SU carb for moped?

[basil_brush]

In my researches on carbs I've read about the SU (or "constant depression") carbs that were used on many cars. It's said that they perform better than anything else at getting the right mixture and best fuel economy. I read that in Japan they've mostly abandoned standard slide carbs on their bikes and gone over to constant depression ones. They are said not to have as much punch as other carbs, but then again powerful acceleration is not something I need. Does anyone have any useful comments on SU carbs? Anyone know if they would perform as well on a 2 stroke as on a 4 stroke? Anyone know where I could buy one or has one they could sell me? I've only found one company that sells them so far, but those are for vintage bikes and don't have usual mounting flange like the common slide carbs - and they are a staggering £200! Ouch. There must be loads of them around, especially if the Japs have gone soppy over them, just can't seem to find any. Google and ebay just turn up stuff about car carbs rather than bike ones. Smallest SU I've found is 1.25 inch which is still way too big for my little 2 stroke.

[stinkwheel]

I know of only one person with an SU carb on his bike. It's fitted to an MZ250 single 2-stroke. It has a cable fitted to the throttle valve in place of the vacuum arrangement.

I seriously doubt a 50cc engine will generate enough vacuum to lift the throttle on an SU.

Pretty much any car carburettor will be an SU. Such as fitted to minis (original ones).

It is more usual to fit a CV (constant velocity) carb to a motorcycle. You could probably re-jet one off a Honda CD or CM 125 fairly easily (125cc twin with a single carb so the carb supplies a 62.5cc pot).

EDIT: For clarity, either the CDS or CM 125 was fitted with a CV carb, I forget which. The other had a slide carb.

Look up Wahl Philips Fuel injectors. Although you are unlikely to find one for sale, they made small bikes go like shit off a wet shovel (and consistantly fail to start).
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[kestrel]

Many Brit bike owners in the 70's converted to SU carbs because once set up they were a lot less troublesome than the old Amal Monoblocs, which suffered badly from slide wear, SU's were fitted to many cars and so they were readily available from the local scrappy and parts were cheap. The Triumph Thunderbird was originally fitted with Amals and in 1952 changed to SU as standard. I recently made a manifold to install a single SU on an XS650 project bike, fires up ok but as yet not set up for the road.

.

[smegballs]

[basil_brush]

Correct me if I'm mistaken, I gather that the main benefit of the SU type is that it responds to the demands of the engine; unlike the slide carb which responds to the demands of the driver. Seems like a good idea, so just wondered how it would fair on small engines such as mopeds.

[virus]

[smegballs]

i just had a look on pigeons nest and found it. i assume its the same guy.
Looks a bit complicated linkages for the time being, i think i'll sort the electrics before i
worry about a new carb

[Kickstart]

Hi

SU carb is basically a type of CV carb. Responds to air going through the carb to operate the slide with the needle on it.

Most large Japanese bikes used CV carbs for decades before the recent switch to fuel injection. Last large non limited production bike I can think of that had slide carbs was the original slab slide GSXR750 (been a few since then but mainly homologation specials, as slide carbs tend to give rather more power and are easier to tune).

Never seen a 50cc bike fitted with a CV carb, nor a production 2 stroke. And suspect that even if they would work in theory you would struggle to find one small enough (realistically I would suspect you would want something around 20mm / 0.8 inch).

Why do you want to use one?

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[basil_brush]

Just eternally curious. I was under the impression the SU were better at metering fuel than other carbs, including CV.

[Kickstart]

Hi

Well, they are basically a crude CV carb.

Not sure that on a 50 there would be anything like enough variation in throttle positions to make any real difference. Most 50s seem to be basically ridden with full throttle or no throttle.

All the best

Keith
____________________
Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[basil_brush]

Haha, very true actually, so not much point on a 50.

So I am correct in understanding that constant velocity is an improvement on the constant depression design, and that CV is the current state of the art for standard production machines?

[Kickstart]

Hi

Basic idea is that the flow of air through the carb causes a pressure drop, and this is fed to an area above the slide so that the slide lifts due to this pressure drop. Faster air = more pressure drop, so the slide lifts further. Hence basically the needle is positioned correctly for the combination of throttle position and revs.

CV = constant vacuum.

Not really state of the art. Doubt there has been any real development done on CV carbs for 20 years. Almost everything is injected, or cheap and using slide carbs.

All the best

Keith
____________________
Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing