Holley 1904 rebuild, tuning and knowledge base.

1959 - 1962, Mighty Mite, M422, Wanted, USED PARTS FOR SALE, and Knowledge Base
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lawverdict
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Holley 1904 rebuild, tuning and knowledge base.

Post by lawverdict » Sun Jun 23, 2013 10:21 pm

I do not have time to write this up and post all at once, so I will post in parts as time permits. Disclaimer: you are responsible for your own safety. I do not profess to be an expert on anything and there are many theories on rebuilding carbs and tuning. I am not stating that my methodology is correct, it's just the way I elected to proceed. If you want to rebuild the carb and tune as recommended by the factory, you would probably be best served by following the factory manuals. My post(s) will be written with the novice in mind and the terminology I use will be based on my experience. To the extent I can locate the component in the manual, I will use the same terminology. Rebuilding this carb is a relatively simple process and even if you have little or no mechanical experience, you should be able to complete the task. If you have rebuilt this carb, you might wish to bypass this post as I think it will be a lengthy post and you will already know the content. If you are aware of more precise terminology or more efficient methodology, by all means, post it.

My MM had a significant miss just off idle and I checked for vacuum leaks, the ignition timing, spark plug wires, distributor cap, point gap, vacuum, spark plug gap, ignition advance, ignition voltage, accelerator pump timing, accelerator pump discharge, choke adjustment, choke fast idle cam, fuel screw, throttle linkage, idle setting, and cylinder compression. Other than vacuum, compression and accelerator pump timing, all were within specification. Before you delve into rebuilding the carb, insure that all other components are within specification. Don't be concerned if you don't know how to check the foregoing, read through this post and most of the topics will be discussed in sufficient detail (If I ever finish.) Alternatively, I believe most of the foregoing is discussed in the manuals.

To my knowledge, unique things about this model carb specific to the MM include a magnesium fuel bowl, aluminum throttle body, and a vent at the top of the fuel bowl. Be very careful during disassembly and assembly as parts can be very difficult or expensive to obtain. Variations of this carb were used on International Harvester truck and tractors, Fords in the 1960's, and other vehicles. There are good write ups on International Harvester forums and I will post a link when I find it. I will also post a link to a company in Florida that offers a rebuild kit specific to the MM.

Before beginning, let engine cool down to room temperature. Disconnect battery. Have fire extinguisher close. Loosen clamp securing air cleaner hose onto carb elbow and remove air cleaner hose. At carb elbow, remove breather to distributor and breather to fording valve. Remove fitting on carb that runs to air cleaner

Image Loosen clamp holding carb elbow to carb and remove carb elbow from carb.

Place rag under fuel fitting into carb. Remove vent fitting on carb that runs to the air cleaner. Remove fuel fitting, using wrench to hold the brass fitting stationary while turning the fuel line fitting.

Remove throttle linkage at the carb.

Remove choke cable.

Remove two nuts holding carb to manifold. Remove carb. Tape manifold closed or place clean rag in manifold.


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Re: Holley 1904 rebuild, tuning and knowledge base.

Post by lawverdict » Thu Jun 27, 2013 9:40 pm

Use 11/16 wrench to hold brass fuel fitting in position while turning fuel fitting with 1/2 wrench. Failure to follow this procedure may result in damage to carb and brass fitting which may result in leaks. Same procedure during assembly.
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Loosen screws identified in photo below. No need to remove screws completely, just slide choke cable out.
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Throttle rod can be pried off with a screwdriver.
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Remove 9/16 nut on each side of carb.
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Sequentially remove four screws securing fuel bowl. By sequentially, I mean turn each screw 1/4-1/2 turn then go to the next screw until tension is relieved. These fuel bowls can warp from improper assembly/disassembly and if the screws are installed with excessive torque, the body of the carb can warp. On assembly, I apply anti sieze to the screw threads and gently hand tighten the screws. I cannot overemphasize that improper installation of the fuel bowl cover will result in leaks. If the cover does not easily remove, gently tap the cover with a rubber mallet, wood block or handle of the screwdriver.
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Remove the brass fuel fitting and insert a screw driver into the housing. Make certain that the screwdriver does not contact the housing as the threads are fragile and can be damaged.
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Re: Holley 1904 rebuild, tuning and knowledge base.

