SU Carb Conversion?

Back in the 1960s, we got the idea from reading the manuals that the damper oil needed to be light.

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Absolutely true Hudson29, however it was some tuners who started that ball rolling, SU always recommended light engine oil (mono days). We fell into that trap until we started started to time and record the car's performance.--- A big eye opener.
Cash

PS The SU HIF 6 I'm sure has temperature sensitive jet and you'll find it'll be a right bugger to set up. It's actuated by a bi-metal disc you might be able to remove or jam it.

Cash

PS The SU HIF 6 I'm sure has temperature sensitive jet and you'll find it'll be a right bugger to set up. It's actuated by a bi-metal disc you might be able to remove or jam it.

Cash

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Neither of my SU's have temperature sensitive jets although I do know they were fitted in some car applications for emissions reasons. The jets in mine are just normal HIF type adjusted by a screw on the side of the carb & dead easy to set up.
Tim

geo46er said:

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

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You might be right, but good comparative info is hard to come by. In my particular case I happen to have one of these kits on hand from 20 years ago. Its worth looking into using it.

Vintage Paul

TimG said:

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.

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Thanks Tim, pictures would be a help. The linkage on my kit will not work long as designed. Also, I would love to see how you fill the dashpots without removing the tank. The easier this is to do, the more often it is likely to be done!

Vintage Paul

Many years ago, I had an MG 1100 sedan which was the hot version of the Austin/Morris version and had twin SU's. IIRC, they were either 3/4 or 7/8" bore. The inter-carb linkage we something Heath Robinson would have been ecstatic about (Rube Goldberg for Americans).

I did an engine rebuild on that thing and I tried to wire the two carbs together before I took them off. Once they were off the whole linkage fell apart. I've found 5000-piece jigsw puzzles that were easier to figure out.

I finally got the 1100 running again, but I decided to sell it, before we moved from Seattle to Hampton, Virginia, to save myself any more grief.

The SU kit looks very interesting, and I am sure with a different intake manifold design, the clearance issues could be resolved very easily. SU carbs always used to be quality parts but now they are made by the same people that make Amal carbs, so not sure on how good they are now?

There is never likely to be any chance of SU kits being marketed today, as these would be superior to the Amal being sold by the same manufacturers, and likely to impact on the sales of the far cheaper to produce Amal carbs (read higher profits!).

I think it was in the 70's that Commandos could have the SU carb conversion, used to hear of them now & again.
Amals & SU's are British & suit a British bike.
A friend used to have an SU on his Royal Enfield twin, seemed alright, very light on the throttle.
Weber would be good if you could set the carburation, but less MPG & more MPH.

Last time I spoke to Burlens they were looking at introducing their own SU conversion for Commandos. They used to be close partners with SU and took over the company when SU closed in the early 1990s. They supplied the carburetters for the Phoenix kit and they still supply SUs for Harleys. If you spoke to them you would find they are big time classic car and bike enthusiasts - part of the conversation revolved around how soon they could acquire a Commando and who would get to do the riding!! E-mail them if you think re-introducing an SU conversion would be a good idea - it will only encourage them!!

How many carb kits for classic bikes do Burlen supply at the moment? I got the impression they didnt have the facilities or know how needed to development and test this type of thing, and simply supplied OE equipment type carbs jetted and set up as originally?

Interested to hear if they do supply complete kits though, as I would think there would be a lot of interest in a well designed and properly developed SU kit for Brit twins.

I think Dave M is correct, a little machining could improve the clearance issue a lot, however, I would remove about 1/16" from each face of the manifold without changing the angle, this way the carb would be 1/8" closer to the engine which would give you enough clearance.

Jean

Jeandr said:

I think Dave M is correct, a little machining could improve the clearance issue a lot, however, I would remove about 1/16" from each face of the manifold without changing the angle, this way the carb would be 1/8" closer to the engine which would give you enough clearance.

Jean

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This is most likely the best solution. It would give a bit more clearance and that may do the trick. I certainly have nothing to loose now other than time. The throttle linkage is going to need a redesign, it was pretty poorly thought out as it came.

The question I had when I started the thread was IF it was worth bothering with. As one person has pointed out, there are other carb conversions on the market that are working well and may be better than this SU. I'm still unsure what the answer to that question is. It might be worth trying if the machine shop costs are not too steep and the linkage improvements not too difficult. I do have a sentimental fondness for SUs.

