S.U carb question

Does anyone out there run his Commando with a single SU carburator ? which type of SU is prefered for the 850 commando, HS4, HS6 or other ?
Also how to set up this carb, (needle/jet types etc).
I would like to hear what your experiences are with this type of carburator.

Thanks, Peter.

Looks like no is the answer to this one. Good luck.Try the new fuel injection instead and post a review.

I think I've seen pics of SUs on Nortons, I just can't remember where.

Might be tough to search on this site as the two letters might not work, but you can Google and get some stuff.

carb-conversion-t2404.html

Arizona Speed and Sport sold SU kits for the Nortons for many years. It was their own development. They quit selling them many years ago and although I have not personally used one of their kits I always heard they ran very well. I don't know where you would find any info anymore. Jim

HS4, i did som research a few years ago. I found nothing but good things about the SU carb. I'll have to dig up the settings. From what I remember it was just a needle and a spring, and a bunch of magic. I remember amazing mileage out of it too.

PM Gino Rondelli I'm sure he ran an SU for a few years.
I reckon the prefered needle is a little too weak judging by the very good mpg reported.
Cash

If you're an NOC member, try contacting Dave Fenner. He experimented a lot some years ago with the SU and came up with some very definite ideas about settings.

I remember a kit named Phoenix?? I am not sure a SU will perform really well: the depression created by a twin would be not constant enough to keep the carb piston from bouncing up and down... I tried a SU on a FXR shovelhead years go and was not happy with the results! I always wanted to adapt a HD carb on my Commando (vacuum model or not) but never got around doung it!
Philippe

In their day, SUs were excellent carburettors but mainly used on cars, not so much on bikes. I converted several European Fords to SU carbs in the 1980s to improve their fuel economy. I would suggest that a single Amal with chrome brass slide would be the best single carb option because they are easy to get hold of and the set-up is straightforward because so many people have done it before.

The SU works well on parallel twins,Triumph had one as standard fit on a 650 and the very last Mk3 Commando was to have an SU.

Engine oil in the dash pot helps control the piston, but, perhaps LAB has a point with 360 cranks and single carbs :lol:

Cash

Brian Pollitts experience of using an SU on a BSA Rocket 3

http://www.lightning-spares.co.uk/Tunin ... 20carb.htm

comnoz said:

Arizona Speed and Sport sold SU kits for the Nortons for many years. It was their own development.

Click to expand...

prmurat said:

I remember a kit named Phoenix??

Click to expand...

I don't know about the "Arizona" kit, but an SU carb was certainly adapted for use on the 1976 Commando prototype-the "Norton 76" by Bernard Hooper (Hooper Engineering) who then sold the Phoenix Commando SU carb kits after NVT went bust, but the Phoenix kit certainly hasn't been available for some years now.

If I was considering the heresy of doing away with the original pair of Amals, which I am not/would not, I'd be looking hard at a set of flat-slides.

The SU on a Norton always intrigued me. The trouble is there is very little space. It looks like if you can fit it the manifold would have to be extremly short. The sharp bends required would likely effect the top end. I find a single Mikuni 34 mm doesn't hurt the top end much on my 850, but the 850 is not really a high reving bike anyway.

There is a lot of info about it here, inclucing pictures of the Phoenix kit

http://nortonownersclub.org/support/tec ... tors-other

That is the Norton Owners Club.

There is a lot of info about it here, inclucing pictures of the Phoenix kit

http://nortonownersclub.org/support/tec ... tors-other

That is the Norton Owners Club.[/quote]

Veri interesting reading...I've not heard the words Brit'Iron forum foe a long time!!! Too bad it had to come with the BS of the infamous Charles Lipton!!
Philippe