Post by lawverdict » Sun Jun 30, 2013 10:52 pm

Gently remove float assembly. Inspect float for any leaks. Note vent to atmosphere on top of carb bowl. Upon reassembly, make sure the gasket is matched to allow venting. This vent vents to center vent in carb barrel.
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Inspect float needle and needle seat for any imperfections. The function of the float needle is to stop fuel flow when the float rises to a preset height. The float tab is used to adjust float height, the setting of which will be discussed later.
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Remove three screws securing enrichment valve actuator cover. Remove cover and enrichment valve actuator. The center vent identified in the photo below is the center vent identified in previous photos in the carb fuel bowl.
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Remove three short screws and two long screws securing metering block to carb body. Remove metering block.
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Metering block removed. (Enrichment valve actuator should already be removed.)
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Re: Holley 1904 rebuild, tuning and knowledge base.

Post by lawverdict » Wed Jul 10, 2013 7:41 pm

Examining the metering block, the orifices from left to right are a vent to atmosphere, the main and enrichment circuit, the idle and transition circuit and the accelerator pump circuit.
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Examining the carb body, fuel flows through these orifices into the carb barrel. Orifices from left to right correspond with the metering block: fuel pump circuit, Idle and transition circuit, Main and enrichment circuit, and vent to atmosphere.
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The other side of the metering block (main jet removed).
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Fuel flows through the metering block, into the carb body and exits the barrel at the locations indicated.
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General operation.
Idle/transition circuit: fuel is first metered through the main jet, travels through the metering block, is again metered by the small orifice on the carb body, flows through the carb body and is again metered by the fuel screw in the throttle body and is again metered by the idle orifice and the transition circuit slot. Note that when the throttle blade is fully closed, the transition circuit slot continues to meter fuel into the carb barrel. I clean all circuits using compressed air. I typically remove the fuel screw for inspection. The fuel screw should not be bent, galled or otherwise damaged. When removing or installing the fuel screw, be careful not to damage it any way.

Main circuit: fuel is metered by the main jet on the metering block, enters the carb body and is discharged into the barrel from the orifice just below the choke blade as shown in the photo above.

Enrichment circuit: the enrichment circuit bypasses the main jet in the metering block. When fuel flows through the enrichment circuit, fuel flows through the metering block into the carb body through the main circuit and is discharged into the carb barrel from the main circuit orifice just below the choke blade. The enrichment circuit only flows fuel when low manifold vacuum is present. The poppet valve on the enrichment circuit is depressed by the enrichment circuit actuator and is always open until high manifold vacuum pulls the actuator up and closes the enrichment circuit valve. In operation, when the throttle is open, or the engine is under load, low manifold vacuum results and the enrichment circuit flows extra fuel to meet the air demand. At idle and cruise, high manifold vacuum results and the valve is closed so the engine runs efficiently. I have seen the enrichment valve called the "economizer valve' on the International Harvester forum. I don't know the correct terminology, so either will suffice. As I understand it, the poppet valve is not serviceable, but I believe there is a person on the International Harvester forum that can replace the poppet valve. In fact, there is a lot of good information on the Holley 1904 on the International Harvester forums. Manifold vacuum is picked up by the orifice below the throttle blade, is directed through the carb body to the actuator cover on the top of the actuator and pulls the diaphragm, lifting the actuator and closing the poppet valve. Similar in operation to a "power valve".

In the photo below, the enrichment valve cover is shown. Manifold vacuum is picked up in the throttle body below the throttle blade, is directed through the carb body to the area indicated in the photo below. The vacuum acts on the diaphragm which moves the actuator up, which closes the poppet valve. Check diaphragm for leaks, hole, tears and replace if necessary. If this circuit does not function correctly, engine will always be running rich. I will discuss checking vacuum and function later.
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Accelerator pump circuit: The accelerator pump is actuated by a mechanical linkage that is a function of throttle movement. As the throttle arm is moved, the adjustment rod actuates the cam which pushes the accelerator pump plunger. The plunger acts on a diaphragm and causes a ball check valve to close, forcing fuel through the metering block to the carb body to the orifice in the carb barrel.
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Accelerator pump diaphragm. Make sure there are no holes, leaks or tears in the diaphragm.
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To disassemble, remove the clip holding the cam onto the carb body, remove the two cotter pins holding the adjusting rod. Upon assembly, I grease the cam pivot. I use compressed air through all orifices.

