Stromberg S4 idle speed
Posted: Thu Apr 25, 2019 10:12 am
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.
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.