SU single carb kit?

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Apr 19, 2005
After Lilah (my '73 850 interstate) ingested bits of both Amal slides (a week apart), I'm thinking about a single carb kit. And as I like the unusual, I'm drawn to the SU carb. I've seen several commandos on the web that have SUs or are thinking about having SUs, but have found no specs or kits. Any help is greatly appreciated, and of course I'm open to suggestion.

Can't help with the SU question but If your really adventurous you may look into using a single Keihin CV. there's a ton of them on ebay for small money and I beleive they're 39 or 40mm. they are used on late model Harley Sportsters and being CV give good throtle response despite being fairly large bore. Fabricating the manifold will be a challenge as well as getting the linkage andjetting sorted but theyre a lot more available than SU's.


I have not fitted an SU but have done some home work. I believe the model of choice is an HIF6 1 3/4" choke size. This is a later model obtainable from a wrecker of old english relics of the 70's. Some of the bigger models from the BLMC range should have this carb. Reading an SU tuning book, this carb needs to have the anti-backfire apendage removed from the throttle butterfly, if a modicum of performance is required. These carbs are just right for the home tuner, as the needle is responsible for mixture control right thru the range. If you get really technical, you can make up your own needle profile with emery paper and a drill press, type in SU carbs and or Suggest you check Captain Nortons notes on the net.


Thanks all.

I do have access to a keihin CV carb, so that is definitely an option. The HIF6 is a welcomed step in the right direction and a great lead. I'm a little surprised at the 13/4inch throat as I seem to remember the single Mikuni having a smaller bore.

Now to find a manifold or two.
I believe the SU conversion was done years ago by a company in England called Phoenix. I don't believe they are still in business.
SU carb

I seem to have misplaced the magazine, but my girlfriend brought back a bike mag from a visit to England in '76. November/December time frame, I think. Cover story was about the preproduction '76/77? Norton. Complete story with test ride and changes to the bike. Only two I remember are Mag wheels and an SU carb. It was reported to have about 11% increase in power and about 10% better fuel milage. Test rider raved about the throttle response/power, if memory serves.
Styling change: all angles, no curves. Like the Japanese bikes.
My personal and shop experience, (no SU's), was that the twin Mikuni carbs were way better than the Amal's and single Mikuni setups. The bikes started easier, ran better at part throttle, and made more power wide open.
My experience with the twin Mikunis are that they flow pretty well, but if the bike sits for a couple weeks the floats tend to stick. I have a set on my hot rod 850 and a set of well fettled Amals on my Interstate. The sleeved Amals are no problem and idle nicely with no sign of hesitation. I'd really like to try some Dellorto pumpers or 35 mm Keihin FCRs, but for money and time. Anybody have any feedback on these?
Ron, the Dellorto's have very strong return springs due to the pumps being spring loaded. They are pushing up on the slide assy, hence the strong springs to control them. Typically, they come with throttle grip tubes that require about a half turn to open the carbs fully. Not fun. These carbs are tough on the wrist, no matter what throttle is used. I usually put a paddle grip on to help alleviate the strain. This is the square body carb. The round body pumps are different, but the effect is the same. Strangely enough, a lot of racers used to disable pumpers, even on Japanese bikes. Don't recall why, although fuel mileage will be affected. If you ever look down the bore of the round body while twisting the throttle, you will be astounded at the amount of fuel being tossed into the port!
Yes agree the dellorto's will increase your wrist strenght. I had a Duc with 40mm pumpers. Now racers sometime disable the pumps because they run bigger mains. The pump actually is there to allow a leaner overall mixture to be used without hessitation whilst accelerating. The amount of squirt going down the choke probably looks gruesome. But if you could poke your head down an Amal or other non pumper carb when it is working hard, my guess is there would be a lot of juice flowing also.
The fuel in an Amal is suppose to well up behing the spay tube, so when a drastic change in pressure occurs, acceleration, this upsets the fuel bubble and it shoot's down the spout.

Cheers Richard
Thanks, I'm aware of the spring pressure, I have 40 mm pumpers on my 900SS bevel. I know that the Ron Wood dirt trackers ran Dellortos and I have seen pictures of a couple street bikes with them, but was curious if any one had any real world experience on a Norton.

I am in the process of putting 41 mm Keihin pumpers on my rubber Ducky (900 SS SP). The reports of increased mid-range and top end are impressive. Can't wait till I get it back together. Again, there is a kit for 35 mm Keihins for a Norton and I have heard that their height makes them difficult to mount. Didn't the Dreer 880's use them? Any real world experiences?
Hi Ron, Yes, the Dreer has FCR 35's and yes I have real world experience. The frame has many mods, one being the center arched gusset below the backbone has been removed and additional gussetting done on the sides. This allows clearance for the air filter.

Sudco makes the kit. They are linked together on a common shaft with a push / pull throttle. The action is very smooth and precise with what I consider a moderate to light pull.

The carbs work very well for what they are intended. They are a racing carb. Things are a little different in the road / street world.

There is no enrichening device for cold starts. So one must prime with a few twists, then attempt to kick without moving the throttle AT ALL. Every little movement injects more fuel usually resulting in a flooded condition, This now requires WOT to start. It will start, minimum, 5 kicks.
Once it is warm, I am still learning when to prime and not to prime.
Yes, we are still getting to know each other.

These carbs combined with the quick action Motion Pro, YZF throttle make for a VERY responsive ride. But for every action, there is a reaction.
When droning down the Interstate and the pavement becomes a little ripply, it is hard to keep the throttle steady as any minute movement causes rpm change which causes lurching which seems to amplify with the ripples. Reminds me of my early dirt bike days trying to learn the Whoop-de Doos.

I have idle issues and it is a little soft from idle to 1/4 throttle, combined with big valves, big ports, big cam, Barnett clutch, tall first gear, stop light take offs are interesting.
Blast offs are not a problem, but civil take offs are.
I realize this may be more a reflection on the general tune of the bike than the carbs as I am working on some things and the carbs will be the last item I deal with.
But in the real world, stop light stalls with a kick start is NO fun.

Kenny worked very hard to get an e-start on his bikes and I can see why.

On a racing bike these aren't even issues. In the real world of road use, they are.

I like the carbs, life is just different.
I have got an 850 74 tweeked in with a UFO and a dialajet on a mic with a 1/4 turn throtel it is just so sweet and get 's 50 MPG . Like you say every bump in the road spins the rear, absolute hell in the gravel. But so much pleasure comming out of turns on smooth black top. Another problem is it's propensity to pop it's carb of it's rubber manifold on certain starts. norbsa Sorry for the spelling
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