Carb Tech

[Miller]

Can someone give me some resources or information on carb jets, specifically why you would switch primary and secondary configurations for different driving circumstances (dailying, drag racing and roll racing)?

[twistedfocus1647545489]

I'm not really a carb guy and maybe I don't understand the question fully, but are you talking about litterally changing your jet sizes for each of your different driving situations?

 

I would think that if it is tuned properly (ie. properly sized jets, power valve, etc) it should do all of the above just fine. Your power valve can be based off of the vacuum your engine makes at idle and is essentially big a vaccum operated jet that shuts at low load, thereby reducing the fuel flow. If that's right I would think you should still have acceptable drivability, even if your engine demands fairly large jets.

 

I think the rule of thumb is half the idle vaccum (ie. if it idles at 12" vaccum, then use a 6" PV).

 

tl;dr: Needs more EFI. :gabe:

 

Edit: Here's some good general reading.

http://www.holley.com/TechService/Library.asp

 

http://www.junkyardgenius.com/holley/tune01.html

[Miller]

I wouldn't modify the jets aside from sizing the correctly. My foot is in/out of the car, that's it. I think I pretty well got it figured from what's in there I am going to need to change up the set. Edited July 25, 2012 by Miller

[blazer427X3]

I still have a couple Holley books I got from Jegs years ago. One is Super Tuning and Modifying Holley Carbs and the other is a 4150/4160 tuning/repair/mod book.

 

I'll probably be at the next C&C, if those might help let me know and I can bring them.

[Green Bastard]

I'm not really a carb guy and maybe I don't understand the question fully, but are you talking about litterally changing your jet sizes for each of your different driving situations?

 

I would think that if it is tuned properly (ie. properly sized jets, power valve, etc) it should do all of the above just fine. Your power valve can be based off of the vacuum your engine makes at idle and is essentially big a vaccum operated jet that shuts at low load, thereby reducing the fuel flow. If that's right I would think you should still have acceptable drivability, even if your engine demands fairly large jets.

 

I think the rule of thumb is half the idle vaccum (ie. if it idles at 12" vaccum, then use a 6" PV).

 

tl;dr: Needs more EFI. :gabe:

 

Edit: Here's some good general reading.

http://www.holley.com/TechService/Library.asp

 

http://www.junkyardgenius.com/holley/tune01.html

 

i thought the power valve just puts in a shot of gas at a quick opening of the throttle plates (to keep the car from bogging). i believe you may be referring to vacuum secondaries, in a 4 barrel carb (if you have vacuum secondaries) the less throttle, the less the secondary plates open, the more throttle, the more they open. i could be wrong, im not an expert on carbs, but thats how i understand it.

[Mattsv8]

Power valve opens when vacume drops below a certin point. It stays open till vacume is above its set point. Holleys usually have a 6.5" pv. U can pull fuel from the mains and use pvs to add it in at wot. This is how u get lean airfuel numbers idle part throttle. The squirters work off the accelerator pumps and give a shot of fuel when u first open the throttle this fights of a lean bog.

[Patterson]

i thought the power valve just puts in a shot of gas at a quick opening of the throttle plates (to keep the car from bogging). i believe you may be referring to vacuum secondaries, in a 4 barrel carb (if you have vacuum secondaries) the less throttle, the less the secondary plates open, the more throttle, the more they open. i could be wrong, im not an expert on carbs, but thats how i understand it.

 

You're thinking about the accelerator pump.

[Miller]

Ya I'm just wondering what jets I should start with in the primary and secondary for a lightly modified 351w

[Patterson]

Start at 68/72. Primary and Secondary respectively.

[Major_golf]

Start at 68/72. Primary and Secondary respectively.

Call me!!

[Mattsv8]

most carbs come jetted very close to what they need to be at their respective sizes. if your in doubt and your carb is used set it back to stock. also check the power valve restrictor channels cause they could be drilled.

the best way to know where you need to go is buy a wideband and use it to tune. i have one on my s10 and s10 blazer and you can dial them in and get fuel injection air fuel numbers.