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Holley 1904 1 Barrel Troubleshooting

Holley 1904 Accelerator Problem

Hi Mike, I resolved the problem I sent you last week. The problem was a vacuum leak at the base of the carburetor. Replaced the gasket and seems to be ok now. The other problem I am having is when I first start to accelerate the engine hesitates and I have to back off the throttle and pump the accelerator. Once I get going all is ok. I have adjusted the air fuel mixture but the problem remains. Do you think I should install a larger jet to increase fuel flow to match the air intake? If I increase throttle real slow it doesn’t hesitate. Leads me to think too much air and not enough fuel.. Thoughts???

Installing a larger jet is not the answer. Do the steps below and you will probably find the problem.

Check the accelerator pump circuit action by looking down the carburetor (engine off) and pump the throttle. You should see a strong stream coming out of the main discharge nozzle. If it looks ok, then you have eliminated the accelerator pump as the problem.

No strong stream?

  • Make sure the main discharge nozzle is clear. Run thin wire through the discharge hole.
  • Check to make sure there is a gasket under the nozzle and it isn’t broken. You don’t want fuel bypassing the nozzle.
  • Make sure the accelerator pump diaphragm was put together properly. The pump, the return spring, then the economizer body.
  • While you have the diaphragm out, look at the carefully to make sure there isn’t a crack, or tear.
  • The pump stem should have the spring over the stem, then the pump rod sleeve. The check ball should be installed in the small hole. If the spring is missing, or there isn’t enough tension, then the diaphragm won’t get the correct action, filling with fuel when needed.
  • The metering block, or economizer body can get warped over time. Be sure to grind the surface flat, or run a flat file over the surface to make sure it is flat. When they aren’t flat, fuel can leak past the block.
  • The economizer body should have a check ball and a check weight on top of the ball in the economizer body, longer hole. The smaller hole will have the bigger check ball installed.
  • Did you clean the economizer body (metering block). Perhaps there is corrosion inside the holes which might mean the check balls are not sealing.

Good stream?

Assuming your distributor vacuum advance connects to the carburetor, you will need to make sure the vacuum isn’t blocked. Remove the fitting and check for vacuum. No vacuum means the passage way is blocked, or plugged with something. It was a practice with some rebuilders to put a check ball in the passage way to block the vacuum when the port wasn’t needed. Check the bottom of the main body to see if a check ball was put in the hole. Make sure the flange gasket isn’t blocking the vacuum hole on the bottom of the throttle body.

What is the hole behind the fuel bowl for?

That hole is the relief hole for the accelerator pump. This hole is used to vent the accelerator pump when it is filling up for the next time. There is no fuel in the back of the pump, so any fluid coming out of the hole indicates a bad accelerator pump.

Do not plug this hole off.

The vacuum hole in the base is covered up.

Holley 1904, 1 barrel carburetor

As you can see there is also a hole inside the bore that leads to the other hole. When the hole on the flange is covered vacuum is fed from inside the bore. This is as expected. This vacuum hole leads up through the carburetor and out the connection for the distributor advance tube.


After rebuild My carb is puking gas out of the overflow. I have removed and cleaned blown out seat ,checked needle looks fine, verified float was not leaking (sinking) not rubbing on side, leveled the float.

I figured it out the, There was a burr on the face of the seat body that would not allow the copper gasket to seal. Filed it flat and its fine.

Another Flooding Situation

I got this rebuilt Holley 1904 carburetor with order #28223 in August 2014. The truck has been driven about 1,200 miles since the rebuilt carb was installed. The truck had an electric fuel pump a Fram inline fuel filter and an Offy fuel pressure regulator set to 3 psi. Last weekend I removed and replaced the electric fuel pump with an original style mechanical fuel pump. Retained the pressure regulator and cut the pressure to 2 1/2 psi.
I made this change because fuel had been overflowing out tge vent in the top of the carb and running down the manifolds. After the fuel pump switch, the truck started and ran – and fuel promptly overflowed the bowl and again ran down the manifolds.1st of all I think you need more than 2 1/2 lbs of fuel pressure, but that isn’t your problem. You are flooding (getting too much fuel in the float bowl).

This can be caused by several things.

  • Dirt in the needle & seat, float gone bad, etc.
    • Test the needle & seat by turning the carburetor upside down and blowing through the inlet. If it is leaking, then try applying a thin coat of Permatex Anarobic to both sides of the washers that reside on the seat.
  • Damage from ethanol if it sat around a lot.
  • Gas turning to varnish from sitting, coating the inside of the carburetor.
  • Fuel may be leaking around the needle & seat into the float bowl. Some kits use a metal washer on the seat which doesn’t always seal well. Treat the washer with Permatex Anarobic.
  • The biggest problem with carburetors these days is the lousy gas, sitting around a lot and the ethanol.
  • I would try to blow a little air into the fuel inlet to see if you can dislodge any dirt that may have gotten into the carburetor.
  • Thank you! Blowing air in the fuel inlet seems to have stopped the flooding. Turned the pressure regulator up to 3 psi also.

Vacuum Ports

The vacuum port on the top of the carburetor body near the fuel inlet is for the distributor advance.
The port in the throttle body is for the spark valve. Not all Holley 1904 carburetors have this.