As Cash rightly says I ran a Phoenix set up for a few years, first inpressions were that it was brilliant, superb MPG (75mpg easily attainable) remember thats UK gallons though. Biggest PITA was starting sometimes, it seemed to need a fair amount of petrol in the tank to start easily, sometimes it was fine. One trip to the Begonia Rally in Belgium I left Perth, Scotland rode to hull (350 miles) went to the rally (not too far in Belgium) and only filled up when I got back to the UK, around 450 miles I think (I've got the earlier bigger 6+ gallon tank)
I ditched it for a while after I siezed travelling up the A1 at 90mph into a strong headwind with a mega hangover after a Nottingham branch camping weekend. It was far too lean, switched to a 34mm Mikuni and never looked back....until I wore the Mikuni out 50K miles later, dug out the SU and spent a couple of months playing around with it again (last year) but as I was heading to a very hot Austria for the International Rally I chickened out and got myself a 36mm Mikuni and consigned the SU to the drawer again.
My own opinion is that they are a bit of a PITA but return incredible MPG, if you want to "make progress" over 70mph then I think that no matter what they run lean especially when its hot, topping up the dashpot is also a right PITA.
As pertol prices start to get ridiculous again here in Europe I'm tempted to fit it to one of my other Commandos again, if you need particular details I can dig it out (when it gets warmer) and check out the needle etc
Remember that the rotaries used SU's so they can't be all bad, posibly just me trying to run it for max fuel economy.

just me trying to run it for max fuel economy.

Gino,

Spoken like a true Scot

All the best to all
Cash

Seems a real shame that no one seems to have bothered to properly set up an SU carb to work properly on Brit twins! I get the impression that those selling SU conversion kits simply supplied then with whatever needle and spring came with the carb, and made no effort to do proper testing and fit the proper parts.

In the past I've had to deal with SU carbs on MG's and for their time they were a good solution. That said, I wouldn't try to put one on a Commando for a couple of reasons, first keeping the dash pot filled w/ oil would be a real pain in the butt since you would probably need to remove the gas tank to do it. Secondly they were designed to operate in an automobile and may not be able to operate effectively in the much harsher vibration and temperatures motorcycles live in. Lack of tuning knowledge might be another deterrent when it comes to motorcycles.

The big advantage an SU has is that its a constant velocity carburetor that allows over carbureattion but still prevents the drop off the cliff power when you whack the throttle open.It also could give better fuel mileage since it only allows the slide to move up in direct relationship to the air flow. This was an excellent engineering approach at least until fuel injection was perfected. Many of the pre FI Japanese superbikes used CV Keihins to allow massive oversized carbs on them to maintain rideability and some semblance of fuel economy and air quality.

A few years ago I picked up a Sportster CV carb for $25 with the idea of seeing if it could be used on a Commando. The carb is a stock Keihin with what appears to be a 40mm bore. The Keihin uses a diaphragm to modulate the slide movement versus the piston (or slide) in the SU, seems to be not a lot larger than a MK II or Mikuni and was used on a motorcycle. The biggest problem aside from getting the jetting right and tuning seemed to be designing a manifold that wold be compact enough to fit the assembly into the space allowed. Other things got in the way so I never went further with this but it seemed like a good idea since there are a huge number of these carbs out there. Every HD guy that bought a Sportster soon realized how frighteninlgy slow the bike was and immediately went to their dealer to drop a couple thou more on the "Screaming Eagle" kit. I would have thought the screaming part came from the poor sucker that just dropped $10k or more on a HD and now had to pony up another $2k to out run a Vespa. :lol:

Scooter

Is there anyone here who has one of those Phoenix manifolds? What do they look like on the inside. How bad are the bends in the inlet tract. It would be really interesting to see a picture.
It is pretty obvious that these can work well. They worked well on lots of British cars. Often those cars had one carb feeding just two cylinders. Often those cylinders were side by side in a four cylinder so that would be equivalent to a twin with a 180 crank, almost the worst possible combination you would think. A lot of people did get them to work really well on Harleys too.

The great thing about these carbs is that all the fuel goes through one huge hole. It is almost impossible for it to get plugged. About the only thing that would eventually go wrong was the throttle shaft and bushing.

I would love to have try set up on my 850.

Nigel