On Fri, 18 Nov 1994, W. Ray Gibbons wrote:
> For example, air has to get into the engine to get an accurate reading.
> On a car with SU or Stromberg carbs is it sufficient to open the throttle,
> or must one also block the pistons in the up position? Which did you do,
> Scotty?
At cranking speed (as when doing a compression check) ample air can flow
past the pistons, since they never completely close off the air flow.
Ditto the throttle butterflies. When they are "closed" they still pass
enough air for the engine to idle, so they will pass enough to fill the
cylinders at the lower-than-idle cranking speed in a compression test.
> (For many years, I've prospered with the philosophy that serious problems
> are more often electrical than due to fuel delivery, and I've considered
> Carburetor to be a French word meaning "don't touch me." But maybe I will
> have to rethink that now that I have a car with SU carbs. Clearly it is
> nice to know how they work.)
Once you understand the operating principle of the SU carb, it is a
wonderously simple instrument to maintain. However, your philosophy is
still valid. If the SUs aren't worn out (worn throttle shafts and/or
bushings, for example) and if they are kept clean inside and out, and if
they were properly adjusted not too many years ago, then they are the
_last_ thing you should tinker with if the engine isn't running right.
Ignition system problems are more likely to be the cause.
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Chip Old 1948 M.G. TC TC6710 NEMGTR #2271
Cub Hill, Maryland 1962 Triumph TR4 CT3154LO (daily transportation)
feold@umd5.umd.edu
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