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Carb basics


daverclasper
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Hi

 

My understanding, is that when carbs are balanced for air intake (throttle buterflies), this is for idle and makes little difference at higher revs.

 

Is this also the case with mixture settings please (whether single or twin carb).

 

Any advice great thanks

 

Cheers, Dave

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yes the main effect of imbalance is at low throttle openings , when wide open its immaterial.., more critical at idle as this is where you adjust the idle mixture , you can set the idle screws to a fixed position like 1.5 turns in from fully closed or use various tools to measure the air flow on each carb.

so the flow over the throttle plates is near equal.

 

mixture is set at idle on Stromberg and  SU constant depression carbs .

how this is set will affect the whole range of where the needle is sitting . if its rich at idle it will be rich all the way to max needle travel ( thats assuming you have a needle to the correct spec for your engine)

 

on a down draught  fixed choke carb the idle speed and mixture only affects   the idle conditions once the throttle plate is moved away from idle the idle circuit ceases to operate and fuel is controlled by the main and compensating fixed jets 

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Thanks Pete

 

Seemed to have a long term issue with getting a correct mixture throughout the range.

 

My rocker cover breather was venting to atmosphere. Have now fitted an original PCV.

 

Mixture seems a lot better now. At least have a grey plugs colour at cruising, before it was white, if I set ok mixture at idle (though just one plug is quite a bit paler?).

 

Could the valve have made a difference in breathing do you think?.

 

Cheers, Dave 

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As the smiths valve extracts breather fumes into the manifold after the Carbs have done their best

 

anything you do to either cork up free air into the manifold or let more in has a pretty wildy influence

on any carb adjustments you make

 

if you block it you get richer if left open very weak

smiths valves are used on semi sealed breathers so if you take the filler cap off expect the engine to race to a higher idle, as air goes direct to the manifold the filler cap needs to be sealed

The smiths valve stays pretty well shut until crankcase pressure allows it to open the fumes get sucked in the manifold, and it modulates all the time to balance removing crankcase pressure without adverse effect on manifold vacuum

well something like that

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