Box Stock

Jets and Adjustment

Skidude

Skidude

Member
What size is the factory low speed jet and what size do most folks run for a purple plate engine? I’m running a factory low speed jet and a .038 main jet.

What type adjustment is the mixture screw? Is it a fuel adjustment or an air bleed? As you can tell, I’ve not yet taken the time to tear into one and see for myself.
 
Per the man himself....
 

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The mixture screw is a fuel screw for the low speed/idle circuit. Turing it out is richer (more fuel) turning it in is leaner. This screw is not a bypass to the pilot jet, in other words the source of the fuel that this screw adjusts must still pass through the pilot jet. Once you are past 4 turns out, it time for a different size pilot.
 
Big Chris said:
The mixture screw is a fuel screw for the low speed/idle circuit. Turing it out is richer (more fuel) turning it in is leaner. This screw is not a bypass to the pilot jet, in other words the source of the fuel that this screw adjusts must still pass through the pilot jet. Once you are past 4 turns out, it time for a different size pilot.
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So if you can turn it out 3 or 4 turns and nothing changes at idle, You should go smaller pilot?? Am I thinking right??
 
I would say it's the other way around .
You've opened the fuel passage as much as allowed by the needle .
 
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T-roy said:
So if you can turn it out 3 or 4 turns and nothing changes at idle, You should go smaller pilot?? Am I thinking right??
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Larger pilot. The mixture screw regulates fuel so turns out= more fuel. But remember all the fuel coming to the mixture screw must first pass through the pilot jet. So if you're at 4 turns out and you need more fuel than you must use a larger size pilot. My thinking has been at 4 turns out you are at the upper limit of the screw's adjustment. Look inside the venturi, see where the mixture screw's needle protrudes into the venturi and regulates the additional "hole".
 
Nothing like a performance trends software with the inertia dyno to get every pony and all the torque you can get out of the motor you would be surprised what a thousands in jet size changes !!!!!
 
triton03 said:
Nothing like a performance trends software with the inertia dyno to get every pony and all the torque you can get out of the motor you would be surprised what a thousands in jet size changes !!!!!
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The main jet change from an .039 to a .038 represents a 5% change in surface area of the jet. So this is, in fact, a relatively large change. Sometimes you can "jump" right over the sweet spot. Once you understand the relationship of how the pilot and main interact (as 1fasttiller eluded to above), and realizing that the pilot always flows fuel, even at WOT. Tuning with a finer resolution than whole main jet sizes can be accomplished.

You'd be surprised what you miss with an inertia dyno that you can see with a water brake.....................but that another topic altogether.
 
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