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

Reply to
bob
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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.

Reply to
Misterbeets

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.

Reply to
Don Stauffer

Rejetting is not necessarily needed. The usual problem is getting the vacuum controlled aspects like metering rods and power valves to open at the correct times to correctly meter or enrich the mix.

Lugnut

Reply to
lugnut

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

Reply to
Dave Baker

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

Reply to
Dave Baker

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