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  #1   IP: 152.163.100.136
Old 08-24-2005, 04:50 PM
BBH BBH is offline
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Runs only on full choke

I have an early model A4 with a Zenith 61 carburetor. The engine wasn't started for over a year, but it now starts and runs good when the throttle is set about 1/4 open and at full choke. When I release the choke, though, the engine thottles down and dies. It remains running only if I keep it at full choke. It restarts easily when choked, but still needs full choke to keep running.

The idle mixture is set at one turn off of its seat, and the main jet adjust is set at two turns off of its seat.

Can anyone help me?
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  #2   IP: 38.118.55.154
Old 08-26-2005, 09:13 AM
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Whenever an engine requires full or partial choke to continue running, it's virtually always the case that there is a restriction somewhere within the carburetor, in most cases within the main jet itself.

Don
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  #3   IP: 64.12.116.134
Old 08-27-2005, 04:58 PM
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Runs only on full choke

Don Moyer:

While working on my Zenith 61 carburetor, I stripped the screwdriver slot in the main jet in which the main jet adjustment needle seats, and cannot remove the main jet to do a thorough cleaning of the main jet passage. I have been able to remove and clean, with a wire, the main discharge nozzle.

I can insert a wire through the main jet and blow through it, but can you recommend a drill bit size to dress up the main jet orifice, which might help the needle to seat better?

Last edited by BBH; 08-27-2005 at 05:03 PM.
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  #4   IP: 38.118.55.154
Old 08-28-2005, 08:02 PM
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BBH,

The diameter of the orifice of the fixed part of the adjustable main jet is .0449".

Don
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  #5   IP: 152.163.100.134
Old 10-12-2005, 02:54 PM
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Trouble starting

Don Moyer.

As a followup to my previous email on my Zenith 61 carburetor, I am still having difficulty starting my early model Atomic 4. I recently installed a new main jet assembly, but that didn't help. Now the engine won't even start with a full choke. I am now reworking all passages, chambers, and orifices on the carburetor for blockages. I have a few questions.

1. How critical is the built-in fuel scavenge feature of the Zenith 61 for starting (and running) the engine? (It appears that the fuel scavenge feature consists of an immovable small-bore tube that extends down into the deep well in the lower housing, and a mating passage in the upper housing that extends approximately 1-1/2 inches up into the opening through which the throttle shaft extends.)

2. Does the venturi tube in the Zenith 61 require a specific orientation when installed? On page 4-4 of your Service and Overhaul manual, in step 2 of "Reassemble the upper housing as follows," you refer to Fig 7 "for proper relationship between gasket and tube," but I am not sure what the proper relationship is, other than that passgeways should not be covered by the gasket or the venturi should not be installed upside down. Also, in reassembly instructions accompanying exploded views that I have of the Zenith 61, it is stated to position the venturi so that its machined flat is towards the fuel bowl, but my venturi has four machined flats at 90 degrees to each other. I am assuming that any one of the four flats can face towards the fuel bowl, but I can't see how the positioning of the venturi affects the operation of the carburetor.

3. In desparation, can I ship the carburetor to Moyer Marine and have MMI determine if the carburetor is in working order, and can MMI actually test the carburetor on an early-model engine?

Last edited by BBH; 10-12-2005 at 02:57 PM.
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  #6   IP: 38.118.52.41
Old 10-12-2005, 05:40 PM
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BBH,

The scavenge tube is critical mostly in the sense that it's required by the Coast Guard. I don't believe that the scavenge tube is playing any part in your current difficulties.

I agree with your assessment of the orientation of the venturi tube. I have never heard of any requirement to have the flat spots facing any particular direction, and the tube is idiot-proof in terms of which side faces upward. It will only go into the lower housing in one way.

All of this, of course, leaves unanswered the question of why your engine will not start.

Before sending us your carburetor, I have a couple questions:

1) When you're trying to start (with the choke fully closed), do you see raw fuel puddled in the bottom of the intake throat?

2) When you remove the 7/16" hex-headed pipe plug from the bottom of the float chamber, does fuel run out?

Don
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  #7   IP: 152.163.100.134
Old 10-24-2005, 06:05 PM
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Trouble starting (Running on full choke)

Don Moyer,

1) When I start my early Model A4 with the choke fully closed, I get some puddling, but not enough to cause fuel to run out of the intake throat. (The engine still does not start when letting it turn over a half dozen revolutions two or three different times.)
2) Some fuel runs out of the float chamber when I remove the pipe plug from the bottom of the float chamber, but not as if the float chamber were full of fuel.

