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Classic Engines, Modern Fuel

Comments on Topic: Piston lift check

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Submitted by Anonymous

I followed everything in the excellent book but for one small thing.

I have always understood (and found) that if you lift the piston(s) of SU or Stromberg carbs and the engine stops, it's running weak. Most of this book affirms that belief - 144.
The chapter which decribes the effect of springs supports that characteristic too.  The stronger red spring holds the piston down, and enriches the mixture; the weaker blue one does the opposite (page 100).

So, I am puzzled by the 'Balancing the mixture' check on page 143 which seems to say the opposite.  Lift the piston and - 'if the engine stalls, that carb is running rich relative to the first.'
The test is done without the damper.  Is that relevant?  Is the damper cap required to seal the volume above the piston of an SU?

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Submitted by The Author

I am glad you have enjoyed the book. 

The description of the carburettors in the book is what happens when they are running normally. Manially lifting the piston upsets their operation and will cause the mixture to become exceptionally rich. If the carburettor was already running rich this will cause the engine to stall. This is true if there is one, two or more carburettors.

I apologise if the description is confusing what I was describing is that if you lift the piston on one carburettor and the engine does not stall, then lift the piston on the second carburettor by the same ammount and the engine stalls. The second carb is running rich relative to the first.

Using the method of lifting the pistons to set the mixture is a very crude way of tuning the carbs, The most important thing is to set them so they are approximately correct AND for multiple carburettors, they all behave in the same way. I.e. they are balanced. You can then fine tune them for normal running using the colour of the spark plugs.

Removing the damper has virtually no effect on the normal running of the carburettor as the guide for the suction piston is a very close fit. Virtually no air can enter the suction chamber through the guide. The only reason for removing the damper is to make it easier to accurately lift the piston. 

Colour-tune plugs give a better guide to the mixture, although personally, I have not found them any better than "lifting the piston". A lamda probe in the exhaust is better still and a rolling road the best way to tune the carburettors. 


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