Carbs: Cv's vs. Vm's

[jimfj]

I made an interesting observation over the last week and wanted to share and hear other opinions on the subject.

Background: I own an 82 GS850G with the CV carbs. I have done all the cleaning, dipping, colortuneing and o ring replacement on my carbs and have done the valves and several synchs. The bike runs well. Plugs are the color they should be..tan. I have put 4000 miles on the bike since May of this year. Bone stock.

2 weeks ago I purchased a 79 GS850G with the VM carbs. I and other KC Gsers have been working to get this bike running doing the same things as stated above. It is running very nicely now and has the stock airbox and a V&H 4 into one on it. I believe the jets are stock as well.

After riding both bikes, I have found that the 79 has better throttle response in the lower rpm ranges. The power just comes on smoother and the engine revs easier than the 82 with Cv's. Very crisp feeling all through the rev range. I thought this might be the pipe freeing things up and smoothing things out.

For comparison I rode another 79 850G that is stock and in good tune, and the same thing was true of the smoothness and eagerness to rev. Crisp as well. Kind of stifles the pipe theory.

My question to other Gs riders that may have had expirience with same model bikes, different carbs is "Am I expiriencing a difference in the way the carbs affect the performance or do I have a tuning/carb problem that needs to be addressed?" The bike is crisp above 4K and on to redline. At about 3000 the engine seems to run a tad "rough", crispness isn't there.

Opinions appreciated and ideas considered.

Jim

[Billy Ricks]

The VM's don't have the butterfly in the way and don't rely on vacuum to raise the slides the way the CV's do. Your right hand controls the slides on the VM's. Even on the '83 750 the hot mod at the time was to get a set of 29mm VM's.

[rudy]

CV's are never gonna be as crisp as VM's. Both my Yammies have CV's, and I can notice the lag, as little as there is. It takes a bit of time for the vacuum to raise the slide, whereas the VM's are instant. Just the nature of the carbs. I've noticed that with the CV's, they do tend to get better in the higher rev ranges. I would expect this is because of increased vacuum from more air movement.

[Nessism]

The cams are different on the CV carb bikes also - timing is set for low rpm grunt compared to the early cams. Not sure if this has anything to do with what you are noticing but it could be related.

[Dan Ruddock]

I love my 28mm VM pumper carbs. With accelerator pumps the response is instant. One day I noticed I had to use the choke to start it and it wasn't as crisp as usual and found the nut for the pump linkage had fallen off, what a difference. It's now back on and back to it's old self. Dan

[49er]

The difference in cam timing between the models will have some effect.
The biggest difference though is probably due to the inlet porting diameter differences. The larger port diameters on the CV heads and the bigger carb throat size, won't flow as efficiently at lower rpms as the smaller ported 79 heads with their VM26mm throats. It's certainly an advantage to have no butterfly valve and shaft causing turbulance throughout the mid to upper rev ranges.

[TheCafeKid]

I personally think it has alot to do with the very different design of the two types of carbs in general.

My theory, as I have been taught;

All carbs work off the vaccum of the motor in one way or another. They rely on the vaccum created in the cylinders to draw air in thru the carbs, and across the jets to draw fuel out of the bowls and into the combustion chamber.

Think of the VMs as a simple two barrel carburetor that youd find in an old car. While the rolled on throttle response seems instant, because you are in direct controll of the slide, which acts as the primary and single butterfly valve in those carbs, a sudden "wack" of the throttle on those particular carbs will cause the bike to bog out. The reason being is that the slide acts as the resistance to the motors ability to draw air, keeping the vaccum up in the cylinders. Wacking the throttle open suddenly allows the bike to draw as much air as it wants, and the vaccum is essentially eliminated. Without that vaccum, the bike doesnt draw more air, and there is no flow across the jets to pull fuel into the combustion chamber. So the bike bogs out for a second, and then its able to catch up, and suddenly it gets a massive amount of fuel and air and shoots off.

Now, Think of the CV type as a FOUR barrel carb, with a vaccum opperated secondary valve. Your throttle is directly linked to the set of butterfly valves in the carbs. When you roll the throttle on, the butterflies open in ratio. When the butterflies are open far enough, the resulting flow of air passed the bottom of the slides, which have a hole in the bottom of them, causes a low pressure area within the slide and diaphram, causing the slide to lift, and thereby allowing even more air flow to pass under it, causing it to lift more... And of course, the air flow is what pulls the fuel into the cumbustion chamber. Now, when you WACK the throttle open on the CV carbs, the butterflies will be completely open, but the slides have not yet started to lift, so there is still some resistance there, and the motor is less likely to dump vaccum and bog out. You'll feel some hesitation, but not that sudden blahhhhhh like you get when you slap open mechanical slide carbs. The hesitation will depend on how long it takes for the slides to start to lift, and allow more air and fuel to be picked up. Jet kits suggest to enlarge the hole at the bottom of the slide to allow the slide to be picked up more quickly, so that the increased jet size doesnt rich out the mix in the combustion chamber.

On your VM28SS "Pumpers' the purpose of the accelerator pump is to help eliminate that bog when you wack the throttle open by giving a small shot of gas down the throat of the carb to keep the revs on the motor up, inturn keeping the vaccum up....

