SU carb conversion

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Jun 1, 2007
Just throwing a topic out there for discussion- Has anyone or does anyone use an SU carb on their Norton?? Ive seen the kits and read the reviews, but would prefer the jury on this forum to set the issue straight. What are the spacial issues as far as any mods being made for fitment? What's the benefit if any? I had them on my old E-type Jag--easy to tune and repair, but I had a country mile of room around them to work with.
I had dual SUs on my MG 1100. Man, what a PITA they were. Because of the east-west engine orientation, the carbs were between engine and firewall and nearly impossible to get at. The were so small (13/16 choke, I think!) that they were too close together to get one of those balancer gadgets in.

They have died in the US because of emission rules. They're very high emitters of unburned hydrocarbons because of the way they handle transients.

The worst example I have encountered was the Japanese copy (Keihin?) installed in a twin-cerb set-up on a Datsun 260Z my daughter had. It was a lastgasp at trying to get this type of carb to meet the emission rules of the time. There was a double-skinned inlet manifold with coolant piped through it, then each carb had a coolant heated base around the jet that the needle operates in.

They do give enhanced performance, but the amount of messing about you have to do isn't worth it, IMO.
My 750 had an SU on it when I bought it (and a 24t gearbox sprocket) but I replaced it with Amals when rebuilding it so I can't claim any riding knowledge.

I believe from the experience of others that it is possible to get them to run nicely (and very economically) on Commandos but it takes a lot of work and a good supply of needles.

Once they're set up, there is the probem that the pulsing from the Norton engine seems to empty the dash-pots quickly and the cap for the pot sits almost tight against the frame spine (I have heard of owners drilling and tapping a side feed into the dash pot but never seen this mod).
And the Norton SU kits haven't been available for a long while, they were originally produced by Bernard Hooper (Hooper Engineering).
Another thing I rememberes is that the high vibration level of the engine/transmission assembly that is allowed by the Isolastic system causes frothing of the gasoline in the float chambers. It might also contribute to the oil loss from the dashpots.
I ran a SU on my 850 Interstate for a while, very economical (it was the kit supplied by Hooper) 75 - 80 MPG pretty amazing, some loss of power for roll on overtaking. It could sometimes be a pain to start if the gas level in the tank was low (never looked into why) I took it of after a run up from York back to Scotland on the A1 into a strong headwind at around 90mph (I had a hangover and was desperate to get home) caused it to nip up, luckily I've experienced piston seizures before (used to race TZ Yamahas) so managed to whip the clutch in quick kill the engine and coast to a halt without too much damage, I let the bike cool down then rode home more carefully. After that I changed to a Single Mikuni which gets 45 MPG, now that gas in the UK is getting so expensive I've dug out the SU again and considering refitting it!! :D
If you do go ahead and fit the SU and you can't get a richer needle try a slightly stronger spring, or a little stretch might do, I think the stronger ones are yellow. Use 20/50 in the dashpot to ensure good pickup, thin oil leans off the rich squirt that's needed.
On later SUs the needle and jet wears out like an Amal. When the tickover is adjusted to compensate for the wear the rest of the needle runs lean.
Love SU's stuck them on all kinds 'cept a Norton.

I've been running an SU conversion from Pheonix, which was Hooper's company, for several years now with no problems. It has lots of pull down low like a single carb but runs like twin Amals when you open it up. I believe it is 44mm. Lately I have been having a surge at constant throttle openings and suspect that the needle might be wearing? I'd like to sort it as I do like the 75 plus mpg.
Interesting stuff with auto carbs on the old Commando. Here in Australia, in the late 70's, someone made a manifold adapter for a Weber DCOE (twin choke) that apparently worked amazingly well, probably due to the numerous adjustment steps in these carbs.

These kits were very expensive and also made available for the later parallel 760 Bonnies and then just faded away.

I guess if you had the time and inclination, it would be a fun project to make one as sourcing Weber DCOE carbs is pretty easy and lots of tune up shops have expertise in them.

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