My thoughts have turned to an air leak in my fuel system, either at the connection between the output of the fuel pump and the input to the carburetor, or at the carburetor itself. ( I have checked for leaks up to the electric fuel pump, using a priming bullb in the fuel line prior to my Racor fuel filter, but have not detected any leaks in that part of the fuel system.)

A question on air leaks: How can you detect air leaks that include the fuel pump and the carburetor?

(I recently purchased a main jet assembly from MMI, and the accompanying instructions stated to avoid overtightening "...the 1/2-inch hex head of the needle assembly, or the sealing washer is likely to extrude out from under the hex head and cause a leak." Is this "leak" a fuel leak or an air leak, or both?)
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  #8   IP: 38.118.52.41
Old 10-24-2005, 10:16 PM
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BBH,

Your evidence at this point is less than clear, so I'll answer the easy question first: If the gasket around the main passage plug leaks, you will experience a fuel leak, not an air leak.

My best guess as to what is preventing your engine from starting is that you have ignition problem, not a fuel or air problem.

If you remove the drain to the float chamber after cranking the engine with the choke closed for a few seconds (20 or so), the fuel that puddles in the bottom of the intake throat will subtract a bit of the fuel that is left to drain out after you remove the drain plug.

If you have a vacuum leak in the induction system (which is what I assume you mean by an "air leak"), it seems to me that you wouldn't have fuel puddling in the bottom of the intake throat of the carburetor. A vacuum leak sufficient to prevent starting should also preclude the puddling of fuel.

Don
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  #9   IP: 205.188.116.138
Old 01-02-2006, 03:06 PM
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Trouble starting (Runs only on full choke)

As of late, I have not been able to start my early-model Atomic at all, much less on full choke, and I am becoming increasingly frustrated. I have cleaned the carburetor and retimed the engine at #1 TDC, but I can't get even the first sign that the engine wants to start. I have good fuel flow and what I think is a good spark at the #1 spark plug.

I had previously removed the manifold and had a machine shop give it a hot bath and mill it down in some areas. I did not use a gasket sealant on the manifold-to-block gasket when reinstalling the manifold.

According to the Service and Repair Manual for the Atomic 4, if the engine won't start you can have an air leak at the intake manifold, with the solution being to "Tighten bolts (replace gasket if necessary)."

1. Is the gasket they are referring to the manifold-to-block gasket or the mounting gasket between the carburetor and the manifold?

2. Can a blown manifold-to-block gasket result in no starting? (Due to not pulling fuel up into the intake manifold, I assume.)

3. Is there a way to tell if there is a blown manifold-to-block gasket without removing the manifold? (Wishful thinking?)
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  #10   IP: 38.118.52.41
Old 01-03-2006, 08:23 AM
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BBH,

Here are the answers to your specific questions:

1. Is the gasket they are referring to the manifold-to-block gasket or the mounting gasket between the carburetor and the manifold? They are no doubt referring to the gasket between the manifold and block.

2. Can a blown manifold-to-block gasket result in no starting? (Due to not pulling fuel up into the intake manifold, I assume.) The short answer is probably yes, but I consider this to be an unlikely cause of your problem.

3. Is there a way to tell if there is a blown manifold-to-block gasket without removing the manifold? (Wishful thinking?) Yes. Remove the spark plugs, and hold your thumb over each hole and check for compression in each cylinder. If the compression is sufficient to blow past your thumb no matter how hard you press, then remove the flame arrestor housing and hold your hand over the intake of the carburetor while someone cranks the engine for a couple more seconds. You should feel definite suction against your hand, and there should also be raw fuel puddled in the intake throat of the carburetor by this time. If you feel suction but see no fuel, the reason your engine is not starting is fuel starvation. If you feel suction and see fuel in the intake throat, the reason your engine is not starting is somewhere in the ignition system (could be timing if someone has recently messed with your timing.

Here is a short checklist we prepared recently for non starting engines:

1) Close raw water through hull as soon as it's determined that the engine is not starting within the normal time.

