View Full Version : S.U. carburettors

12-06-2002, 06:11 AM
Hello all,
it would seem that S.U.'s are not popular over your side of the world, with so many after market Weber conversions and remarks like "pesky S.U.'s."
In my experience, they are reliable and do not need a lot of maintenance other than cleaning and keeping the dashpot oil topped up. Also their spares availability, even for very old carburettors is excellent.
Some quick points:- find the settings for your car, e.g. for H or HS carbs, the jet setting (Only one jet, must be a bonus) is two full turns down from the top. This is a good starting point. If the piston is sticking, clean it and inside the piston housing with solvent only, DO NOT use an abrasive at all, even metal poish. If it does not drop with a click when you lift it (manually with the air cleaners removed)then the jet needs centering, done by screwing the jet fully up, slacken the jet clamp, push the piston fully home and tighten the jet clamp then reset jet to run position.
Sorry, this post is getting a bit long, but with the basics understood they are a good instrument, especially for a road car. (Webers, especially DCOE types make the car less flexible even if they give more top end)
What do I drive?, Triumph 2.5 saloon, er, Lucas petrol injection. (Basically TR5 engine)
Alec graemlins/hammer.gif

12-06-2002, 09:47 AM
SUs are incredibly simple, especially once you understand how they work.

I think one of the things people have a problem with isn't merely the carb, but knowing most SU setups have two or more carbs, which complicates things a bit but is still relatively simple to deal with.

Gary Pope
12-06-2002, 10:46 AM
I have a pair of HS6's on my TR6 and have found them to be very easy to work on and maintain. Adjusting the mixture is very simple as is setting the air balance with a gauge. The carbs perform well at high speed open throttle situations but not so good for driving around the town at low revs.

I think they may be prone to bad idle but that maybe due to just old age and wear. I think the 6 cylinder cars would benefit from a triple setup, but getting a manifold isn't easy.

Perhaps the H1F6 carbs would work better, I like the idea of an integral float bowl.

12-06-2002, 02:44 PM
Thanks Aerog\Gary, I had expected some adverse comments really.
Regarding your TR6, Gary, S.U.'s normally are good at low revs as they automatically choke down with small throttle openings so give good atomisation. Also idle is normally good. (you ought to hear my P.I. engine idle, but it smooths out with just a touch of throttle)
One point, as you mention, is wear especially of the throttle spindles, which then draw in air at idle. The other things that gave trouble are modifications made to give better emissions control like the poppet valve in the butterfly and the waxstat main jet. If I had them I would dump them for the plain butterfly and the simple main jet. Being an American spec car it may have both. Incidentally, we have the American market to thank for the injection system that works so well on the TR engine, as Triumph mistakenly thought that was the only way to achieve the standards required by yourselves. In the end the Strombergs were adequate and the Lucas injection wasn't.
I take your point regarding a three carb set up but the later inlet manifold looks to be good as far as distribution goes. Compared with my MK 2 Jaguar (Also on twin 1 3|4" carburettors) it is miles better. I do use, however, a 6 into 3 into 1 exhaust manifold set up which works well.

Alec graemlins/cheers.gif

[ 12-06-2002: Message edited by: piman ]</p>

12-06-2002, 10:14 PM
Aloha Piman,

I have two cars with SU carbs, MG TF and a TR3A and would not consider changing them. I've also had an Austin Healey 3000 Mk III with SU's and was very satisfied with them. The SU needs a some periodic maintenance, replacing seals and gaskets, topping up oil and cleaning but once they are set up little else is needed. Replacing the throttle shaft bushings is generally the fix to incosistant engine idling. I think Aerog got it right, many people don't understand the simplicity of how the SU works. Weber sound exotic and racy plus they look similar to Holley carbs.

I've found that the SU Tool Kit available from Moss Motors is very useful. The jet centering tool makes reassembly easy.

As to balancing dual carbs,I use the wire pointers from the SU toll kit. The pointers are inserted in the dash pots before starting the engine. If the pointer stay even as the revs are increased, the dash pots are raising at the same rate, therefore the same amount of air going through each carb. This test can be done without removing the air cleaners. I have a moto-meter, but I've only used it once or twice.

Care must be taken when replacing the jets to get ones acurrately bored. Since the needles that meter the fuel are measured in 1/1000th of an inch, if the jet bore is not percise you will have problems with idle setup.

Safety Fast,

12-07-2002, 06:54 AM
Hello Dave,

I like the idea of the pointers to balance the carburettors. I must admit it's new to me, but like all good ideas, very simple. I have always used an air flow gauge which is only practical at idling as you move the gauge from carb to carb. (Of course the throttle can be wedged to give different 'idle' speeds)
I have seen adverts for motorcycle units with multiple manometer tubes which monitor all chokes simultaneously, which seems better. But your S.U. device seems ideal.



