Consider this o-ring :

enter image description here

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

enter image description here

For completeness sake, here are the real parts :

Where the o-ring goes :enter image description here

Where the little hole leads : enter image description here

The other end of the path is at the top of the venturi : enter image description here

Another hole : enter image description here

Which leads there : enter image description here

Little hole in caps where o-ring goes : enter image description here

And for completeness, what goes inside : enter image description here

Both-side view of the diaphragm : enter image description here

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 ?


1 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).

  • 1
    This is exactly the explanation I had in my head, expect that in my head the real names were replaced with "little holes" etc ;) .. I'll edit my question with the real names when I come back from work and follow up on this. No problem to get back correctly the rest of the carb together. Aug 25, 2015 at 15:20
  • 1
    Believe it or not, I gained 30km/h top speed and and insanely faster acceleration. Thanks for your help. Aug 26, 2015 at 12:26
  • 1
    @Jean-FrançoisSavard - The acceleration doesn't surprise me, but the top speed does! Good deal :D Aug 26, 2015 at 14:42
  • 1
    To explain more about the top speed, I was not able to go over 5 rpm without that o-ring. It definitely worth the 4$ CAD even tho that seems a lot for a such little piece. Aug 26, 2015 at 14:44
  • 1
    $4 CAD = 30km/h ... I'd say it's well worth it ... If someone out on the Bonneville Salt Flats could get 30km/h more out of their vehicle for that price, they'd be all over it. Good deal. Aug 26, 2015 at 14:50

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .