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I've read up on these and the theory behind how it works sound ok. but i only see then for the 49 cc midi bikes. what gives?
 

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Boost bottles are only useful on piston skirt engines which lack a reed to prevent flow back into the carburetor.

The reed valve inducted engines as featured on the '47cc' bikes don't need boost bottles - the reed valve does the same job :)
 

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I don't know if it's completely useless, Though, I have heard that before from people that know more than me..


Try running the bike without any air filter on it. If you rev it up you can see exhaust coming back through the intake. I can't prove this happens on all bikes, but mine has done it with both stock and CF reeds. This makes three big performance losses,



1. Less oxygen. The exhaust has already been burnt, leaving little or no oxygen left. Sucking that back in is just a waste of space.

2. Hot Gases. Heated gas has increased volume. This means that you are getting less gas back in per stroke. (39cc of cold air has more oxygen than 39cc of hot air)

3. Rich state. The air coming in, going out, and coming in again has passed by the carbuerator 3 times. Not very efficient or accurate, you can imagine. You can "tune "this out with the adjustable jet, but this is only a band-aid to the real problem.

I might try making one and see if it really does help, I've seen people say yes and no. You can make it cheap enough, and if it does what it could, it should improve all round somewhat.

Btw, I wouldn't spend money on a boost box. Just make one for like $.50
 

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PMantix84 said:
I don't know if it's completely useless, Though, I have heard that before from people that know more than me..


Try running the bike without any air filter on it. If you rev it up you can see exhaust coming back through the intake. I can't prove this happens on all bikes, but mine has done it with both stock and CF reeds. This makes three big performance losses,



1. Less oxygen. The exhaust has already been burnt, leaving little or no oxygen left. Sucking that back in is just a waste of space.

2. Hot Gases. Heated gas has increased volume. This means that you are getting less gas back in per stroke. (39cc of cold air has more oxygen than 39cc of hot air)

3. Rich state. The air coming in, going out, and coming in again has passed by the carbuerator 3 times. Not very efficient or accurate, you can imagine. You can "tune "this out with the adjustable jet, but this is only a band-aid to the real problem.

I might try making one and see if it really does help, I've seen people say yes and no. You can make it cheap enough, and if it does what it could, it should improve all round somewhat.

Btw, I wouldn't spend money on a boost box. Just make one for like $.50
:) Couple minor corrections for you re:1 and 2...

The fuel/air mix coming back out past the reed is not exhaust. It is unburnt fuel/air which should be forced up through the transfer ports into the cylinder, but is instead leaking back out the reed.

On 3, you are exactly right - the disruption of airflow across the carb needle results in a mixture different than what is intended.
 

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Yeah, you're right..


When I saw this coming out I bent over to smell it. I guess it never crossed my mind that the exhaust it right there too.

You'll get that when you work on cars a lot and just tinker on a little bike occasionally. ;)

Thanks for the fix
 
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