SU Carb Conversion?

Back in 1992 when I was first finding out had bad off my Newly purchased Norton really was and beginning to pull bits off for rebuilding, I responded to an ad in a magazine for an SU carb conversion. I do not recall where the ad was, but the kit was made by Phoenix Motorcycles Ltd. in England and came with an SU HIF 6 carb, manifold, throttle cable, air cleaner mount & a foam sock air cleaner. The claims for it were something along the lines of more power, better milage and better running and I had high hopes for the conversion. The factory was supposedly going to fit them to the new models that never made it to production.

This project ended up taking about 8 years off and on to finish and when the SU was finally fitted to the Commando, some issues cropped up. First the throttle linkage was poorly designed requiring a very sharp bend in the cable to work. This would need reworking! Worse was to come . . .

When the kit was installed on the motor with its new Mk III isolastic conversion, there was almost no room between the vacuum dome on the carb and the frame web plate:





After puzzling on this for a time it seemed that the lack of clearance was an insurmountable problem. I did not know how much the motor would rock fore and aft, but it was probably more than the 1/16 clearance I had. Machining the manifold down did not look too promising either, there was not a lot of metal there to work with. Reluctantly, the SU was replaced with a single Mikuni which worked out rather well.

Still, I like the idea of the SU and today dragged it out of the storage cabinet and shot these pictures:













Has anyone had any experience with the SU carbs on a Norton? Is it worth pursuing this conversion now?

Vintage Paul

The only reports i've read were them converting away from SU d/t sluggish response.

Looks interesting, what size are they Paul. ?
Any chance of making up a drop down manifold to help the height issue.
Would need a bigger Air cleaner I think, praps shorten the Ram tube, to get a longer one.?
AC.

I ran the same kit for a few years, great MPG, slight loss of power especially when roll on overtaking. Thinking of re fitting it again and persevering to get it better, some guys swear by them. I reckon at least 70MPG is possible if you keep it below 75MPH. I run mostly Mikuni's now, better fuel consumtion than the twin AMALs and not much if any loss of power.
With the price of fuel the SU has to be worth a try again I suppose.

I recall that the clearance for the 'dashpot' at the top of the SU was/is always an issue and some claim that the clearance can be partly down to frame-manufacturing tolerances. Most reports claim improved MPG but some loss of performance.

I've run this conversion on my 850 roadster for many years now, probably the best mod I've made to the bike, I can almost do unmodified interstate mileage ! Yes clearance is a bit tight, mine actually squeezes the gromit that the loom runs through but after nearly 30,000 miles has caused no problems. I dumped the original foam air filter in favour of a K&N that fits straight on & have to top up the dash pot with 20/50 now & then (a syringe & length of washer tube saves a lot of hassle for this) but thats it, never needs any other attention. Nobody beleives the mpg you can acheive, I reckon 70-80 mpg as an average & far from a loss of performance I think mid range is improved, might lose a smidge at the top but I never go there anyway & idle is always reliable. I did find starting on a low tank a bit of a problem at first but found my petrol taps a bit restrictive so opened the holes through them to provide a better head of fuel which cured that one. I cant recomend it enough ! infact, I've just finished rebuilding another Commando, this time an 850 interstate, guess what carb I've fitted ! Oh, & just to qualify, I have run Commandos on standard set up, mk2 amals, both single & twin & also Mikunis so have had first hand experiance of all. Mikuni would deffinately be my second choice !

There was a lot of discussion at N-V about switching to SU, but we had other things on our plates as the company was starting to unravel. The main concern was with the high vibration level on the engine assembly causing the SU metering piston to bounce around too much. I suppose the much heavier oil that Tim uses in the piston dashpot solved that issue. It used to be light sewing-machine oil (5 weight or less) in SU's on cars.

Could you not chamfer the manifold a bit on both mating faces, so that the carb is angled down a fraction, a few degrees might make all the difference to the fit .

I remember SU carbs fondly from my MG & Triumph days. They had a variable venturi size that allowed them to adjust their size according to need and rarely caused any trouble.

Hobot - Was the sluggish response to the throttle inputs or in acceleration?

Aussie Combat - A crony who has done a number of Jag projects pronounced this one to be 1 1/2. If I proceed with this conversion, I'll measure it to confirm.

Gino & mwoo - Was the loss of performance just at the top end of the rev range?

TimG - This is a very hopeful report. Does this modern ethenol fuel bother anything in the SU? Are these kits still available? Is the throttle linkage worked out better than the 1992 version which was very crude? Can you please post a few pics of your setup if it is different from mine?

frankdamp - What advantages & disadvantages did N-V encounter with the conversion? Did N-V make the manifold or buy it from an outside vendor?

dave M - Yes, there is some room for careful machining but not a lot. The inlet tract already looks quite restricted, this would only get worse. How much clearance should I be looking for on the frame/dashpot? How much does a Commando engine rock fore & aft on its Mk III isolastics?

I really like SUs and would like to complete this conversion. The Mikuni works fine, but the SU seems to offer more for those of us who spend little time at WOT.

Vintage Paul

Hobot - Was the sluggish response to the throttle inputs or in acceleration?

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Duh I don't know why or if could be over come some how, just less responsive that plain carbs is all I read. These have been tried on the Drouins and dropped I think for lack of response expected. Bruce McGregor removed his vacuum throttle carb off his Drouin 850 I think for similar reasons. I know when I hopped up my 406cid V8 in big ole van I got better hits with wiring the throttle butterflys together on Quadrajets and Holley 4 brls than letting engine vacuum wait on doing it. This did require more throttle control by me not to bog it down on takes offs but wasn't very hard to do to get more pull than soft vacuum operation. Now if were taking mileage mastering then vacuum throttle rules and if not racing sounds wonderful to me for old man commuter thats still a nice thriller.

SUs are a great carb and correctly set should give good throttle response and midrange. I'm pretty certain SUs fitted with a black damper top should be used with engine oil whereas the rare white top is for thin oil. If oil thinner than engine oil is used with a black top, on snapping the throttle open the carb throttle piston ( I can't remember what it's called) will fly up, the venturi effect lost and acceleration suffers.
We got a lot of HP out of the old A series motors on twin SUs

SU recommend 20 or 30 straight or 20/50 oil.

Best of luck.

Cash

hobot said:

I know when I hopped up my 406cid V8 in big ole van I got better hits with wiring the throttle butterflys together on Quadrajets and Holley 4 brls than letting engine vacuum wait on doing it. This did require more throttle control by me not to bog it down on takes offs but wasn't very hard to do to get more pull than soft vacuum operation.

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The throttle on an SU is still manually operated. The dashpot makes the venturi itself get larger or smaller. You get the advantages of a big carb when you need it and a small carb at other times. SUs are not the only carbs to work this way. I have a 1926 Hudson with a carb that works just the same way but for the oil damped dashpot. Hudson used a large brass bit that moved more slowly.

Vintage Paul

cash said:

I'm pretty certain SUs fitted with a black damper top should be used with engine oil whereas the rare white top is for thin oil. If oil thinner than engine oil is used with a black top, on snapping the throttle open the carb throttle piston ( I can't remember what it's called) will fly up, the venturi effect lost and acceleration suffers.
We got a lot of HP out of the old A series motors on twin SUs

SU recommend 20 or 30 straight or 20/50 oil.

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Back in the 1960s, we got the idea from reading the manuals that the damper oil needed to be light. I used to fill the dashpots on my old 1969 MGB every week with 5w, more often if on a trip. Maybe it would have stayed longer if a heavier oil had been used. I seem to recall that different weights were called for for different states of tune. I like the idea of using the same 20-50w in the carb as in the rest of the bike.

Vintage Paul

I've often wondered whether the Stromberg would be a better bet than the SU, due to it's much lower height? I've never seen any reference to this in connection with Commando's but just a thought,

Dave.

daveparry said:

I've often wondered whether the Stromberg would be a better bet than the SU, due to it's much lower height? I've never seen any reference to this in connection with Commando's but just a thought,

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The only Stombergs we got were the awful smog models that were not worth the powder to blow them to hell. They crippled a whole generation of British sports cars just as the Japanese were finding their feet. I sure hope they were better elsewhere!

Vintage Paul

I had several cars in the sixties and sevnties with Stromberg carbs on and never had any issues with them. Variable choke, similar to SU but a diaphragm instead of dashpot and piston, the diaphragms could deteriorate after a time though,

Dave.

Are these kits still available?

TimG - This is a very hopeful report. Does this modern ethenol fuel bother anything in the SU? Are these kits still available? Is the throttle linkage worked out better than the 1992 version which was very crude? Can you please post a few pics of your setup if it is different from mine?

Haven't noticed any problems with ethanol & haven't heard any reports of it affecting SU's. The kits are sadly no longer available, Pheonix dissapered several years ago but spares are available for the SU carb. My throttle linkage looks much the same as yours except the arm that the cable attaches to, yours looks to be fairly parrallel with the butterfly spindle, on both of mine it's cranked away from the spindle a few degrees & the outer end then set accordingly to remain in line with the carb. Hope that makes sence. Essentially, the bends in my opperating arms are about 75 degrees where as yours look about 90 degrees, this increases the distance between the cable pivot & the spindle giving a fairly straigt cable pull. I'll try to get some pics for you but looks like I'll have to remove the tank, so when I've a few moments spare.
With regard to the carb size, it should be 1.75 inch, yes, it does seem huge for a little 828cc motor, but it works ! Don't worry too much about the clearance problem, as I said, mine is also very tight but I've had no probs, There's hardly any fore & aft movement, just up & down & there's room for that. I've experimented with different weight oils in the dash pot & found 20/50 to be best, & it's whats recomended, thin oil just vanishes in no time & thicker oils do take the edge off acceleration. I think multigrade copes with the high temp better also. I get a small flat spot when the dash pot needs topping up, about every 800-1000 miles so not a big issue, especially as I have now figured out how to do it without removing the tank. I have some Pheonix tech data & setting up instructions I could scan & email to you if you dont already have them

Regards, Tim

With SU carb almost don't matter their size as the vacuum level determines throttle and/or venturi opening so may never really hit WOT if engine can't draw it open on its own. Wonder if an accelerator pump would give best of both worlds, instant spunk with great cruise mileage.

Greetings,
The way I see it is you could spend time messing around with an SU carb, but both Mikuni and Keihin manufacture a number of CV carbs that are probably not only cheaper, but better fitting and easier to get parts for, some available with accerlerator pumps.

GB