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                                                     Holley Rebuild - Info - Date Codes 

                                                                                                            Revised Jan 17, 2012 - Updated 4 digit Date Chart

Click on thumbnails for larger picture

Other Holley info after the rebuild section

CARB SPECS

Holley 3310-2 / 780 CFM Rebuild

This is my spare carb so at the time my 780cfm did not require a rebuild, so I just used my spare to take apart and snap shots.

The principle is the same except there is no rear metering block on a 3310-2 750 cfm Holley.

I will add 780 pics when I need to tear down my 780 for problem resolutions.

 780

3310

remove 4 fuel bowl mounting screws. If your going to just swap jets or change gaskets while the

carb is still on the car, then remove one of  the lower bolts 1st and drain the fuel into a funnel/container.

Front Bowl/Metering Block

pry front metering block & remove gasket and remember orientation for new one

780 front metering front block/bowl/baffle

  3310--->

Remove power valve views using a 1" socket (780)

Now test the power valve for leaks.

Place a spark plug boot tight against the front and suck the air out and using your tongue see if it stays closed. If it holds, then it's still good.

Clean gasket surfaces and clean all holes/passages with carb cleaner and blow passages out with compressed air, then install power valve.

Place in a vise with wood on each side of the vise  jaws so you don't damage it and using a 1" socket install/torque.

Note gasket type.

When installing, tighten to the following specs.  For power valve with multiple drilled fuel opening, torque to 40-50 inch pounds.

For power valve with 2 rectangular fuel openings, torque to 100 inch pounds.

3310 front metering block - note gasket orientation pins

jet side

metering valve side

Rear Bowl/Metering Block (no metering block on a 3310-2)

Remove 780 rear bowl, clean parts, and remove power valve, then reinstall power valve

Remove front idle mixture screws, 2 per front metering block, and change cork gaskets. You can use a 7/32" drill bit to push it all the way in.

Install screw all the way in clockwise until it lightly touches the end (don't over do it or the point will get damaged.

Turn both screws out 1.5 turns counter clockwise for initial startup.

Jets

  780--->

There are 2 per metering block front/rear (none on the rear of a stock 3310-2)

Using a thick straight screwdriver, remove both jets, there are no gaskets here.

Front 780 jets are 70 and rear are 76.

Needle and Seat  

Using a wide screwdriver, unscrew and remove the lock screw. Then turn the nut counter CW to remove the needle/seat.

Note paper gasket on both sides of the nut.

Needle/seat removed. Replace them unless they are fairly new. Before you install them, take compressed air and blow them clean while

holding the needle off the seat. One spec of dirt will cause flooding.

Before installing the needle/seat, lube the needle/seat O ring with a dab of oil or the O ring will be too tight on assembly and may tear

turn bowl upside down and turn the needle/seat clockwise until the bowl is level, you'll check this later with the car running

Check to see if the float is getting any fuel inside. Remove it and shake it.

Using a flat screwdriver remove the float.

Remove the from and rear (780) fuel level site screws and note a metal gasket.

Some kits today are using a plastic washer. I myself use the metal ones, because

I don't trust the plastic since it doesn't compress.

Front accelerator pump

<--- 780-->

Using a square head screwdriver, remove the pump cover and note diaphragm orientation and spring.

If the gasket is old, it'll be hard and won't be able to flex so replace it.

Secondary Vacuum

780--->

Side view of secondary vacuum linkage/manual choke if equipped as on the 3310

3310 - Remove side choke mount and note paper gaskets and replace from kit

Note the cork gasket, if this isn't installed then the secondaries will not open & you will have a vacuum leak

Remove secondary vacuum diaphragm housing. Note spring orientation, it snaps into the post.

Check the chart below for correct spring color (plain color are used from speed shop carbs)

Also note chrome ball will be here on 71+ Holley's

Fuel Filter

Remove fuel inlet fitting, note metal gasket. If your using the stock fuel filter then it'll be the hard brass ones.

Don't forget the spring if your using this location. If you don't want to wear the fuel line attachment bolts

even when using a fuel line flare wrench then install an inline fuel filter.

Body - using a wire (guitar string) clean out these holes

3310 Rear

Remove rear fuel bowl

The 3310-2 has no metering block but a plate to keep costs down, remove plate using a square screwdriver and lift and remove gasket.

For the 780 remove the bowl/metering block the same as you did for the front.

  780--->

With the bowls off and metering blocks or rear plate, remove all philips screws from the throttle body and note gasket orientation

780 rear bowl/float/metering block

This screw is your secondary stop screw and must be set off the car, see Step 2 below after the rebuild section.