When assembling, the clearance between the plunger and cam must be minimal. I use .005 inch. Clearance is obtained by bending the adjusting rod. The throttle and choke should be fully closed when measuring clearance (photo shows choke open). Often, it is best to set the idle prior to making this adjustment. Be gentle with the adjusting rod, to my knowledge, replacements are not readily available and fabricating a new rod would take a little time. Many times, a hesitation is caused by the accelerator pump actuating too late, so check the clearance carefully. Looking down the barrel of the carb, ideally, fuel should be seen from the accelerator pump orifice as soon as the throttle is actuated.
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Last edited by lawverdict on Fri Jul 12, 2013 5:46 pm, edited 1 time in total.

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Re: Holley 1904 rebuild, tuning and knowledge base.

Post by Bill H. » Thu Jul 11, 2013 9:49 am

Absolutely wonderful information! Thank you so much for taking the time to post this.
Bill H.

"Each shall seek his own kind, in other words, a bird may love a fish but where would they build a home together?" Tevye, Fiddler on the Roof

1952 Dunbar Kapple M100
1953 Willys M38A1
1962 AMC M422A1
1965 Stevens M416B1 X 2
1967 Kaiser M715

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Re: Holley 1904 rebuild, tuning and knowledge base.

Post by lawverdict » Fri Jul 12, 2013 2:44 pm

Thanks Bill. I am trying to get this write up done, and hope I can work on it over the weekend.
Mike

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Re: Holley 1904 rebuild, tuning and knowledge base.

Post by lawverdict » Sun Sep 29, 2013 9:16 pm

I have not been able to update and finish this post until recently, so I hope that I can pick up where I left off and that I do not forget anything. Below is a photo of the throttle shaft, idle adjustment screw, choke fast idle cam and choke shaft.
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There should be very little, if any, free play in the throttle shaft bushings or the choke shaft bushings. The choke fast idle cam causes the throttle blade to open when the choke cable is pulled; thus, resulting in a fast idle. Upon installation of the carb, make sure that the choke is all the way open so that the idle adjustment screw seats on the flat part of the fast idle cam; otherwise, you will experience an artificially high idle. At the same time, make sure that the choke cable does not bind and holds the choke all the way open.

I am not going to cover reassembly of the carb, with the exception of setting the float, as you can reverse the disassembly instructions. Below is a disassembled accelerator pump return spring assembly. To disassemble, slightly compress the spring and shake the ball bearing out of the hole in the plunger. To reassemble, turn the hole in the plunger to align with the notch of the plunger retainer, compress the spring, drop the ball bearing in the hole and release the plunger.
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Setting the float. I set the float to close the float valve when the top of the float is 3/8 inch from the top of the carb float bowl. I will discuss verifying the float level in the tuning section which will follow later.
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To set the float, orient the carb upside down, push the float (toward the throttle body)until the valve closes and measure the distance between the carb float and the top (away from the throttle body) of the carb fuel bowl. Bend the adjustment tab on the float until you achieve the desired setting.
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Before installing the carb on the engine, at sea level, I suggest setting the fuel screw open 1/2 turn from closed. Install the carb on the engine, insuring that all mating surfaces are clean, that the carb spacer is in good condition and that all gaskets are in good condition. Attach all fuel, vacuum and vent lines. Attach throttle rod and confirm adjusted to close throttle blade completely. Attach choke cable and confirm choke adjusted to open all the way.

Tuning.

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Re: Holley 1904 rebuild, tuning and knowledge base.

Post by lawverdict » Tue Apr 08, 2014 9:16 am

I haven't had time to complete the tuning section, but I received a note from a mm owner stating that fuel is entering the 5/16 vent line from the carb to the air cleaner. There are many conditions that can cause this, including an improperly set float level, leaking float needle, leaking float, vacuum in the line, excessive fuel pressure, other vents clogged, etc.

I'll set forth a method to quickly check the fuel level in the carb bowl while the mm is running. The posts above show the vacuum diaphragm for the economizer valve on top of the carb secured by three screws. Remove the vacuum diaphram and associated hardware. Have a fire extinguisher handy. Have an assistant and prior to starting the engine instruct the assistant to immediately turn the engine off upon your signal. Signal the assistant to start the engine. Observe the fuel level in the bowl at idle. If the fuel level continues to climb up and out of the carb bowl, one or more of the above referenced issues may need to be addressed. I suggest you remove the carb and inspect per above posts. If the float level remains within specifications (I show 1/2 " from top of carb body and 5/16 from inside carb bowl per above photos) and fuel continues to enter the vent hose, I suggest you replace the vacuum diaphragm and disconnect the vent hose from the carb. Start the engine again. If no fuel comes out of the vent, try different rpms. If still no fuel out the vent, try holding different levels of rpms until fuel comes out the vent. Then disassemble and inspect carb. At this point, I would guess that one or more vents are clogged, but wouldn't rule out other causes.

