Unstable SU HS6 carburettors

[Jean-Marie Raedts]

My garage adjusted the SU HS6 carburettors of my TR4A and the CO emission was 3.8%. However, after 500 km the CO was 7.8% and the carburettors were readjusted to 3.9%. After 100 km I checked the CO at home with my Gunson meter and the reading was still around 3.9%. A few days later I drove 15 km to the technical inspection (MOT) and there the value of the CO emissions had again increased, now to 5.4%. The maximum allowed is 4.5%.
The carburettors were completely overhauled by Burlen 6 years ago.
I've always had this problem, could it be that the adjusting screws loosen while driving due to the vibrations of the engine?

Thank for your inputs. 

Jean-Marie

[RobH]

Are you sure the choke arrangement is allowing the jets to raise to the fully closed position every time?  It can be stiff and if they stick slightly the mixture will be rich.  The jet adjustment nuts should not move under normal vibration. 

[BlueTR3A-5EKT]

If the floats are punctured or sinking, would that cause over fuelling or would the float chamber flooding be the obvious sign?

The Australian trick…..Blank the emission valve pipe that goes into the inlet manifold with a ball bearing so the engine fumes do not enter the combustion area of the engine.

Edited May 5, 2023 by BlueTR3A-5EKT

[Jean-Marie Raedts]

Thanks for your reply, I was out and could not react earlier.

The choke cable doesn't always push far enough, which prevents sometimes the jets to raise to the fully closed position. This is certainty a part of the problem. The cable itself is thin and not stiff enough and cannot, even after greasing the mechanism, always push the mechanism far enough because the cable bends. It has a diameter of 1,1 mm, is this not too thin?

The garage, indeed adjusted the float level of one of the float chambers, because there was some fuel above the jet bearing when the piston assembly was removed. This must be solved now. I do not understand the Australian trick.

Jean-Marie

[Motorsport Mickey]

Jean-Marie,

The interior engine emissions (oil mist etc) from the rocker cover on your engine on your TR4a will be fed by a pipe into the inlet manifold for burning by the engine. This will cause your engine exhaust emissions to increase because the air coming from the engine is oil soaked and will burn in the engine increasing the engine exhaust emissions.

Blocking off the feed pipe into the inlet manifold (a large ball bearing that can't pass through the pipe and stops the passage of oil soaked air) will reduce the engine emissions because the engine then will running on clean air...the engine will run cleaner and may meet your maximum 4.5 % figure.

Mick Richards

[Lebro]

The choke cable should not need to "push" the jets fully up, the springs on the carburetters should do that.

Do you have a solid wire cable, or stranded ? I have stranded, & it works just fine.

Bob

[Jean-Marie Raedts]

Mick,

thank you for the clear explanation, I will do that before I go to the next technical inspection (MOT) next year.

Bob,

I have a stranded cable but after pushing in the choke button, sometimes it is bended (see picture). After just touching it, the springs on the carburetters moves the jets fully up. I did greased the mechanism, but it clearly needs more attention because the friction is still too high to let the springs on the carburetters move back the jets fully up by itself.

Thank you both for your valuable inputs.

Jean-Marie

[Waldi]

Dear Jean-Marie,
it may indeed be just a matter of getting the jets move more easily. WD-40 (or similar product) and some help my fix this. 

Groeten,
Waldi

[ctc77965o]

Agree : stuck jet is most common
If crankcase breather arrangement is working as intended, it shouldn't affect idle emissions.

[Lebro]

Are the jets correctly centered ? If not they could stop the piston (& needles) from dropping fully down.

Bob

[BlueTR3A-5EKT]

Assistance here

https://sucarb.co.uk/technical-hs-type-carburetter-description-adjustments

[Jean-Marie Raedts]

Bob, I think that the jets are correctly centered, but I will check it when I open the carburettor next time.

Dave, the original crank case breather is removed by my garage mechanic. There is a direct connector from the rocker  cover and the airfilters (see picture). I understood that blocking that flow will help to bring down the CO percentage, which I will do before the following inspection in 2024.

Jean-Marie

[Jean-Marie Raedts]

Peter, thank you for the article. Very helpful.