Remove nozzle pump screw and 2 paper gasket

Then remove the needle

Pump discharge needle orientation (paper gasket on each side of the pump nozzle)

Clean all parts with carb cleaner and assemble in reverse order and setup

While WOT is held open, using a flat screwdriver, push the lever down and take a reading, it should be .015, if you don't do this you will have a stumble when you press the accelerator

If while idling you give it some gas and it has a stumble and the .015 doesn't fix it then you need to test the power valves. My rear one was shot when this happened in July 2011.

turn in lightly until they stop, turn out 1.5 turns. When car is idling, set until you get the highest RPM or

highest vacuum reading with a vacuum gauge, then do the other side then repeat the drivers side.

Remove while idling and the gas should be at the bottom of the threads. If you nudge the fender with your leg, it should just trickle out.

If gas is coming out too fast, reinstall site screw and lower float

With car running, using a box wrench, hold nut so it doesn't turn, then loosen lock screw just enough so you can turn

the NUT 1/4 turn clockwise (this nut is connected to the needle/seat and lowers it or it can raise it to raise the fuel level in the bowl).

Rev the throttle once and wait 5 sec. Open fuel bowl site screw and check the level.  If it's still too high, repeat procedure. If  the fuel level is too low,

then the nut needs to be turned counterclockwise to raise the needle/seat thus raising the float.

Other Info

Exploded view - Part numbers - Variable parts listing

4 speed 1970 Z28 780 cfm Holley List 4555 GM # 3972121

Auto 1970 Z28 780 cfm Holley List 4555 GM # 3972120

Holley Quick Setup Steps:

1a. Accelerator pump. At wide open throttle the pump lever should be set to .015 of an inch (feeler gauge). Undo your throttle return string if your doing this on the car. Have a friend pull back and hold throttle lever wide open as if the gas pedal is floored or tie it down but make sure it's still wide open.

1b. Using a long slot screw driver push down on the pump lever hard and hold while using your other hand measure with a feeler gauge the space between the lever and the pump adjusting nut. If you need to adjust then let go of everything and adjust the nut to get .015. Hold the bottom nut and turn the top nut clockwise to compress spring and make the gap bigger or counter clockwise to make the gap smaller. 

2. Secondary throttle stop screw adjustment Must have carb off car. Set as follows: Back off adjustment screw until the throttle plates are fully closed (secondaries). Turn adjustment screw until it just touches the throttle lever and turn 1/2 turn more to position the valves.

3. Float level While car is running. Float level should be to the bottom of the fuel bowl site screws. Have a rag handy, then loosen the lock screw a bit and hold, then turn nut clockwise to lower float level or counter clockwise to raise. A 1/4 turn makes the float move 3/16". You can give the accelerator a shot to hurry up the change you made. It may be hard to see the fuel at the bottom of the site plug hole. What I do is nudge the fender a bit and if the fuel drips out of the site plug then that's perfect.

4. Idle Set idle at 800 rpm or 1000 or what ever your specs call for.

5. Secondary diaphragm spring  You should start off having a yellow secondary diaphram spring to see how fast the secondaries are opening. Don't need the chrome ball in the secondary vacuum base (70 did not use one). Don't forget the cork washer on the sec vac base that mates to the carb or the secondaries will not work.

6. Mixture screws Turn in until the engine starts to stumble and back out screw 3/4 turn. If your cruising and the car feels like it's surging then richen (turn counter clockwise) the air/fuel mixture screws 1/16" on both sides and go for a drive. Do this until the surge is gone. I know you probably don't have a surge problem but keep this in mind for future problems. Some guys like to turn the screws out (richen) until they get the highest vacuum reading or highest rpm but I think this ends up being too rich. You can smell the fuel at the tail pipes at idle which means your too rich unless your car won't idle unless your on the rich side.

7.  Metering Block (To remove metering block)

1. Remove one of the lower screws that holds the fuel bowl to drain the gas from the bowl  (use a pan to catch it) don’t let the gas run on the intake. 

2. Undo the fuel line to the front bowl if your working on the front bowl/metering block..

3. Pry off the bowl carefully so you don’t damage the gasket.

4. Remove metering block,  careful with the gasket.

5. Use a wide flat thick screw driver to unscrew the jets. 

The 2 jets go into the 2 small holes and the power valve goes into the centre hole. Remember to use a gasket for the PV and torque to specs.

8.  Power valve If you have to change the power valve then use an adjustable or socket. Place in  a vise with wood on each side of the vise  jaws so you don't damage it. Tighten to the following specs.  For power valve with multiple drilled fuel opening, torque to 40-50 inch pounds. For power valve with 2 rectangular fuel openings, torque to 100 inch pounds.  

