007, 6:07 pm Post subject: Re: VM Carb tuning

Too rich can hurt things too.....

John

Xumi · Posted: May 22, 2007, 6:10 pm Post subject: Re: VM Carb tuning

INXS650 wrote:

I am assuming the motor can only use so much fuel. It's kinda like putting a Holley 850 double pumper on a stock 6 cyl. It'll work, but not too good.

A bit simplified way to look at it.. but somewhat accurate. The motor is basically a giant air pump. As it draws more air, the carb's job is to ALLOW fuel to flow into that air. The jet's job is to meter how much fuel goes into that air at a given throttle range (Pilot jet for idle to 1/4, needle jet and needle taper/position for 1/4 through 3/4, main from 3/4 to WOT). If the jet is too large, then you end up with a rich fuel/air mix, which doesn't burn optimally. Also take into account that the fuel does NOT stop flowing from the other jets, so it is cumulative. If you change the pilot jet, it richens up everything. Change the needle it changes from 1/4 on up. Same with the main jet, but more affect at WOT.

xsjohn · Posted: May 22, 2007, 6:32 pm Post subject: Re: VM Carb tuning

Simplified....hello rich mixture goodby top end......hello lean mixture goodby top end.....

John

ttmaniac · Full Member Joined: Dec 22, 2006 Posts: 468

Just a touch of richness on the bottom end....hello awesome top end. Better to be a hair rich than a hair lean is what I say. Don't wanna cook it. By the time I hit 5000 rpms its right how I want it which is leaning out a bit to ideal top end conditions (tan plug). And throttle response is excellent. I am a hair rich on the bottom and still have good throttle response. The plugs will go black if I run 5th gear at 45 mph. I got it set for the rock n roll mode.

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