Carbeurators

Just Holley 4150 & 4160 for now

THE O-RING ON THE NEEDLE AND SEAT, AND THE DIAPHRAGM IN THE ACCELERATOR PUMP, CAN BE DAMAGED BY ETHANOL IN FUEL UNLESS AN ETHANOL COMPATIBLE MATERIAL IS USED

Power Valves - Vacuum activated fuel enrichment valves

Holley carburetors utilize a vacuum operated power enrichment system

• The purpose of a power valve is

  • the accelerator pump supplies a shot of extra fuel to cover the rapid increase in air flow during the transition from partly open throttle to fully open throttle
  • for it to add the needed extra fuel the engine needs to transition and maintain the correct fuel/air ratio when you are under wide open throttle.
  • restrict additional fuel flow until the vacuum drops to a known level stamped on the power valve rim
  • allows additional fuel flow, that comes in as the velocity of air through the carbs venturi speeds up and the vacuum reading drops, it opens,

• It is not intended to be an enrichment circuit during normal throttle cruising.

• Each Holley power valve is stamped with a number to indicate the vacuum opening point. For example, the number “65” indicates that the power valve will open when the engine vacuum drops to 6.5" or below.

• there's a number stamped on the power valve (EXAMPLE 6.5), that relates to the VACUUM reading the carb will need to drop below (in that case 6.5" of vacuum) before the power valve opens and adds additional fuel to what the carbs jets already allow

• the higher the number the faster the power valve opens as the vacuum drops as the throttle opens

• Power valves are screwed into a metering plate between the carb fuel bowl and the carb body in many carbs

POWER ENRICHMENT SYSTEM

The power enrichment system supplies additional fuel to the main system during heavy load or full power situations. Acceleration - the transition from partly open throttle to fully open throttle

Event 0) at idle or light cruise, the car produces more vacuum because the throttle blades are almost closed restricting flow into the intake

Event 1) The Driver presses the gas pedal, which rotates the carb's throttle linkage clockwise/open Causing both of these events to occur at the same time:

Event 2a) The Accelerator pump sprays an extra shot of fuel into the venturies

- Covers the transition from part to fully open throttle
- The cam actuates the lever which pushes up on the diaphragm
- Fuel is sprayed against the outside of the boosters for better atomization
- Only primaries for a "Vacuum Secondary" 4150/60; both primaries & secondaries for a "Double Pumper" "Mechanical Secondary" 4150/60

Event 2b) The throttle blades open causing a

- Rapid INcrease in air flow 
- Rapid DEcrease in vacuum level
- as you open the carb throttle blades vacuum drops off and flow increases, at some point the carbs increased air flow requires additional fuel to maintain the correct fuel/air ratio

Event 3a)

- the accelerator pump has mostly run out of stroke

Event 3b)

• the power valve opens to flow extra fuel during the transition period and while the accelerator pump is also adding fuel, . Under WOT the vacuum of the engine will drop to a vacuum reading of 1.5 or even lower near zero and the power valve will then open adding the extra fuel needed during WOT.
• the power valve allows additional fuel to flow, as the plenum vacuum drops.
• the diaphram on the power valve senses the throttle is open and opens a valve which allows a richer fuel/air ratio mixture to flow until you lift off the throttle .
• The power valve senses low vacuum and opens up and allows more fuel to flow through the carberator allowing a richer and better, less detonation prone power producing mixture, and thus you don,t get a big "BOG" or cough when the carb suddenly ingested more air and the carbs jetting alone can,t keep up with demand,
• the power valve controls the point where that fuel starts to be added using the drop in intake vacuum as the trigger to release more fuel.

Event 4)

- as the vacume reading increases the power valve senses the increase and then it closes
- Leaning its mixture out for only partial throttle open operation used for steady speed operation like durring low speed cruising.
- the power valve fuel supply flows fuel until you lift off and the vacuum in the intake plenum increases, and shuts it off.
- keep in mind the power valve supplys extra fuel as the vacuum drops as the throttle blades open in the transition process from idle to full throttle, 

Event 5)

- and the carbs fuel delivery internal fuel routeing goes back to standard jetting

Tuning power valves

• keep in mind there's many guys chasing carb problems that are eventually traced to the ignition, or fuel system, verify the damper and timing tab really do reflect TDC and your ignition advance is working correctly, and verify you have 5 psi of steady fuel pressure at the carb and the floats are set correctly and the cars valves are adjusted and there's no vacuum leaks before you go swearing its solely a carb problem

• If you are lean or rich during normal driving conditions, you need to tune the jets, not the power valve.

