Carburetors

Just Holley 4150 & 4160 for now

Note: If non-ethanol compatible materials are used for the needle-and-seats' O-rings or for the accelerator pumps' diaphragms they could be damaged by ethanol gasoline

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

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

• Before adjusting the carb's jets, or valves

  • eliminate any possible issues with the vehicle's ignition
  • eliminate any possible issues with the vehicle's fuel system
  • Verify the damper and timing tab really do reflect TDC
  • Verify the ignition advance is working correctly
  • Verify you have 5 psi of steady fuel pressure at the carb
  • Verify the carb's floats are set correctly
  • Verify that the cars valves are adjusted (valve lash?)
  • Verify there aren't any vacuum leaks

• TUNE based on the results of the car under actual operating conditions, don't assume anything

• 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,
• Reading spark plugs helps a lot

• Changing ignition advance curves helps a lot

• While tuning your carb you'll most likely need to change the

  • carb jets
  • power valve
  • accelerator pump cam
• These are less likely needed to be changed

  • squirter size
  • accelerator pump size
  • booster type

Power Valves - Vacuum Activated Fuel Enrichment Valves

power valves and accelerator pumps make up the Holley 4150/60's vacuum activated fuel enrichment system, AKA "power enrichment system"

• The power enrichment system (which includes power valves)

  • Supplies additional fuel to the main system during heavy load or WOT
  • Helps maintain a target AFR only while transitioning from idle/cruise to heavy-load/WOT
  • Note: PVs are not intended to be an enrichment circuit during normal throttle cruising when manifold vacuum is high

• How they work

  • They have a spring and diaphragm that restricts additional fuel flow until the manifold vacuum falls below a certain value
  • They're screwed into the metering plate which is located between the fuel bowl and the carb's main body
  • Each Holley power valve is stamped with a number to indicate the vacuum opening point, for example

    • "65" indicates that the power valve will open when the manifold vacuum drops below 6.5"Hg (=< 6.5"Hg)
  • The higher the vacuum threshold number, the faster the power valve opens, e.g. an 85 will open sooner than a 55

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

  • For each PV rated at a certain vacuum threshold, there's a "Standard" & "High" flow version which indicates orifice size
  • Use standard flow for small blocks, up to 350 cubic inch
  • Use high flow for big blocks, 396 and up

Power Enrichment Timeline of Events

• Event 0 - Idle or light cruise

  • Manifold vacuum is high because the throttle blades are almost closed, restricting flow into the intake
  • The carb's main jets provide sufficient fuel to maintain the target AFR
• Event 1 - The Driver presses the gas pedal

  • This rotates the carb's throttle linkage clockwise ("open")
• Event 2a (concurrent w/2b) - The Accelerator pump begins to spray an extra shot of fuel into the venturi

  • This produces a richer, less detonation prone air/fuel mixture during the transition from partly open throttle 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
  • This happens in just the primaries for a "Vacuum Secondary" 4150/60 and in both the primaries & secondaries for a "Double Pumper" "Mechanical Secondary" 4150
• Event 2b (concurrent w/2a) - The throttle blades open

  • Manifold vacuum rapidly DEcreases

    • Under WOT, it could drop all the way down to 0"Hg - 1.5"Hg
  • The velocity of air going through the carb's venturi speeds up

    • This rapid INcrease of air flow requires additional fuel in order to maintain a target AFR
• Event 3a - (concurrent w/3b) - The accelerator pump stops spraying fuel into the venturi

  • The accelerator pump has run out of stroke
• Event 3b - (concurrent w/3a) - The power valve opens

  • The lower manifold vacuum triggers the PV's diaphragm/spring to open its valve
  • More fuel flows through the carb providing a richer, less detonation prone air/fuel mixture
  • This prevents bogging and hesitations caused when the carb's main jets alone can't keep up with the fuel demand
• Event 4 - The Driver lets off of the gas pedal
• Event 5 - The throttle blades close

  • The velocity of air going through the carb's venturi slows down
  • Manifold vacuum INcreases

    • The throttle blades are almost closed which restricts air flow into the intake
    • Typical values are 11-20"HG
• Event 6 - The power valve closes

  • The higher manifold vacuum triggers the PV's diaphragm/spring to close its valve
  • This leans out the air/fuel mixture
• Event 7 - The car returns to partial-throttle, steady-speed operation, low speed cruising, or idle

  • The carb's idle circuit and/or main jets provide sufficient fuel for the target AFR

Tuning power valves

• The initial power valve rating should be 1/2 of the manifold vacuum reading while the car is at a steady cruise or idle

  • 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 foot on the brake
  • The reason to start with a PV rated at 1/2 of the car's idle manifold vacuum is so it doesn't open while the car's idling or just off idle. That would cause an excessively rich mixture.
  • Note: There are only "x.5" power valves, i.e. there are no "x.0" power valves. If the vacuum reading is an even number, select the next lower power valve.

Recommended PV Sizes (to start with)

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

• Some internet ppl say

  • getting a manifold vacuum reading at steady cruise is better than idle
  • Vacuum at cruise (Vc) is about equal to vacuum at idle (Vi) less 4"Hg (Vc = Vi - 4, "If you take CRUISE vacuum, divide it in half and add about 2" you will be close)
  • Some internet ppl say that 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.
    • you'll 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.

Broken power Valves

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

the most common cause for a busted power valve is a carb backfire

• If you suspect a blown power valve, use this simple test:

  • At idle turn your idle mixture screws all the way in. If your engine dies the power valve is not blown

Accelerator Pumps & Squirters

select a 30cc or 50 cc accelerator pump volume

the accelerator pump cam will control how fast the carbs linkage starts to cover the transition between idle and fully open venturi 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

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