Setting up single SU

a15htn

a15htn

New Member
Hi all,

Does anyone know the where abouts of, or know, the steps to set up a single SU ?

I just can't get mine right...... Too lean now too rich...... So, I'd like to start from scratch.

Anyone? Please?

Thanks
J
 
........ Change of plan...... The answer has to be a colourtune!

I'll pick one up from halfrauds tomorrow and see how it goes.
 
Yes Colourtune is the best way. Pretty straightforward with that and makes a big difference when set up properly :D

Dave
 
Yep, a colourtune is the answer!

With a single carb engine you could also use a gas analyser up the exhaust. That doesn't really work on a twin carb set up though - it would tell you that you were off, but not what to adjust to bring the engine back to purrfect.

Chris
 
I used a Lambda meter for my SC, unfortunately not a piece of kit that everyone has but if you know anyone in the motor trade or even better in motorsport they may be able to blag you a session with the meter. The advantage to this is it gives you an accurate measurement over the whole rev range and can tell you if you're going lean or rich and with a tacho at what revs so you can fine tune needle selection etc.
I have my car set up now to be running spot on mixture round town and just a tad rich when at motorway speeds.
 
Well I got my colortune and as a little bonus I bought a gunsons automotive meter (to give accurate rpm, as I have no tacho.... And it was reduced to £20 :p )

So armed with timing light, meter and colortune I managed to get the ignition spot 4deg, the idle to 750, then the mixture a tiny bit rich....... That spot between blue and yellow that sort of looks a bit whitish.

Something I did learn was that if the oil in the carb dashpot was too thin the mixture is fine until you hard rev it, the needle drops properly and the mixture goes rich beyond belief!
Wipped off the dashpot, gave everything a clean (boy was it mucky) reassembled and topped up with some thicker oil.... Needle drops nicely now..... Redid the mixture to just off blue to yellow - I'd rather be rich than I think.

All in all about an hour....... Im getting fond of having all the right gear and some idea !!!!!!!
:roll:
 
Whatever the thin oil was or wasn't doing, I think the biggest benefit was cleaning the assembly, so that the piston drops properly, closing the jet off in idle.
 
The piston lifts with the increased vacuum when you floor it, as it lifts it pulls the needle out of the jet and allows more fuel through. The dash pot oil is there to dampen this lift, hence a thicker oil slows the lift and gives a weaker mixture.
 
I always believed the oil slowed the piston rising , so the air speed was greater through the carburretor , hence the suction greater at the jet, and therefore more petrol was sucked out richening the mixture ?
If I'm wrong I've been putting ATF in my carbs for the wrong reason yet still getting the desired result
 
As I understand it, this issue has a lot to do with what happens in theory, and what happens in practice.

In theory, the viscosity of the dashpot oil is irrelevant to the piston's movement. A set amount of vacuum will result in a set amount of piston rise, and a set station on the carb needle. All the damping does is affects how long the piston takes to get it to its maximum height for a given vacuum.

In practice, your foot is never still on the pedal, the road surface is never 100% flat, there is always a wind, and it is rare for these parameters to remain constant for long enough to allow the piston to reach its optimum height before everything changes again. At this point, the damping comes in.
Low viscosity and the piston will move quicker. It is therefore more likely to get to the correct height before a change in vacuum, and will be correctly set for more of the time, but every tiny blip of the throttle at idle will waste excessive amounts of fuel as it jumps upwards.
Higher viscosity, and it will be leaner for longer under vacuum, benefitting economy, although pinking may occur under hard acceleration. But as the damping is double-acting, the piston will also be slower to fall, thereby staying richer for longer on the overrun, and taking time to settle to a smooth idle. So it's swings and roundabouts with this approach.

I'm personally a fan of experimenting with these things - every car and driving style is different, so find what fits!

