Stromberg S4 idle speed

[69S4]

My Stromberg S4 has always 'suffered' from a higher hot idle speed than it should have. At the moment it's idling - with the carb stops backed right off - at around 1300 rpm. It's not a problem in that it's a reliable and even 1300 rpm but as it should be around 900 there's something not quite right. Over the years I've looked at just about everything I can think of to track the issue down but - obviously - to no avail. Any insight into what might be the cause will be gratefully received. Here's a list of what I know / have checked / have done:

Carb set up - mixture is about right based on drivability / plug colour. Needles + jets + diaphragms have been changed numerous times. Spindle seals have been changed, float height is manual correct, butterflies close completely when bench set and the spindle clamps are not holding one butterfly slightly open. The air bleed screws are shut. The temp compensators are set to start opening at 40C + fully open at 50C. The choke mechanisms fully shut when the cable is pushed in. There is free play in the throttle cable + an extra return spring to bring the carbs back to 'rest'.

Timing - static is set at 9 BTDC (using a strobe light).

Air leaks - I've traced the vacuum pipes through the car and replaced all the rubber connectors. The T piece has been replaced twice with commercial versions and once with a DIY one. The vacuum pods have been replaced and there's no wear holes in the tubing. Only thing not looked at is the dash vacuum switch. The vacuum line to the servo is intact with no leaks. The carb O rings have been replaced more times than I can remember but a leak there gives a wandering, unstable idle. The idle is stable and even, just too fast.

The car runs fine from idle - no signs of it being rich or weak and drives well otherwise.

If you've read this far, are still awake and have any ideas about where I should look next I'd be grateful to hear suggestions. I'm going to look very carefully at timing to start with and check that the TDC mark on the pulley is actually correct.

[oldelanman]

Hi Stuart,
Have you checked that the fast idle screws are not in contact with the choke lever cams when the choke is off and the throttle fully closed ? The setting procedure in the manual is done with the choke fully rich and a 0.6mm drill holding the throttle open and this doesn't ensure that there is clearance when choke and throttle are closed.

Also have you checked your tacho is accurate ?

Apologies if you've already considered these possibilities.

[69S4]

Not bad - you got one out of two! I have checked the choke lever cams. In fact I?ve reset them several times as I fiddle with the amount of fast idle when the chokes are working. There?s plenty of clearance. However, your other suggestion was much closer. I?ve always assumed the tacho was accurate as the reading corresponds with the gearing and road speed. I checked it earlier today though with an electronic clip on tachometer and at idle speed it?s reading 200rpm high. The error diminishes as the engine speed increases and by 2200 rpm it?s accurate. So my 1300 is a real 1100. It still won?t drop below 1100 though - when hot (it?ll stall when cold).

I checked the ignition timing again - exactly 9 degrees at slow cold idle, and checked the pulley marks with a dial gauge down the plug hole - exactly correct. So it doesn?t look like the timing.

[oldelanman]

Another thought .... do you have an electric fuel pump ? Maybe the fuel pressure is too high.

[chickenstock10k]

It may be worth taking a quick look at the dash light switch. I?ve found that might can sometimes get stuck between on and off which allows for a small vacuum leak and the vacuum lines to the switch were old and crumbling when I bought the car. It?s worth checking, since it sounds like you are chasing down a vacuum leak.

[69S4]

Another thought .... do you have an electric fuel pump ? Maybe the fuel pressure is too high.

Yes I do have an electric pump and the fuel pressure is, well who knows. It feeds through a pressure regulator but the regulation side of things is just as mysterious. I can twiddle the pressure screw but without a gauge it's meaningless except by trial and error and I've no idea what feed pressure the carbs need anyway. I'll twiddle it in the downward direction and see if it makes a difference.

It may be worth taking a quick look at the dash light switch. I?ve found that might can sometimes get stuck between on and off which allows for a small vacuum leak and the vacuum lines to the switch were old and crumbling when I bought the car. It?s worth checking, since it sounds like you are chasing down a vacuum leak.

I think that's worth trying as well. Easiest way to do it might be to remove the vacuum line from the engine end and block off the port temporarily. That way I'll bypass all of the vacuum system. If it turns out to be the issue I can then replace it bit by bit. I'm away for a couple of days from this afternoon on a two wheel trip but next week I want to try and sort this out once and for all.

[oldelanman]

Another thought .... do you have an electric fuel pump ? Maybe the fuel pressure is too high.

Yes I do have an electric pump and the fuel pressure is, well who knows. It feeds through a pressure regulator but the regulation side of things is just as mysterious. I can twiddle the pressure screw but without a gauge it's meaningless except by trial and error and I've no idea what feed pressure the carbs need anyway. I'll twiddle it in the downward direction and see if it makes a difference.

After a bit of Googling around ... 1.5 - 2.5 psi seems to be the recommended pressure suggested on other forums and Moss Technical have this:-
"Both SU and Zenith-Stromberg constant depression carburetors feature a needle and valve seat assembly to control the amount of fuel entering the float chamber, which is effective up to 3.5 psi of fuel pressure. At higher pressures, the needle will fail to seat properly allowing the chamber to flood."