What could be the result of this missing o-ring?

Question

Consider this o-ring :

And this full diagram if you need a more global view :

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For completeness sake, here are the real parts :

Where the o-ring goes :

Where the little hole leads :

The other end of the path is at the top of the venturi :

Another hole :

Which leads there :

Little hole in caps where o-ring goes :

And for completeness, what goes inside :

Both-side view of the diaphragm :

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What could be the result of this missing o-ring ?

My hypothesis is that a bad acceleration would occurs since air flow would be corrupted.

Am-I right and why ?

Answer 1

Actually you could very well be correct. The passage you are pointing out with the missing is an air passage which acts as a pressure reference. The diaphragm (last picture) is actually the accelerator pump. As the butterfly opens, more air is sucked in, causing a high pressure point at the top of the venturi (picture marked as: The other end of the path from the inside ). This forces the air through the hole and tube and behind the diaphragm which pumps fuel into the incoming air. This gives a larger boost of fuel, which allows richens the air up a bit until the carb venturi fuel inlet can catch up.

With the o-ring gone, there is no air reference (or very little) going on. The diaphragm cannot pump the extra fuel which the engine needs. When this o-ring is back in place, I would surmise you'll see a great increase in how the bike accelerates, as long as the rest of the carb is put back together correctly. The o-ring looks like you could replace it from pretty much any o-ring kit (it doesn't look like it's anything special).