Higher Elevation

I have a 1904 with an electric fuel pump on my 1942 Willy’s MB Jeep. Seems to work fine driving on streets and highways. The Jeep looses power, acting like it’s running out of gas, when I take it off road 4 wheeling. The Jeep will stall and takes a bit to get started again. When it does start back up it puts out a lot of black smoke out of the exhaust. I love at 5000’ in elevation and go off roading up to 7,500’. Above 7000’ the Jeep runs very rough and dies frequently. I’ve made adjustments to the air-fuel adjustment screw with only minor improvements to the jeeps operation. Looking for some help:)

This would be the normal reaction when going up to high elevation. The jet is probably jetted for lower elevation and when getting up to 7,000 too much fuel is being passed.
One possibility is that the jets are metered for sea level, which would have been the default. At 5,000 we would reduce the jet size by .002. Perhaps that has never been done. I would suspect that a jet for 5,000 would also work at 7,000.
If we had the carburetor number we might be able to find the sea level size, otherwise it’s a guess.
At 5,000 watch your plugs. They should burn tan, or gray. Dark brown or black means they need to be reduced by .002.

Adjusting the idle mixture has no affect other than for idle.

I purchased a Holly 1904 carburetor kit from you. I received the kit. Disassembled the carburetor, cleaned, and installed new kit. The engine is a 223″ in a 1956 Ford f100 pickup. I tuned the carb., timed engine. It idles good, runs well. I shift through 1st, 2nd. Pulls strong. But once into 3rd gear after engine is up to operating temps. Under load, with pedal to the floor, it loses power and shudders. Let off and ease throttle,it runs okay until full throttle under load. I’ve checked mechanical fuel pressure and it is putting out 4p.s.i.,checked coil and switched to a known good one, still the same results, checked intake manifold for leaks, none to note. It acts in this manner only when hot, in 3rd gear under load, flat ground or up an incline. I’ve checked and adjusted air fuel with a vacuum gauge. I’m not sure what I’m missing. Any ideas for me to check?

First thing that comes mind is the power circuit. Make sure the economizer diaphragm stem isn’t bent to the side of the power jet stem and is the stem straight. With low vacuum, or at high speed the economizer stem is down which pushes on the stem and allows extra fuel in.

Did you put a file across the economizer block to make sure it is flat? Any warpage will cause fuel to leak by and back into the bowl.

Is the float level with the carburetor upside down?

Doesn’t Run Unless Choke is Closed

This can be caused by a vacuum leak, or starving for fuel.

Vacuum leak – Spray carburetor spray cleaner around the mounting, intake and any other vacuum source. If the RPM changes you found the problem.

Lack of Fuel – Float should be level with the carburetor turned upside down.

Look for kinks, or plugged fuel line.

Weak fuel pump- measure pressure. Should be about 4 lbs.

Plugged passage. If the problem is at idle, check the passage coming from the idle mixture screw.

Does Your Carburetor Have One of These?

Holley 1904
Connects to some air cleaners.

What are all the small passages for?

Holley 1904 Metering Block

Won’t Idle

  • First take care of the ignition system. People tend to blame the carburetor for any problem when the electrical should be the 1st thing to look at. Once the electrical system is good then look at the carburetor.
  • Check for vacuum leaks. Check around the mounting gasket, throttle body, intake manifold and any vacuum lines coming from the carburetor, or intake manifold.
  • Check the throttle shaft for looseness. Spray some carburetor cleaner around the ends of the shaft. If the RPM smooths out, you found the problem.

Are you getting too much fuel at idle?

  • Determine if fuel is dribbling out of the main discharge at idle. That shouldn’t be happening and is probably due to the discharge check ball not seating.
  • The needle & seat could be leaking. Treat the seat gaskets with Permatex Anarobic.
  • Remove the idle mixture screw and blow out the passage. Put a finger over the idle mixture hole in the bore and blow through the mixture hole. Is air coming out of the top?
  • Blocked passages might be caused by old fuel turning and coating the inside of the carburetor. A rebuild/cleaning would be called for.

Rebuilt My Carburetor But Still Floods

Aside from you doing something incorrectly there are a couple of ideas.

Did you test the fuel pump for proper pressure – about 4 lbs.

Is dirt getting into the carburetor. Many rebuilds go bad because the fuel filter, or tank is dirty and polluting the carburetor after it is cleaned.

Did you test the needle & seat to make sure it’s sealing. The 1904 is notorious for leaking around the seat. See above for flooding comments.

How Can I Fix Flooding

Float Bowl Leak

Most 1904 carburetor these days have warped bodies where the float bowl attaches. The biggest problem with the fit of the 1904 floatbowl is not the bowl itself. Rather, it is the presence of so much warpage in the main body surface to which the bowl mounts. I know of no other way to fix this than the way that I use: put the main body in a press, heat it up and press straight the “ears” to which the bowl mounts. Yes, after I get it as straight as it is going to be I dress the gasket with some Permatex Anaerobic Gasket Maker, but that will do you little good without a reasonable straight main body.

Carburetor is Empty After Sitting Overnight

  • Engine is getting hot and when turning off, the gas boils out of the carburetor.
  • Gas is dripping from the bottom of the float bowl when they are usually warped. Use a cork gasket, or straighten the carburetor.
  • Gas cap is non vented and should be vented. Easy enough to check after a long run, loosen the gas cap and if non vented it will probably suck in air from excess vacuum in the tank.
  • After running, turn the engine off and look down the carburetor. Is the main discharge dripping gas? If so, it is being siphoned into the engine. Vent is plugged, or wrong gas cap perhaps. The check ball in the metering block may be leaking.

This is an early version of the IHC Holley 1904 carburetor, typically from the 1950’s. The valve on the carb bowl is an external adjustable main jet. These are not replaceable with any parts currently manufactured. Replacement bowls are also not available.

Updated on 12/01/2021

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