Michael
 
Dave is right, it actually is a little more complicated than it seems at first glance.
The mixture strength at any given time, is not only determined by the profile of the needle exposed at the top of the jet, but also by the air speed between the jet bridge and the bottom of the piston. This space is called venturi port and in contrast with almost all the other types of carburretors, in an SU it is variable due to the moving piston. So, for a given throttle opening and engine rpm, the smaller the size of the venturi (i.e. the lower the piston is) the higher the air speed will be at the top of the jet, and the harder will pull petrol from the float chamber.
The damping of the piston in an SU does the same job that the accelaration pump does in all the other carbs. At an instant throttle opening, it delays the rise of the piston in order to maintain the smaller venturi and high air speed above the jet and in essence to produce a momentarily rich mixture. Of course the piston will start to rise to match the increased revs of the engine. Without piston damping, the piston would rise very fast, not allowing for the momentarily enrichment, something that we feel as a "flat spot". If someone knows what he is doing (and has plenty of time!), could play with different oil viscosities in the dashpot in order to alter the damping rate of the piston and fine tune the throttle response to his taste.
 
Excellent description, chaps. And now you know why the SU is God's Own Carburettor! Set up intelligently it comes very close to matching EFi! Pity there isn't an equivalent to replace the Prince of Darkness's distributors to do the same for ignition timing without resorting to electronics!

Chris
 
The damping of the piston in an SU does the same job that the accelaration pump does in all the other carbs. At an instant throttle opening, it delays the rise of the piston in order to maintain the smaller venturi and high air speed above the jet and in essence to produce a momentarily rich mixture. Of course the piston will start to rise to match the increased revs of the engine. Without piston damping, the piston would rise very fast, not allowing for the momentarily enrichment, something that we feel as a "flat spot".
Click to expand...

Interestingly I slightly disagree. The dampers have a certain amount of slack, free movement on the shaft, which means the piston can move a short distance very rapidly without being damped, this allows for the accel enrichment.

I don't think the speed of air over the jet/needle makes much difference, with the needle in the way sucking harder will only bring a slight increase in fueling, whereas moving the needle even a small amount will have a much greater effect.
 
Having just read around a bit, I stand corrected, slightly. In that the slack is there to allow the piston to rise quickly and let more air in, then it is damped which increases the airspeed over the jet and hence pulls more fuel. So a combination of both effects.

As Dave suggests, if it rises too quickly the air flow could effectively stall and not draw fuel from the jet at all, although at this point the piston will fall again due to lack of depression, and you get an oscillating piston, hence the fine balance between too thin an oil and too thick.
 
webmaster said:
I don't think the speed of air over the jet/needle makes much difference, with the needle in the way sucking harder will only bring a slight increase in fueling, whereas moving the needle even a small amount will have a much greater effect.
Click to expand...

But Richard in carburettors it is only the air speed over the jet that pulls the fuel into the engine. Without sufficient air speed at that point nothing will happen no matter how big is the jet openning.
Also i am slightly suspicious about the slack in the damper as an enrichment means theory. Actually i just went out and had a look at the carbs in my TC, the slack is negligible and certainly won't change the needle profile enough to enrichen the mixture for a thirsty accelerating engine. Just remember the lifting piston method to determine the air / fuel ratio in SU equipped engines. Lifting the piston for as much as probably the damper slack is, will only make a tiny difference in an idling engine. I doubt it could do anything to an accelerating engine.
Another interesting point is that the damping rate is not the same for both piston directions. It is much higher for a raising piston, and lower for a dropping piston. This is effected through the shape of the damper, and i actually think that the slack in the damper is there to allow the oil to flow through it for a falling piston, rather than around it for a rising piston. Thick dashpot oils (i.e. 20W-50) in cold weather produce too much damping in a falling piston, resulting in some spectacular flat spots after wide open throttle - high engine revs gearchanges as the piston cannot return fast enough to its new - lower position to match the lower engine revs for the higher gear.
 
Yes, it appears I was a bit mistaken :oops:

As you say it looks like part of the function of the slack in the damper is to open/close the passage of oil through that section, and give the different rise and fall rates. I did also read that it was to induce an initial increase in airflow.

Must be my age, or my computing background, but I understand fuel injection far better than carbs. :LOL:

I spent ages trying to get my old 2.2TC to run straight, it would be fine when cold, but as it warmed up the carbs would go out of balance. I rebuilt them several times with new needles / jets etc, even tried balancing the flow / piston lift, off the car using the vacuum cleaner as a vac source. I suspect they were just badly warn.
 
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