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Carburetor conundrum

  • Thread starter Thread starter sbrink97007
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sbrink97007

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me again, and still confused about carbs. i have read through the forums and found mentions of this information: BS series, VM ?series? , CV (constan velocity), round slide. my root question is "What is a constant velocity carburetor?". followed by how is that different from a round slide carburetor? what is the difference between a BS series and a VM series?

i have a 1978 gs400
it has the carbs that are referenced for that model on bike bandit.

also, on my bike there are 2 cables from the throttle that work a rocker between my 2 carbs.

the VM series carbs appear to have a different set up (1 cable from the ?top? of the carb).

is there any reason to chuck my old carbs in favor of the new vm series round slide carbs? There is some weeping from round the drain plug (?new gasket?) and around the float bowl (new gasket) as well as there is some sticking on one of my throttle cables (?lubricate?). But really I am looking for more performance (via the information from the old Clymer manual(see my other post)).


thanks for all your help on this
 
I'm no carb expert, others can probably shed more light than I can, but here's my $.02...

All the Suzuki GS bike (except the GS400) used VM (non-constant velocity) carbs through 1979. Starting in 1980 they all used the BS (constant velocity) carbs. The GS400 actually used the later BS carbs from the beginning. The switch was made to help comply with emissions regulations that were starting to phase in for motorcycles in the late 70s and early 80s.

I believe both models came with single or dual throttle cables.

I think the VM carbs have slightly better throttle response, at the cost of more emissions.

As far as your 400, leave it the way it came from the factory. If the throttle cables are sticking, make sure they're lubed, and make sure they're adjusted properly. If one is too tight, it binds, sometimes enough so the spring can't return it to idle.

If you want more performance, I'd get a bigger bike. There are big bore kits, and higher flow air filters and exhaust systems and carb jet kits for the bigger bikes, especially the 750, 1000 and 1100, but not much available these days for the 400.

Here are some more links for carb info and theory:

http://www.motocross.com/motoprof/mo...1/carb101.html
http://www.dansmc.com/carbs.htm
http://www.dansmc.com/carbs2.htm
http://oldmanhonda.com/MC/Rcarbs.html
http://www.mikuni.com/pdf/vmmanual.pdf
http://www.mikuni.com/fs-tuning_guide.html
http://www.mikuni.com/fs-performance_guide.html
http://www.sudco.com/ see the diagrams section
 
CV carbs use a rubber diphragm in the top of the carb to control the attitude of the slide based on the amount of vaccum created in the intake. The more air that is requested from the airbox, the higher the slide goes. A small port in the intake manifold of the carb body goes to the diaphragm and fills it like a balloon when the engine spinns faster. The advantage is that the slide only raises the needle as high as it needs to for the air that is entering the engine. This makes for better fuel economy and less emmisions becasue you can't feed more fuel into the engine than air. The disadvantage is slower throttle response, and why many CV carb owners who modify thier bikes for performance go with other carbs.

I've been meaning to scan and post a few pages from my shop manual that have huge cross sections of the CV carbs and exactly how each circuit works. I'm sure anyone who has the factory manual knows what I am talking about. Nothing has ever helped me understand carbs better than that.

I'll try and scan those pages tonight.
 
Thank you PTM & Jethro,

i wish it was just as easy as "get a bigger bike" because i would, but ..., i have to make incremental changes to what i have. The big bore kit for the 2 valve 750 is the same big bore kit for the 400, and wiseco sells the pistons for that individually (on order :)), mac makes a 2 - 1 exhaust (will buy), and then lastly is this the carbs and air filter pods.

i now understand that i have "BS Series" Constant Velocity carbs because i have the rubber diaphrams on the top of the slide (that are round :-s) and the throttle control rotates a butterfly(?) valve inside the carb. This in turn allows for more air which inflates that diaphram causing the slide to be pulled up. This allows more fuel in response to the more air.

i believe, from the diagrams and the vm_carb_rebuild.pdf, that there is not a butterfly valve in the "VM Series" Round Slide carb. The slide itself, incorporates a "cut-away" that controls air flow and needle height. In the "VM Series" carb the throttle cable physically pulls that slide up measuring air flow and fuel flow.
 
I think you are on the path to enlightenment my friend!
 
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