carburetor main jet
02-10-2009, 01:36 PM
#1
carburetor main jet have a question regarding the main jets in a carburetor,
not sure if the make or model carburetor will make a difference but for my question use a generic holley 4v model 4150/4160.
what does changing the main jet size actually do?
does it only change the air-fuel ratio for a given volume of air passing through the venturis?
this is regarding taking a carb and throwing it on different size engines, such as a 450cfm holley 4v carb that is set up to run on a 5.0L engine. It has whatever number jets. I should be able to take that carb and put it on a 5.7 or even 7.4L engine and the engine should run just fine until the carb is not letting enough air through so the engine will be choked and will never reach the power output it should. But up until the carb size chokes airflow, it should maintain the correct air/fuel ratio for whatever rpm and load?
thanks
not sure if the make or model carburetor will make a difference but for my question use a generic holley 4v model 4150/4160.
what does changing the main jet size actually do?
does it only change the air-fuel ratio for a given volume of air passing through the venturis?
this is regarding taking a carb and throwing it on different size engines, such as a 450cfm holley 4v carb that is set up to run on a 5.0L engine. It has whatever number jets. I should be able to take that carb and put it on a 5.7 or even 7.4L engine and the engine should run just fine until the carb is not letting enough air through so the engine will be choked and will never reach the power output it should. But up until the carb size chokes airflow, it should maintain the correct air/fuel ratio for whatever rpm and load?
thanks
02-10-2009, 04:30 PM
#2
The main jets feed the main ventri, fuel is drawn through by the air velocity going past.
There is basically a second set of jets on a Holley called the power valve channel restrictions. These are a pair of set diameter holes that feed additional fuel in when the power valve looses vacuum. (high load)
Going backwards, but the idle feed restrictions remain effective well off of idle and feed below the throttle blades.
The problem with a small cfm carb on a big engine is the idle feed restrictions uncover more because the blades need to be opened up to allow more air in just to get a big engine to idle with a small carb. If you put a small enough carb on a big engine you can start pulling fuel from the PVCR's, and main jets at idle, or off idle, basically when they're not supposed to be supplying fuel.
This screws the whole carbs design up.
Getting tired of typing, basically yes, the smaller carb becomes a problem when it can't supply enough air.
And main jets are only fully effective at WOT. (unless of course you put a really small carb on a huge engine)
If you put a huge carb on a small engine, again it won't work as designed. It most likely won't be able to pull enough velocity past the venturis to pull fuel from them.
There is basically a second set of jets on a Holley called the power valve channel restrictions. These are a pair of set diameter holes that feed additional fuel in when the power valve looses vacuum. (high load)
Going backwards, but the idle feed restrictions remain effective well off of idle and feed below the throttle blades.
The problem with a small cfm carb on a big engine is the idle feed restrictions uncover more because the blades need to be opened up to allow more air in just to get a big engine to idle with a small carb. If you put a small enough carb on a big engine you can start pulling fuel from the PVCR's, and main jets at idle, or off idle, basically when they're not supposed to be supplying fuel.
This screws the whole carbs design up.
Getting tired of typing, basically yes, the smaller carb becomes a problem when it can't supply enough air.
And main jets are only fully effective at WOT. (unless of course you put a really small carb on a huge engine)
If you put a huge carb on a small engine, again it won't work as designed. It most likely won't be able to pull enough velocity past the venturis to pull fuel from them.
02-10-2009, 05:40 PM
#3
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The main jets feed the main ventri, fuel is drawn through by the air velocity going past.
There is basically a second set of jets on a Holley called the power valve channel restrictions. These are a pair of set diameter holes that feed additional fuel in when the power valve looses vacuum. (high load)
Going backwards, but the idle feed restrictions remain effective well off of idle and feed below the throttle blades.
The problem with a small cfm carb on a big engine is the idle feed restrictions uncover more because the blades need to be opened up to allow more air in just to get a big engine to idle with a small carb. If you put a small enough carb on a big engine you can start pulling fuel from the PVCR's, and main jets at idle, or off idle, basically when they're not supposed to be supplying fuel.
This screws the whole carbs design up.
Getting tired of typing, basically yes, the smaller carb becomes a problem when it can't supply enough air.
And main jets are only fully effective at WOT. (unless of course you put a really small carb on a huge engine)
If you put a huge carb on a small engine, again it won't work as designed. It most likely won't be able to pull enough velocity past the venturis to pull fuel from them.
There is basically a second set of jets on a Holley called the power valve channel restrictions. These are a pair of set diameter holes that feed additional fuel in when the power valve looses vacuum. (high load)
Going backwards, but the idle feed restrictions remain effective well off of idle and feed below the throttle blades.
The problem with a small cfm carb on a big engine is the idle feed restrictions uncover more because the blades need to be opened up to allow more air in just to get a big engine to idle with a small carb. If you put a small enough carb on a big engine you can start pulling fuel from the PVCR's, and main jets at idle, or off idle, basically when they're not supposed to be supplying fuel.
This screws the whole carbs design up.
Getting tired of typing, basically yes, the smaller carb becomes a problem when it can't supply enough air.
And main jets are only fully effective at WOT. (unless of course you put a really small carb on a huge engine)
If you put a huge carb on a small engine, again it won't work as designed. It most likely won't be able to pull enough velocity past the venturis to pull fuel from them.