Vintage Paul, still pondering

hudson29 said:

Jeandr said:

I think Dave M is correct, a little machining could improve the clearance issue a lot, however, I would remove about 1/16" from each face of the manifold without changing the angle, this way the carb would be 1/8" closer to the engine which would give you enough clearance.

Jean

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This is most likely the best solution. It would give a bit more clearance and that may do the trick. I certainly have nothing to loose now other than time. The throttle linkage is going to need a redesign, it was pretty poorly thought out as it came.

The question I had when I started the thread was IF it was worth bothering with. As one person has pointed out, there are other carb conversions on the market that are working well and may be better than this SU. I'm still unsure what the answer to that question is. It might be worth trying if the machine shop costs are not too steep and the linkage improvements not too difficult. I do have a sentimental fondness for SUs.

Vintage Paul, still pondering

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Machining a little off both faces of the manifold & retaining the original angles should be ok if you realy need more clearance,dont take too much off the head side as the center is already quite thin & don't be tempted to alter the angle, especially to lower the filter end as this will result in the suction chamber contactng the frame webb. As I said earlier, you realy don't need much clearance & looking at your pics, you have plenty !!
The throttle linkage will be fine with a tweak of the opperating lever, when I can work out how to do it I'll post some pics of mine, as I said, I have done almost 30,000 miles on this set up & have had NO problems !
Topping up the dashpot without removing the tank is something I have learned along the way, there is enough room to unscrew the damper rod & push it to one side, then, with a small syringe & length of tube I basicaly fill to overflow then draw back on the syringe to suck out the excess, Easy !
As you have the kit why not give it a try ? I wonder how many of the naysayers have had experience with this conversion ? To answer your original question, In my experience, it's well worth "bothering with", I'd not use anything else.
Regards, Tim

TimG said:

Machining a little off both faces of the manifold & retaining the original angles should be ok if you realy need more clearance,dont take too much off the head side as the center is already quite thin & don't be tempted to alter the angle, especially to lower the filter end as this will result in the suction chamber contactng the frame webb. As I said earlier, you realy don't need much clearance & looking at your pics, you have plenty !!
The throttle linkage will be fine with a tweak of the opperating lever, when I can work out how to do it I'll post some pics of mine, as I said, I have done almost 30,000 miles on this set up & have had NO problems !
Topping up the dashpot without removing the tank is something I have learned along the way, there is enough room to unscrew the damper rod & push it to one side, then, with a small syringe & length of tube I basicaly fill to overflow then draw back on the syringe to suck out the excess, Easy !
As you have the kit why not give it a try ? I wonder how many of the naysayers have had experience with this conversion ? To answer your original question, In my experience, it's well worth "bothering with", I'd not use anything else.

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Yes, the engine side of the inlet manifold is very thin. Now that you point it out, I recall it as one of the big reasons I did not feel machining the surfaces of the manifold would give enough clearance. Possibly a trim off the carb side alone might open it up some. Just looking at the pics of the installation, I can't really tell how much room was there. These pics were old school film negs and I scanned prints to get the shots shown. I might have to take the Mikuni off and put the SU back on the accurately measure things up.

Photos of your setup would be very helpful!

Vintage Paul

A pal who is a better searcher than I am found this thread about the SU carbs from six months ago:

carb-question-t8281.html

These is some good discussion and some useful links here.

Vintage Paul

Dare I say it, no no I musn't, but I will, CUT A SECTION FROM THE FRAME WEB. There I did say it, sorry it must be the summer heat or dehydration.

GB

The 77 commando I am restoring came with the SU which looks like it has been on the bike a long time
There was very little clearance between it and the wiring loom grommet but there is no indication of any contact between it and the frame
I had not known about this carby before buying the bike. It was cut into the airbox (in order to maintain mounting brackets for other components) but accessing the filter with the box in place is not an option without dismantling half the bike!
I will not be putting the airbox back in and will have to find some other way of supporting the side cover and other components.
There was no filter on the SU and no room inside the airbox for one which is a bit of a worry as I believe the SU needs a filter
Can't wait to see what the valves and bore look like - not.