• The reason you check the vacuum at idle and take that reading and divide it in half, is so you do not have a power valve that will open at idle as this will tend too cause an excessively rich mixture at idle, or just off idle.
• START the tune with the power valve used being selected too be close to 1/2 the vacume/plenum reading at a steady cruise, at minimum throttle NOT at idle in neutral
• TUNE based on the results youll see in actual operation, and you don,t assume anything here, your almost certainly going to need to change the power valve, jets and accelerator pump cam based on the results youll see under actual operating conditions.

• An accurate vacuum gauge should be used when determining the correct power valve to use.

• A competition or race engine which has a long duration high overlap camshaft will have low manifold vacuum at idle speeds.

• Holley's instructions might be misleading (use a PV 1/2 idle vacuum)

  • They will ALWAYS lead you to choose too low a PV number, which in turn will force you to use too large jets, too much pump, etc.
  • youll generally want to select a slightly higher number power valve if your getting a flat spot.
  • Keep raising the PV number until that flat spot clears up.

you take the vacuum reading at idle and cut it in 1/2 then jump up or down one -two numbers depending on how the plugs look, the engine runs and what your fuel/air ratio meter indicates example if your pulling 16" at idle that would be a power valve in the 6.5-9.5 range youll also see advise saying that Basing the PV selection on idle vacuum is BAD advise that has floated around for a long time. If you take CRUISE vacuum, divide it in half and add about 2" you will be close. read what holley says

• THE Only way to truly know how the combo of power valve, booster type, accelerator pump cam, squirter, accelerator pump size and carb jets will work on your combo, is to try a couple different combos and see how the 60' times and the engine feels as you accelerate, and both experience and knowledge of how to read spark plugs and change ignition advance curves will help a great deal.

• If the vehicle has a manual transmission, take the vacuum reading with the engine thoroughly warmed up at idle in neutral

• If the vehicle is equipped with an automatic transmission, take the vacuum reading with the engine thoroughly warmed up, idling in gear with fott on the brake

After determining the manifold vacuum in inches-mg, select a power valve thats 1/2 of that value

Note: There are no "n.0" power valves, there are only "n.5" power valves. If the vacuum reading is an even number, select the next lower power valve.

Recommended Starting Sizes

• There's no "4.0" PV and 3.5 is next lower

Broken power Valves

• If you have a carburetor older than 1992 (or you have experi- enced an extreme backfire) and expect a blown power valve, use this simple test. TEST: At idle turn your idle mixture screws (found on the side of the metering block) all the way in. If your engine dies the power valve is not blown

  • power valves have a diaphragm that can be ruptured
  • a BLOWN or busted power valve will leak fuel in an irregular manor,and not regulate flow, so if it does not test correctly, you'll need to replace it, the most common cause, for a busted power valve is carb back fires

Accelerator Pump

• select a 30cc or 50 cc accelerator pump volume
• the accelerator pump cam will control how fast the carbs linkager starts to cover the transition between idle and fully open venturies when the engine could go lean
• remember that a larger squirter size does not give you more total fuel, it only allows more volume to move through the squirter faster, but this only tends to result in a shorter duration squirt, of fuel before and during the transition,as the power valves and secondary jetting add fuel and air flow

"Standard Flow" PV vs a "High Flow" PV

• There's a standard & high flow rate version for each power valve
• match the fuel flow rate of the power valve to the engines needs

  • standard flows work well for the small blocks (up to 350 cubic inch)
  • high flows work better on big blocks (396 and up)

Holly "List Numbers"

I think that for each model number there are many variations and each of these variations has a different "list number"

I think that list numbers are the same as part numbers

How to Identify Your Holley or Demon Carburetor

List numbers are stamped; they are not raised like a casting number

List numbers are typically hand stamped on the front of the choke tower to the right of the vent tube.

Examples:

	80457 <-- List Number
	1571 <-- Date code

	4779-9 <-- List Number
	  1008 <-- Date code

	3577185 <-- Optional OEM part number	
	4790    <-- List Number
	2124    <-- Date code

• List numbers for the most popular models

  • 4150, 4160 square flange
  • 4165, 4175 spread bores