[SqDancerLynn1]

I think the VM's are a lot less trouble than the CV's-- more forgiving

[TheCafeKid]

I think the VM's are a lot less trouble than the CV's-- more forgiving

I agree in many respects Lynn. Before Steve taught me in the ways of CV carbs, I found that the VMs made ALOT more sense to me as everything was purely mechanical. It was right there in front of you, and adjusting things made more sense. Syncing them, to me, is easier, as EACH carb has its own sync screw. The CVs share sync screws, and messing with number one changes number 2 and 3 and 4, and it can be very infuriating to sync them just so. At least in my experience. Also, on the VMs, you have a Fuel screw and an Air screw, and each adjusts exactly what it is. On the CVs you simply have a Mixture screw, which adjusts the amount of the preset mix of fuel and air getting to the pilot circuit. That can be a bit confusing until you understand it as well...

[Nessism]

Back when I had the two 550 (one CV and one VM equipped), the CV bike ran cleaner and had a more free running nature. The difference was not dramatic but noticeable. The CV bike had shorter gearing so that might have had something to do with it but no doubt in my view, the CV carbs are an improvement in driveability.

[dpep]

My question to other Gs riders that may have had expirience with same model bikes, different carbs is "Am I expiriencing a difference in the way the carbs affect the performance or do I have a tuning/carb problem that needs to be addressed?" The bike is crisp above 4K and on to redline. At about 3000 the engine seems to run a tad "rough", crispness isn't there.

Jim

All of the 850s I have owned have been 1980 or later so I can't answer your question directly, but for my bikes the power comes on like a rheostat across the rpm range though I admit I don't spend a lot of time at 3000 rpm and below.

In what gear are you when you notice the roughness? Every motorcycle I have owned seemed to have certain rpms where the harmonics where either more or less....well, harmonious. Cruising on the highway I usually can find a "sweet spot" that may be only a couple hundred rpm away from where there is noticeable vibration.

[Don-lo]

The best carburation I have experienced in a long time comes from the CV carbs on my 80 850G: smooth and responsive everywhere. I did carefully clean and dial them in. The biggest problem with CVs is that the diaphrams will eventually give out.

[jimfj]

First, thank you all for your replies and comments and keep them coming!

"In what gear are you when you notice the roughness? Every motorcycle I have owned seemed to have certain rpms where the harmonics where either more or less....well, harmonious."

I don't really have to be in gear for this to happen. Idling in the driveway from 2200 to 3300 will produce the feeling of vibration and a feeling almost like the engine suddenly went out of tune. Sound changes a little as well. Sort of like the engine needs a mix screw adjustment at this range. Idle is beautiful when warm. Upper ranges are wonderful. 75-78mph at 6.5-6.7 K is great.

Back when I had the two 550 (one CV and one VM equipped), the CV bike ran cleaner and had a more free running nature. The difference was not dramatic but noticeable. The CV bike had shorter gearing so that might have had something to do with it but no doubt in my view, the CV carbs are an improvement in driveability.

Nessism, this is the exact opposite. I thought the same thing about the Cv carbs. In our little group there are 2 850's and an 1100G with CV's and two 850s and a 1000 with Vm's. We are all learning very fast but for us it seems the Vm boys have us beat in smoothness and crispness.

Cafe Kid, thank you for the explination, that makes sense to me and hopefully will to others.

Just for clarification on this, I'm not expiriencing a lag, so to speak. Or if I am thats not the biggest difference.

Any ideas on slight modifications to increase the crispness in the low end and take advantage of the low end grunt that the 82 is cammed for?

Thanks again,
Jim

[trippivot]

I made an interesting observation over the last week and wanted to share and hear other opinions on the subject.

Background: I own an 82 GS850G with the CV carbs. I have done all the cleaning, dipping, colortuneing and o ring replacement on my carbs and have done the valves and several synchs. The bike runs well. Plugs are the color they should be..tan. I have put 4000 miles on the bike since May of this year. Bone stock.

2 weeks ago I purchased a 79 GS850G with the VM carbs. I and other KC Gsers have been working to get this bike running doing the same things as stated above. It is running very nicely now and has the stock airbox and a V&H 4 into one on it. I believe the jets are stock as well.

After riding both bikes, I have found that the 79 has better throttle response in the lower rpm ranges. The power just comes on smoother and the engine revs easier than the 82 with Cv's. Very crisp feeling all through the rev range. I thought this might be the pipe freeing things up and smoothing things out.

For comparison I rode another 79 850G that is stock and in good tune, and the same thing was true of the smoothness and eagerness to rev. Crisp as well. Kind of stifles the pipe theory.

My question to other Gs riders that may have had expirience with same model bikes, different carbs is "Am I expiriencing a difference in the way the carbs affect the performance or do I have a tuning/carb problem that needs to be addressed?" The bike is crisp above 4K and on to redline. At about 3000 the engine seems to run a tad "rough", crispness isn't there.

Opinions appreciated and ideas considered.

Jim

mechanical slide carbs are more performance oriented, with some drawbacks on economy and smooth power delivery. CV are better ,smoother, and get much better miles per gallon. they mix the fuel and air much better . for you to have a set of mechanicals that are better than a set of CV something is NOT right.