Anyone Using CV Carbs?

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sportster_restorer

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I've been following several posts about carbs. I was just wondering if anyone has any experience using CV type of carbs. One thought is adapting a single carb manifold to use the 40mm CV carbs common on Harleys. The Keihin 40mm CV can deliver over 100 HP on big twins and Buells, yet still carburate cleanly on a stock 883 (~50 hp). Also, there seem to be quite a few CV multi-carb sets of 32 or 34 mm on EBay for reasonable amounts. Possible take two of the 32s from a set of 4 from a Japanese bike and use dual CVs.

Harley 40mm CVs you can sometimes get for a song from guys going to S&S carbs

FYI, actually I've been lurking here since the spring when I got a 74 850. I had a 74 850 when I got out of college back in 1976 and loved it but hated the Boyer it had back then. Always wanted another one and found a decent priced model this spring.

You can see from my login most of my experience has been with Harleys, but always longed for that 850 I had back in the day. If only it had started more reliably, might have never gotten rid of it. Always thought is was the sexiest bike.

I'm overseas working right now, will try and post some pics of the bike later.
 
I'll be trying it soon, jeandr made me a set of intakes for these...

Anyone Using CV Carbs?


I was mocking up my '74 monoshock bike when I got the idea. Yes, they are half of a brace of 4 from a jap bike.
 
Some have used SU carbs which are CV type.
As for using a single CV carb frame clearance is always an issue. As I recall the Harley Kehin is a relatively big carb and i believe it would be a tight fit in spite of the throttle pull being located on the side.
The Commando engine has relatively narrow spacing between the intake ports so finding a rack of 2 or half a rack of 4 from a Japanese bike would be a challenge (although Paul's photo shows it can be done).

GB
 
I tried for 2 years to make a 40 mm Hardly CV carb work with a Drouin supercharger. It was a Dog. I Went to a 40mm Lectron by fast by gast and what a difference. Awsum carb, easy to tune, well made.
I have been wondering how a single Lectron carb would work with out the supercharger but on a 1 into 2 manifold. So I guess what I'm trying to say is skip the CV and get a late Lectron ( they are about $300 new),if you want to go this route.
Bruce MacGregor
 
prmurat said:
I was thinking of putting an older non CV Harley carb: they are small, worked well on Sporster and big twin of the 70' and early 80'...one cable, tons of parts and kits...and cheap to buy! Will need a Mikuni 2 in 1 manifold ans an adapter?!

http://www.ebay.com/itm/Kiehn-Harley-Ca ... 3cbf819867

Not as good as a CV, but as easy to pull...
Philippe
Click to expand...
Picked it up for 26$ and will give it a go. I like butterflies :) Hope I wasn't bidding against you.
 
The big problem with CV carbs (at least SU's) is their susceptibility to vibration. Youre relying on the piston that controls the mixture to work properly. On the Commando, the vibration of the engine/transmission module within the isolastics is pretty brutal. One of our mechanics at N-V tried a private mod, putting a pair of SU'sfrom an MG1100 car on his personal bike.

Disaster one! No matter what he did at idle, the carbs got completely screwed up by the engine pod's vibration at anything above about 1200 rpm.

i've been away from the industry details for nigh on 50 years, so maybe modern CV carbs don't use the same principles as the old SU's. How about EFI?
 
Frank, Phoenix sold a single SU carb kit for Commandos, for quite a few years. Several tests of them appeared, they seemed positive, and seem to recall that 60 mpg was mentioned with normal riding - although acceleration was a bit more leisurely to achieve that. So was possible.
Anyone tried them ?

Jim Comstock here had a CV on a big manifold, and seemed to like it.
 
Considering that many of the rubber mounted Harley's used the 40 mm CV, and those engines would visibly bounce around at idle, the CVs don't seem affected that much by vibration. The one on my Buell works great. Think I'd love to find a used 2 into 1 manifold and see about getting the inlet welded up and a larger opening made. Perhaps this might give close to best of all worlds, single carb simplicity but still regain some of the top end power.
 
sportster_restorer said:
I was just wondering if anyone has any experience using CV type of carbs.
Click to expand...

I know a guy here in Germany (regular poster on a German forum) who is running a Bing carb from a BMW on his Atlas and claims it's performing well. Might also be an alternative.



Tim
 
years ago I fitted a Phoenix SU kit to my Commando, best thing I ever did, My mate saw how good it was and bought one to, Phoenix said I would get 83 MPG and thats exactly what I got, so did my mate also, I wonder what happened to that company.
 
Honestly, I'm surprised, particularly with regard to SU carbs. My colleague concluded that frothing of the fuel and bouncing of the needle piston were what was screwing things up, as his SU's were bolted to the head. I suppose that if the carbs are on the frame and there's a flexible connection to the engine that can absorb its movement, then they could be an improvement over the stock Amals. Honestly, the Amals aren't much of an improvement over dribbling the fuel in from a rag, sometimes.

These days, digital EFI is the way to go. Those systems can compensate for so may things that they're amazing. If you ride at significant altitudes or widely varying temperatures, go EFI.

The other caveat is unique to the pair he used. I had an MG1100 after we came to the US and had to rebuild the engine. They were 7/8" chokes, I think, and the inter-carb linkage would have done Heath Robinson and Rube Goldberg proud. Fortunately I knw enough never to seaprate them!
 
frankdamp said:
My colleague concluded that frothing of the fuel and bouncing of the needle piston were what was screwing things up, as his SU's were bolted to the head. I suppose that if the carbs are on the frame and there's a flexible connection to the engine that can absorb its movement, then they could be an improvement over the stock Amals.
Click to expand...

The thing is that whenever there is first-hand info available on SUs the feedback generally is very good. They obviously are a good alternative if you can live with the loss of top end power. I have a Phoenix kit at home which might find its way on the 750. The carb bolts to a manifold - in which you can literally see the horses stumble and suffer - and the manifolds bolt to the head, all solid. AFAIK the SU kits for my other make are also solid mount but I haven't looked into carbs that much as the Mikuni on the Glide is just working fine.

These days, digital EFI is the way to go. Those systems can compensate for so may things that they're amazing. If you ride at significant altitudes or widely varying temperatures, go EFI.
Click to expand...

Well, that is a completely different story and not so much a technical one: There is a stage where improving a Commando leads inevitably to buying a R1 or GSX-R .... :mrgreen:

I personally like the idea of doing all sorts of mods to the old bikes and an ECU certainly is on the list of things I'd love to do. However I can perfectly understand if somebody would like to keep at least the basic technology relatively contemporary.


Tim
 
I ran an SU on my 750 fastback and it was generally a good ride with excellent pickup and solid dependable tickover. The problem with them is starting. You need a good head of fuel to make them suck using a kickstart. Electric start probably a lot better. (or a fuel pump) I was sad to see it go but my right leg was becoming deformed. Now running a single mikuni until my JS flatslides arrive.
 
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