Carb and Cam question

Working on carb problems and in reading a common comment is you need to
enlarge jets if you change to a larger cam (higher lift and longer
duration). Since the carb just mixes fuel with air as is passes by, why
would you need to add more fuel just because of how long the valve is open?

Just curious but if the air/fuel ration is right, how does the cam make a
difference?

Thanks
bobby

The ability of the jet to supply fuel is limited. It is only linear up
to a point. Beyond this, the greater airflow at max rpm would cause a
lean mixture.

bob wrote:
> Working on carb problems and in reading a common comment is you need to
> enlarge jets if you change to a larger cam (higher lift and longer
> duration). Since the carb just mixes fuel with air as is passes by, why
> would you need to add more fuel just because of how long the valve is open?
>
> Just curious but if the air/fuel ration is right, how does the cam make a
> difference?
>
> Thanks
> bobby
>
>
No! Changing the jet size changes the MIXTURE RATIO. You need to
supply more air and fuel, but in the same proportion, to take full
advantage of the cam. However, you will recieve some advantage even
with stock carb.

The only reason to change jets is if increased horsepower will cause
overheating. In that case a slight richening of mixture will cool
internal parts. Generally racing engines are set up pretty rich for
this reason. If this is for a street machine you probably do not need
to change mixture.

Depending on how much lift and duration the cam has, you may want to
increase carb size.

Misterbeets > wrote in message
oups.com...
> The ability of the jet to supply fuel is limited. It is only linear up
> to a point. Beyond this, the greater airflow at max rpm would cause a
> lean mixture.

Wrong. It's the other way round. Carbs self enrich at higher airflow rates
which is why air corrector jets are needed to keep the mixture ratio linear.
--
Dave Baker

bob > wrote in message ...
> Working on carb problems and in reading a common comment is you need to
> enlarge jets if you change to a larger cam (higher lift and longer
> duration). Since the carb just mixes fuel with air as is passes by, why
> would you need to add more fuel just because of how long the valve is
open?
>
> Just curious but if the air/fuel ration is right, how does the cam make a
> difference?

Enlarge which jets? There's often no need to enlarge main jets unless the
choke size has also been increased and no specific reason why the mixture
ratio would change once the engine is at reasonable rpm. However at low rpm
a long duration cam isn't working very efficiently and a bigger idle jet is
often needed to offset the poor "signal" from the venturi. However jetting
is very engine specific and there's never any point guessing. Take the car
to a chassis dyno and have it set up properly.
--
Dave Baker