What size Power Valve to use? To decide what size,  use a vacuum gauge and measure the vacuum at idle and subtract that reading by 2. 

Example: if idle reads 10 then you need a 8.0 or 80.  If the number is not on the side where the wrench grabs it,  then look at  the front face. The first digit is at 2 o’clock and the second digit is at 5 o’clock. Example: first digit=6 and second digit=5. You have a 65 power valve. When the engine vacuum goes down to 6.5  then the valve opens up and lets in more fuel in the carb.

Changing the screw position for the pump cam will give you a different fuel discharge.

                        Q & A Most Common Problems:

All setup steps from above have been followed prior to jumping here.

QUESTION Flooding or have fuel dripping on the throttle plates after engine shut down.

ANSWER Remove the needle and seat from the top of the bowl and take compressed air and blow out all specs of dust/dirt. One spec will stop the needle/seat from stopping the fuel from leaking. Did you also remember to add a gasket and the small piece of steel with a point at one end under the squirter?

QUESTION I have a wet spot of fuel overnight on the bottom of your throttle plates.

ANSWER This is an actual problem I had and was stumped for a while. I removed the fuel bowl and metering block and noticed that the power valve was loose. Place the block in a vise with wood on each side of the vise jaws and tighten to the following specs.

But this is usually caused by percolation. This is when the engine is shut off and the engine temperature rises it causes the fuel to boil in the bowl and leak out of the boosters. There are a couple of things you can do to cure this one is make sure the fuel level is not too high. You can also lower the fuel level about 1/8" below the sight plug hole and this will cure it sometimes. The heat from the engine will rise into the carburetor sometimes and will cause the fuel to boil. Installing a phenolic heat spacer between the carburetor and the intake or a heat shield can cure this. These parts will prevent heat from getting to the carburetor and boiling the fuel.

QUESTION Turning mixture screws clockwise does not lower RPM or stall engine.

ANSWER A carb rebuild is required.

QUESTION Car stumbles for 1/2-1 sec when I cruise and hit the accelerator short or full shot.

ANSWER See step 1a above.

QUESTION Car bogs when I tromp it.

ANSWER Your secondaries are opening too fast. Go with a stiffer secondary vacuum spring. Or  your running with the incorrect squirter and or accelerator cam for your engine.

QUESTION My vehicle has a stumble from a dead stop. What can I do to cure this?

ANSWER The most common cause of a stumble is not having an adequate accelerator pump shot. The first thing to do is to look at the discharge nozzle and make sure you are getting a good strong pump shot. If not then you need to inspect the pump diaphram for a hole or tear. You will also need to make sure that the pump passage is clear from any trash or debris. First you will need to check the adjustment on the pump. To do this you will open the throttle all the way (WOT). Push the pump arm lever down and then adjust the pump override spring to obtain .015" clearance between the pump arm and lever. If you are having a stumble and no black smoke out the tailpipe then you will need to increase the shooter size. If it stumbles and you are getting black smoke from the tailpipe then it will be nessasary to decrease the shooter size.

QUESTION I can rev the engine when it is in park and the secondaries will not open. Why is this?

ANSWER The secondaries will not open by free revving the engine. The engine needs to be under a load before they will open. If you are still uncertain if they are opening or not you can take a normal paperclip and clip it onto the secondary diaphram rod. You will then push it up against the bottom of the secondary diaphram housing, now you will need to go out and drive the vehicle. When you return you will be able to look at the position of the paperclip on the rod. If it is lower on the rod then you can tell the secondaries opened and how far they opened. This is useful in determining if you need a heavier or lighter secondary spring.

F.Y.I The 1970 manual for a 396/375 or 454/450 front jets should be 70. Rear should be 76. 

Holley Vacuum Secondary Spring Info:

CFM Size Chart

780 choke

Rear fuel line support bracket   

Fuel Filter

I use a 3/8 inline filter hidden under the alternator bracket.

GM GF61P part # 854619

Z28 Heat Shield

Heat shield for a big block car (new GM)

    Top                Bottom              Bottom #'s                 Parts Pkg    

Stock 70 Z28 vacuum hoses with AIR pump run from:

TCS vacuum switch connections vacuum hoses  

Accelerator Spring

Left is original GM, Right is a repo

Spring thickness less paint. The paint would probably be 4mm

Spring end inside dimension

                                    Date Codes

1970, July, 2nd week

0348 - 034=February, 8=1978