When I operate the mm at elevations above 6000', I find the mm runs very rich, producing black smoke. I remove the vacuum diaphragm and economizer valve actuator and use a regular gasket with the cover, allowing the economizer valve to stay closed causing a lean condition because no extra fuel flows through the economizer valve. Caution: operating without an economizer valve actuator can cause a lean condition which can instantly destroy your engine! In fact, I do not recommend this mode of operation for anyone other than me. If you elect to try this mode of operation, do so only when you see black smoke out your tailpipe, which for me occurs at approximately 6000' of elevation.

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Re: Holley 1904 rebuild, tuning and knowledge base

Post by mspeters » Sun Nov 22, 2015 7:50 pm

Lawverdict,

Fantasic write-up on the Holley carb.

Did you deduce the cause of your off idle miss ? ? Same symptom for my Mite.

New motor with a NOS carb has a rough stumbling off idle. Good vacuum, idle circuit and fuel adjustment work normally. Great power past mid throttle and above. Fuel system is new: tank, lines, fuel pump. Ignition ruled out. Timing has nominal effect. Next step will be to replace the economizer bellows as a trial test before a teardown.

Float level is too high out of the box, as it dies after a moderate rate stop , but does not seem to be seeping & no hard restarts.

Thanks,
Mark

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Re: Holley 1904 rebuild, tuning and knowledge base.

Post by lawverdict » Mon Nov 23, 2015 7:52 am

Mark,
I will do a full tuning write up when I can find the time. In the interim, confirm that the accelerator pump is actuating immediately upon moving the throttle linkage. Then look down the throat of the carb and confirm fuel spray immediately upon movement of the throttle linkage. I have found that some metering blocks are clogged and that no amount of soaking in carb cleaner and pressurizing with compressed air will clear those metering blocks. I believe I have located the correct metering block plugs and will disassemble those metering blocks to perform a thorough cleaning to determine whether that is contributing to an off idle stumble. I advanced the timing to 9 degrees initial advance and that, along with the carb rebuild, helped solve the off idle stumble. My friend's mite had the same problem, I went through the same process, and he no longer has an off idle stumble. Good luck and update on your progress.
Mike

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Re: Holley 1904 rebuild, tuning and knowledge base.

Post by mspeters » Mon Nov 23, 2015 9:26 am

Mike,

I will let you know what I find.

Accel pump is spray strong. The stumbling occurs at static RPMs, 800-1500.
Compression is uniform in all cylinders, timing light has clean spark triggers on all cylinders when stumbling.
Have swapped out with my spares for for coil, wires, plugs, 151 electronic 'Swiss' module.....no change.

May find some decayed rubber bits in the metering body or perhaps the main jet. I read the holley 1904 manual. It's not too complex.

http://www.tocmp.com/manuals/Carbs/Holl ... C_53B.html


Mark

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Re: Holley 1904 rebuild, tuning and knowledge base.

Post by lawverdict » Mon Nov 23, 2015 9:43 am

Mark,
That manual is very informative and provides the correct terminology. After I study the manual, I will have to edit my posts accordingly. Thanks for posting. I forgot to mention that I arrived at 9 degrees initial advance using a vacuum gauge. I would suggest using a vacuum gauge to tune, as each mite will tune differently depending on altitude, etc.

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Re: Holley 1904 rebuild, tuning and knowledge base.

Post by mspeters » Tue Nov 24, 2015 4:39 pm

Mike,
Thanks for the tips. I had timed by a jumbo vacuum gauge prior. Took apart the carb and cleaned, it was clean inside with channels in the metering body and vents clear. Float level was lowered based on the info in the 17 page IHC thread on this carb. I richened the idle circuit quite a bit and added more timing advance which smoothed the shudder and hesitation until the throttle passed through the idle transition slot. It runs well enough for now after it's warmed up. The timing mark I had painted is incorrect; will need to recheck vs TDC with a dial gauge. May try a larger gap on the spark plugs.

FWIW, There is a Argentina made replacement 1904 carb available through several sellers for those with damaged carbs.

mark


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