CV carbs on commando

baz

baz

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Has anyone tried a CV carbs other than an SU on a commando?
I ran a single CV from Bank of carbs from an fj1200 on my 750
It ran well enough in the middle range but used to empty the float bowl above 90mph
I found this video interesting
Cheapo Chinese carb
 
Not sure why I cropped this photo so much, but that's a Commando head. I seem to recall that this setup would not clear the main frame spine tube. I hacksawed a pair of intakes, what a waste...

CVK01.JPG
 
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Performance depends on how fine the adjustment of mixture can be achieved. The main problem with a carburetor is compensating for loss of vacuum. A CV carburetor still has the needle attached to the slide, and might be more sensitive to vacuum changes. But the adjustment is still the taper on the needle. A quick taper needle will still richen quicker than a slow taper needle. A lot of people seem to feed the throttle on too quickly. You get more mixture but if it is richer, you get less power - depends on the taper. When guys race, it is common to get around the corner and stand the bike up before accelerating hard. It is much better to feed the throttle on from a long way back in a corner using a slow taper needle.
Racing a two-stroke is different from racing a four-stroke. It is more difficult to accelerate from a long way back in the corner with a peaky motor. Steering geometry is important
 
acotrel said:
A lot of people seem to feed the throttle on too quickly.
Click to expand...
CV carbs eliminate that issue, CV stands for Constant Velocity. You can snap open the butterfly valve but the slide raises independently only as the pressure above the diaphragm reduces, keeping the air going through the venturi at a constant speed so the slide itself cannot be snapped open.
 
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baz said:
Has anyone tried a CV carbs other than an SU on a commando?
I ran a single CV from Bank of carbs from an fj1200 on my 750
It ran well enough in the middle range but used to empty the float bowl above 90mph
I found this video interesting
Cheapo Chinese carb
Click to expand...

Out of interest I was going to put a Harley Sportster 44mm CV (Keihin) pumper on mine. I bought a second hand carb and a new manifold from MikuniOz. When I went to fit it, I realized that it would clash with the infamous plate between the frame rails, as the setup is longer than the S.U. I didn’t want to cut it out, so gave up on the idea. I sold on the manifold, but still have the fully refurbed carb if anyone is interested they can have it for £50 plus shipping, which is much less than I paid for it.


EDIT : The CV carb is now sold.
 
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cliffa said:
Out of interest I was going to put a Harley Sportster 44mm CV (Keihin) pumper on mine. I bought a second hand carb and a new manifold from MikuniOz. When I went to fit it, I realized that it would clash with the infamous plate between the frame rails, as the setup is longer than the S.U. I didn’t want to cut it out, so gave up on the idea. I sold on the manifold, but still have the fully refurbed carb if anyone is interested they can have it for £50 plus shipping, which is much less than I paid for it.
Click to expand...
My commando doesn't have that pesky gusset plate
But please don't tell anyone!
 
baz said:
My commando doesn't have that pesky gusset plate
But please don't tell anyone!
Click to expand...
The MOT inspectors over here are super picky, so If I had removed it, and I happened to get an inspector who knows Commando’s It would be an instant fail. (Thank god they have not noticed the extra ISO under the gearbox).:rolleyes:
 
cliffa said:
The MOT inspectors over here are super picky, so If I had removed it, and I happened to get an inspector who knows Commando’s It would be an instant fail. (Thank god they have not noticed the extra ISO under the gearbox).:rolleyes:
Click to expand...
Yep I remember the issue you had with your front brake upgrade
 
cliffa said:
The MOT inspectors over here are super picky, so If I had removed it, and I happened to get an inspector who knows Commando’s It would be an instant fail. (Thank god they have not noticed the extra ISO under the gearbox).:rolleyes:
Click to expand...
I didn't remove the gusset plate
It was a previous owner
The frame was also chrome plated !
Chrome frames are not good in the UK
Especially on a salty north circ road!!
 
Why does the MOT give you guys such a hard time when you upgrade the parts on an old motorcycle? I don’t see the rationale.
Thanks,
Mike
 
kommando said:
CV carbs eliminate that issue, CV stands for Constant Velocity. You can snap open the butterfly valve but the slide raises independently only as the pressure above the diaphragm reduces, keeping the air going through the venturi at a constant speed so the slide itself cannot be snapped open.
Click to expand...
Doesn't the height of the slide depend on the vacuum, and the slide carries the needle ? The mixture richness would still depend on the shape of the needle. For maximum torque through the whole rev range, the balance between comp. ratio, ignition advance and mixture needs to be maintained. A CV carb might be a better answer, but a computerised engine management system with fuel injection is probably far superior. It has often puzzled me, why an 850cc Commando is slower than many modern bikes of the same capacity. With Ducatis, an air-cooled four valve motor on methanol is about as quick as a water-cooled four valve motor on petrol. Bob Brown built the Ducati which Kevin Magee raced in Australia, before he went international. It was the first four-valve Ducati. I suggest the major difference between old and modern is probably the engine management system - not so much porting, cams or the number of valves.
 
With CV carburetors, is there a similar choice of needles to that which is available for VM Mikuni carbs ?
I have never seen a chart of needle options for Amal carbs, which is similar to the one for Mikunis.
Needle shape makes a big difference to performance.- If it richens the mixture too quickly as the throttle is opened, the bike is slower,
 
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YING said:
Why does the MOT give you guys such a hard time when you upgrade the parts on an old motorcycle? I don’t see the rationale.
Thanks,
Mike
Click to expand...
Three reasons come to mind. 1. They are not private facilities, but government testing centres. 2. They charge for a retest. 3. Some of the testers are a-holes. (You can be lucky or unlucky on the day).
 
YING said:
Why does the MOT give you guys such a hard time when you upgrade the parts on an old motorcycle? I don’t see the rationale.
Thanks,
Mike
Click to expand...

Off topic, but I bought a set of Wilwood four piston calipers with drilled and slotted rotors on aluminium carriers ($1700) for my TR4A never giving it much thought.
I found out after they arrived, with them having no dust seals they will most likely be rejected.
In saying that, the last roadworthy I got was where an inspection is done at your home. When it came time for the brake inspection (G meter?) the inspector used my GTI (Golf) to do that test.
Any future inspections will be done by that person.
#
Maybe a Mikuni BST 40mm carburettor (DR650SE) could be used on a Commando.
 
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cliffa said:
Three reasons come to mind. 1. They are not private facilities, but government testing centres. 2. They charge for a retest. 3. Some of the testers are a-holes. (You can be lucky or unlucky on the day).
Click to expand...
They are called Vehicle Standards Certificates here (certs), but only required for initial vehicle transfer. Thankfully, the inspection is performed in private garages... so it's common to have a licensed mechanic "pal" write you a cert for a case of beer ("cinquante" if you're in Quebec)
 
Time Warp said:
Off topic, but I bought a set of Wilwood four piston calipers with drilled and slotted rotors on aluminium carriers ($1700) for my TR4A never giving it much thought.
I found out after they arrived, with them having no dust seals they will most likely be rejected.

Maybe a Mikuni BST 40mm carburettor (DR650SE) could be used on a Commando.
Click to expand...
Or maybe a TM 40
 

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