12-07-2002, 12:04 PM

I've used both the pointers and the flow-guage, both seem acceptable but the comment about the aircleaners (and the flow-guage) is a valid one. Seems to me a matched set of filters shouldn't unbalance the carbs much but who knows images/icons/smile.gif

A friend of mine has dual carbs on his motorcycle - one on each side, without any kind of balancing tube between them. He bought one of the electronic guages thats out there, and it works great. You see both carbs at once without switching anything around.

One thing though - on his carbs they put an additional nipple to attach such a guage so the aircleaner is completely removed from the problem.

12-08-2002, 01:08 PM
After owning two cars with Zenith-Stromberg carbs I'm LOVING SU's!

12-08-2002, 02:08 PM
My Jag has threee SUs and I have no problem with them. They take a bit or care to balance, but once you get them set, they generally stay tuned. Rarely have I had to do anything with the carbs once I get the car in good tune and then set up the SUs.

My Spitfire, on the other hand, has Dual Webbers and I prefer the simplicity of the SUs.


12-08-2002, 03:10 PM
I think webers are overly complicated and mostly overrated. WHen I bought my Spit it had a weber which I prompty sold and replaced with a stromberg. I have just bought a pair of old SUs and plan to rebuild them next week and get them on my car. Also I have a tidbit that may interest you. My friend who I bought the SUs from puts them on harleys around here and apparently the harley owners really enjoy them over the more complicated webers.

Cheers graemlins/thirsty.gif

12-08-2002, 03:15 PM
My father worked at Norton, and came up with putting SUs on a Commando. One machine, the Norton 76, was made as a prototype, (which worked very well) in order to save the Norton brand but the government went with Triumph instead, for reasons I had better not get into images/icons/mad.gif which fizzled.

12-08-2002, 11:18 PM
Now I feel bad cause I'm running Webers.. images/icons/frown.gif

Really though.. On a low HP motor webers are a waste of time, but if you have some cam and some rpm to play with, the Webers ( or Del orto, or Mikuni ) are the cats meow.. They just howl from whatever rpm you are at to redline.. and never skip a beat at WOT..

I'd rather have a single DCOE than twin Su's any time.. Maybe I'm just a glutton for eye wash appeal.. graemlins/cheers.gif

12-08-2002, 11:35 PM
I think Webers get a bum rap regarding complexity. I don't think there's an easier carb to rebuild than a Weber DCO. They are not all that complex. Also, you can replace jets without even taking the air cleaner off.

The large selection of jet sizes, venturi sizes etc is a mixed blessing - there's always a temptation to try to find a better mix than you already have.

Balancing a pair of them can be a trick, but you're effectively dealing with 4 carbs, not two. I found a combination of measuring airflow and using a colortune sparkplug works best.

My comments apply only to the DCO sidedrafts - our one experience with a downdraft wasn't that good.

However, I don't think I'd replace SU's with Webers for street use.

12-08-2002, 11:39 PM
I will say this about the Webers - they definitly look cool sitting on my Spit motor! images/icons/grin.gif

12-09-2002, 01:03 PM
Just to add to the confusion here, I'll add that I'm running an S&S pumper carb on my time trials racer. Not exactly a "purist" solution, but the reason is simple (sort of)…..vintage rules allow me to race the car as a "Sprite" (it's really a 1500 Midget), but only if I keep the single carb manifold (any carb is OK).

The S&S has a 2-1/8 inch bore with fully adjustable idle jet and (optional) adjustable main jet…easy to adjust! (accel pump is adjustable too) I had to cut into the water passage on the manifold to get this carb to fit….took a few hours on the Bridgeport.

The American Zenith-Bendix is similar, and also available from Harley shops (my carb was $199 for J&P).


John Turney
12-09-2002, 06:05 PM
After rebuilding an American 4-barrel (I forget which) and 4 SUs, I find the SUs much easier. I rebuilt and was running 2 HD6s and then went to HD8s. Really simple to tune, expecially with a flowmeter and a pair of Colour-Tunes. They also have a seal at the shaft bushings that keep them sealed even with shaft wear.

When tuning Webers, don't you need a supply of various jets? More parts to buy and try. I will admit that thy guy running 3 Webers was about 4 mph faster than me and my 2 SUs at the end of the 1/4-mile at Tahoe.


12-13-2002, 03:45 AM
Yes you do need an assortment of jets, and maybe chokes, but I find the carbs are pretty forgiving for tuning.. Just as I say that one of mine is hanging up and I have a 3500 rpm idle now...

Back to the drawing board..