2) COMPRESSION: With all spark plugs removed, hold your thumb over each spark plug hole to check compression as someone cranks the engine for a second or two on each cylinder with the starter. An Atomic 4 will usually start if any two cylinders have normal compression as indicated below.

a. Compression sufficient to force past your thumb no matter how hard you press it against the plug hole would confirm normal compression of approximately 85 psi or above.

b. If you can hold your thumb against the compression, but not easily, a compression value of approximately 40 to 50 psi would be indicated, and starting could be problematic.

c. If you feel virtually no compression on any one of the cylinders, the problem is likely a stuck valve.

3) FUEL: Remove the flame arrestor and check for the presence of raw fuel. If the choke is closed completely, there should be raw fuel puddled in the bottom of the intake throat within 15 to 20 seconds of cranking (3 or 4 five-second attempts). If the carburetor intake throat is "bone-dry", after this amount of cranking, the reason for the non-starting is either a problem in the fuel supply which prevents fuel from getting to the carburetor, or some problem within the carburetor that prevents the fuel from getting through the carburetor (most likely a blocked main jet).

NOTE: Though quite unlikely, a massive manifold or carburetor flange gasket failure could also cause the engine to not draw fuel through the carburetor while cranking on the starter. Assuming a good compression check in step (2), remove the flame arrestor housing and hold your hand over the intake throat of the carburetor while someone cranks the engine for a second or two. If you feel good suction on your hand, the manifold gasket and carburetor flange gasket are both OK.

4) Ignition: Remove the secondary lead from the center of the distributor cap and hold it approximately 1/4" from the cylinder head while someone cranks the starter. You should see a good arc between the end of the coil lead and the head that can be stretched to 1/2" or even 3/4". If you see no spark, the reason for non-starting is clearly within the ignition system, most likely a breakdown within the primary ignition circuit.

This quick check of the ignition system does not rule out timing issues. While the Atomic 4 has virtually no history of slipping out of time once the timing has been properly set, if the non-starting follows work that was accomplished on the ignition system, the timing should be rechecked.

5) If all of the above checks prove to be satisfactory and the engine still won't start, the problem is probably somewhere within the secondary ignition system, downstream of the coil. The components within this part of the secondary ignition system are very difficult to inspect, but they are fortunately not very expensive, so I recommend replacing them in the following sequence: plugs, distributor cap, plug wires, and rotor.

Don

Last edited by Don Moyer; 01-03-2006 at 08:25 AM.
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  #11   IP: 76.173.109.184
Old 10-18-2009, 04:47 AM
malicki malicki is offline
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timing

I cant seem to find a detailed post on how to set the timing on the A4. Can someone walk me through each step to make sure timing is correct? I had to pull the distributor stem out of its gear and most likely have lost my place. Ive done what I believe is right but need reassurance cause Im trouble shooting some issue with my engine. I need to rule correct timing out. To be absolutely sure of TDC I did it with the cylinder head off. Made the #1 piston flush at the top of block with both valves closed. Then I aligned the rotor and #1 ignition wire within the D cap. then I turned the Distributor stem counter clockwise until the points began to open. Then I tightened it all down. Should this be correct rough timing?
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Old 09-12-2022, 08:31 PM
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Full Choke Required to Run After Exhaust Replacement

My A4 in our 1981 C&C 32 will run only with full choke after replacing the exhaust. Iíve read many similar posts and have taken a number of steps. It ran well prior, and Iím cautious to pull things too far apart without a logical path from original issues so I thought I would seek wisdom from the group. Here is the situation. The engine was running well early in the season. In July the exhaust rusted through at the top of the exhaust loop, above where water is injected in. I rebuilt and installed exhaust. It would not start. I had spark so I was suspicious it was water in the cylinders (must have got in when pressure dropped when exhaust split and water was still being injected into exhaust). I removed the plugs, added a bit of engine oil to the cylinders (to avoid rust), and let it dry out. I replaced the sparkplugs with new. It would then run smooth but only with full choke. I realized I had not gapped the sparkplugs, so i gapped them to .35. It then ran very rough, (perhaps some oil left in cylinders). I cleaned the plugs, reinstalled, and itís now back to running smooth but with full choke only. Iíve cleaned the flame arrestor. Checked that the choke lever opens and closes choke. Iíve adjusted idle adjustment screw opening it up by half a turn - no change. I closed it entirely and tried at 1.5 and 2 turns out. No change. My next step, I suppose, is to clean the carburetor and check the fuel filter (electric fuel pump). My dilemma is that it worked fine, till the exhaust failed. Working backwards from that, everything like water in the cylinders and fouled plugs makes sense. But not the carburetor.

I welcome this groups collective wisdom.
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