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CV or VM, which is best? :confused:

  • Thread starter Thread starter SeanGSUK
  • Start date Start date
S

SeanGSUK

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I know this must have been asked 10,000 times but I have spent almost an hour searching and can't find anything. So, I have '78 GS550E (main bike) and a '80 GS550L complete bike for spares.
Is it worth upgrading to the CV carbs? I thought they must be better as they replaced the VM22s?
I have the inlet rubbers etc, choke cable, etc, etc, so is it worth doing? VMs seem a little erratic to start/bogs down even though I had them professionally set up.
 
Uh oh. Get prepared.
I'll give you my opinion, but I admit to being biased.
My first GS was a 1978 GS750E. I liked the performance on the road, but I was used to two strokes (RD350) where I could whack open the throttle and wheelie down the road. Not so with the VM carbs, they just bogged down and eventually started working. Sold the bike within a year and got another RD350.
The CVs do not have that problem, but they have issues of their own. For example, if you change the intake or exhaust (my GPz has both mods) you must install a special jetting/needle kit. My bike has a DynoJet kit, which I tuned myself using a wideband O2 system. Using the plug chop method is time consuming and not very accurate. The VM carbs do not require the kit, but they do require rejetting if you make any mods.
To sum it up; both work, both have issues.
I also read that with some you have to swap the heads along with the carbs due to different sizes of each type.
 
I'd stick with the VMs unless you ride where there are large elevation changes, then the CVs are worth their weight in gold. Almost.
 
If you are in the proper gear and have the RPM's up you can whack the VM's wide opened all day long and get instant response. If you do this at lower RPM's and crack the throttle wide opened they are going to bog and/or take their time catching up. The CV carbs are a compromise, but excellent for extreme altitude changes. Cracking them wide opened in any situation does not have the negative aspects of doing the same to the VM's, but they will never have the instant response of the VM's used correctly. Don't expect to drive from sea level to Denver without having to stop and re-tune or re-jet your VM carburetors, depending on how you have them set up.
 
You can put an excelerator pump on the VM's to get a quicker throttle response (not that I've done it..yet). I tell you one thing the CV's are easier to tune, but the VM's are more tunable. That can be a good thing or a bad thing based on what you like. I like the way the VM's look better than the CV's if that matters...what am I saying, of course that matters! And very true about the VM's response at 5000 and up, instant:)
 
Depends on your riding habits. CV carbs will always (when tuned proper) have the correct flow of air/fuel mixture by using vacuum to control the venture size for the best flow.

VM carbs have mechanical slides that "you" control with the throttle.

I love the CV carb as the operation/response is smoother and work well with larger cc 4-strokes. As mentioned VMs are great for 2-strokes because they have the ability to use more gas faster, they intake every 2 strokes instead of 4, and get the RPMs up faster from spinning half the weight.

As far as tuning, the CV carb is harder to tune, but is more tunable then a VM. The VM carb can be throttled around poor jetting as you control the air/fuel flow. Where as with a CV the spring holding the slide down against negative pressure controls it.

With a 550cc I myself would stick with the VM carbs as they are offered in smaller sizes, a smaller carb will always be better then a carb too big. As well as the 550 is lacking the torque to pull through a CV carbs initial snap of the throttle.

That's just my 2 cents.
 
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Still think it's worth a try with the CVs, I guess I can always swop back. I am swopping out my Motad 4 into 1 back to the 4 into 2, I will try the carbs then as the CVs were on another bike with those exhausts.

I'll update as I go one. Wont be for a couple of weeks though.
 
The CV's are most likely the better all around carburetor for touring where you might expect large elevation changes, but It's not an upgrade. It's hard to beat a properly tuned VM carburetor unless you are going from sea level to mile high Denver.
 
OldVet66 said:
It's hard to beat a properly tuned VM carburetor unless you are going from sea level to mile high Denver.
Click to expand...

Mile high Denver is about the lowest I might ride, 14,000 ft is the highest. Working on an all CV and FI fleet, I'm almost there.

VMs suck around here.
 
I have VM carbs on my 1000 and CV carbs on the 1100.
I personally prefer the VM carbs.
And even though parts are getting hard to find for the VM carbs, especially the choke parts for the GS1000, the diaphragms for the CV carbs cost $100 each. :eek:

I do believe the '80 GS1000 quarter mile time dropped to 12.4 with the CV carbs added that model year as the '78 GS1000 was 11.78.
 
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