Carb balancing
Discussion
Hi my newly acquired 1987 xflow Caterham has two Weber 40 DCOE carbs that may need balancing. Three plugs look a healthy colour but one is much darker and probably running rich. The car has a pronounced cough on even mild acceleration. I was thinking of buying a ColourTune kit and carburettor vacuum gauges (the ones with a plastic pipe). Where do they connect to on the Webers?
all you need to get them roughtly right is a length of hose and your ear.
Remove air filter.
put one end of pipe to ear
hold other end of pipe in lip of carb barrel.
Listen to pitch of noise
Now compare to another carb barrel.
Which one has highest air flow?
Adjust to open one or close one as required.
Don't forget that the linkage between the carbs may need to be disconnected so that you are onlt altering the one carb and not all via the linkage.
A lot cheaper that buying a guage and good enough for normal useage.
Remove air filter.
put one end of pipe to ear
hold other end of pipe in lip of carb barrel.
Listen to pitch of noise
Now compare to another carb barrel.
Which one has highest air flow?
Adjust to open one or close one as required.
Don't forget that the linkage between the carbs may need to be disconnected so that you are onlt altering the one carb and not all via the linkage.
A lot cheaper that buying a guage and good enough for normal useage.
davidball said:
The car has a pronounced cough on even mild acceleration...
That doesn't sound like a balancing problem. Balancing just equalises the flow through wth carbs at idle, so that each cylinder is getting equal air/fuel mixture and so the engine runs nice and smoothly. The typical symptom of a badly balanced pair of carbs is that the engine will shake like a dog having a s
t when it's idling.... it won't normally show up in the form of a flat spot, hesitation or spitting-back when you accelerate.A 'cough' on acceleration such as you describe is usually caused by the mixture leaning out. The carbs should be pumping an extra dose of fuel in when you press the accelerator, to avoid this happening, so the usual cause is either incorrectly sized jets or (more likely if the car has been standing, since it's safe to assume that the car was originally set up correctly) jets blocked with sediment or residue. You can get a gadget that allows you to measure the flow rate though the pump jets that will tell you if that's the case, but to be honest you'd be better taking the car to a Weber specialist to diagnose the problem, if in any doubt.
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