Sudden rough running, out of ideas

[alexiskai]

Problem: '29 coupe suddenly started running very rough, not drivable

Notable symptoms:

• Sounds like the cylinders are only occasionally firing, engine rotation very irregular
• Sparkplugs show a lot of black residue
• Exhaust seen coming from somewhere at the back of the engine compartment, maybe from the point where the manifold meets the muffler? Hard to tell.
• At idle, ammeter wobbles constantly
• If left at idle, engine will stall out within 30-60 seconds

What I already tried/examined:

• Lights and starter work fine, so no problem with electrical grounding
• Re-did the timing, checked clearance at rotor and points
• With ignition on, opened points. Spark was seen at the points and a spark jumped from the coil wire to a plug terminal when held nearby. Worked every time I opened the points. (Modern distributor plates installed last month, older coil)
• With engine running, grounded each plug terminal to the block with a screwdriver. Saw spark at each plug.
• Using Nu-Rex "C" spark tool, saw good spark between coil and distributor up to 10K volts
• Fuel line is clear
• Removed and inspected carburetor, seems fine, no deposits, all parts move smoothly (balanced carb is less than a year old)
• Tried various settings for GAV and spark advance, no notable change in behavior
• Tried with air filter off, no change

Anyone have any ideas? I don't have any spare parts to swap in unfortunately. It's bewildering because I just drove it a couple weeks ago and it was fine. I've worked on a few components since then, but nothing in the fuel-air-engine block system. The only system I've messed with is the distributor and coil, but I feel like I've tested them thoroughly.

[WHN]

Try removing gas cap. Re-check gas line flow. Carburetor???

[Joe K]

Condenser?

Necessary for consistent and timely spark creation. Every Model A should have a condenser and screwdriver under the seat.

Joe K

[Jim Brierley]

Always give it a compression check at first! Could be anything from a piece of carbon holding a valve partly open to a blown head gasket, to a intake manifold leak. A compression check will tell you if the head needs to come off, or not.

[30 Closed Cab PU]

Originally Posted by Jim Brierley

Always give it a compression check at first! Could be anything from a piece of carbon holding a valve partly open to a blown head gasket, to a intake manifold leak. A compression check will tell you if the head needs to come off, or not.

X2, but wouldn't an leaky Intake cause the motor to run lean, would not cause plugs to be black? If carboned valve open, would only affect 1 cylinder? Just asking, not trying to be picky, arm chair mechanic here trying to learn.

[Bob C]

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(Modern distributor plates installed last month, older coil)


Did you use the wireless upper and lower plates??


Bob

[larrys40]

Originally Posted by Bob C

(Modern distributor plates installed last month, older coil)


Did you use the wireless upper and lower plates??


Bob

My bet is that is the problem . Get those junk plates out of there . I had two roadsters in my shop Saturday and both of them had that issue. Fixed with pigtail plates right and problems solved.
Larry shepard

[alexiskai]

Originally Posted by larrys40

My bet is that is the problem . Get those junk plates out of there . I had two roadsters in my shop Saturday and both of them had that issue. Fixed with pigtail plates right and problems solved.
Larry shepard

I do have new upper and lower plates on order along with that carb from Bert's.

[The Master Cylinder]

Originally Posted by alexiskai

Problem: '29 coupe suddenly started running very rough, not drivable

[*]Sounds like the cylinders are only occasionally firing, engine rotation very irregular
[*]Sparkplugs show a lot of black residue

Fouled plugs? Try cleaning them.

[Purdy Swoft]

Did the problems start after installing the modern points setup ? If the points and condenser were made in China , that could be the problem . The much higher cost modern points that can be ordered from NAPA and other parts houses are said to work good . Most people probably won't want to pay the higher price . Cost is probably why most don't stock the higher cost parts . Being as there is No advantage to running the so called modern points setup , I went back to the original points setup years ago . The original setup is easier to adjust points gap . I figured why not make it easier on myself . Just a thought .

[alexiskai]

OK, I'll try to take these responses in turn:

• Gas cap: I did make sure the gas cap was venting. Fuel flow is good down to the carburetor.
• Modern distributor: Yes, it is the modern points and wireless plates. I've had trouble with them before, but I looked them over pretty thoroughly this time and can't see any problems.
• Condenser: Could be the condenser, but then why does the spark test good every time I check it? I checked it with the engine off and on. I guess I can't be certain it's working 100% of the time.
• Fouled plugs: I treated the plugs with a wire brush after I pulled them and found them to be fouled. Should I do more?
• Compression check: Do I need a tool for this or can I check it by just sticking a thumb in the plug socket and turning the engine over?

[30 Closed Cab PU]

Compression check- you are looking for reasonable compression readings , and also importantly readings that are within 5- 7 lbs from each other. You really can not tell from the thumb method. The thumb method tells you if you have little or no compression. If you do not have a compression gauge and buy one make sure you it has an adapter that fits the A spark plug hole, a lot/most plugs are metric and supplied adapters do not fit.


The procedure is not difficult, if you search I am sure there is info in the forum. Or if you have a mechanic friend he may have a gauge and can help you. Or pay a local independent shop to do and possibly teach you. Just be aware when performing the test the spark lever must be fully up/retarded and all plugs should be removed.


From your symptoms you have a too rich condition fouling all your plugs, kind of points to a carb issue since it is affecting all cylinders (all plugs black). If running so rich that it stalls in a couple of minutes and you have to clean plugs the problem is pretty intense. If you had an intake manifold issue it would suck air and run lean.


Others that know the fuel system better than me will have to comment.
I am unsure how/why a carb would run rich unless you have an air filter plugged, or the carb is messed up. Could be you have a clogged passage in the carb. newer carb, but you never know when something can get through, or if the carb/fuel sat a long time something varnished up.

Originally Posted by alexiskai

OK, I'll try to take these responses in turn:

• Gas cap: I did make sure the gas cap was venting. Fuel flow is good down to the carburetor.
• Modern distributor: Yes, it is the modern points and wireless plates. I've had trouble with them before, but I looked them over pretty thoroughly this time and can't see any problems.
• Condenser: Could be the condenser, but then why does the spark test good every time I check it? I checked it with the engine off and on. I guess I can't be certain it's working 100% of the time.
• Fouled plugs: I treated the plugs with a wire brush after I pulled them and found them to be fouled. Should I do more?
• Compression check: Do I need a tool for this or can I check it by just sticking a thumb in the plug socket and turning the engine over?

[ronn]

I agree- check your gas flow from tank.

[31Tudor]

I would take the carburetor apart, cleaning and blowing out all passages, and change to the original point set up.

[Jacksonlll]

If the plugs are black and fuzzy, clean the, don't just look at them. I must assume that you did clean them.

[30 Closed Cab PU]

Originally Posted by Jacksonlll

If the plugs are black and fuzzy, clean the, don't just look at them. I must assume that you did clean them.

Was looking for your U-tube videos - can't seem to find them can you post a link to them, please?


Thanks

[1931 flamingo]

Timing gear??
Paul in CT

[Patrick L.]

Good info above. One thing not mentioned was a vacuum leak.
so,
compression test.

condensor test, or, replacement with one of the new burn-proof ones.

I don't have an issue with the modern upper plate as long as its not a wireless unit and it has the higher price quality points.

Improper fuel flow can be a real issue, but, it seems like you have that checked. Running with fuel cap loose is an easy way to check the cap for venting.

Spray the intake area to check for a leak and check to make sure the bolts are snug.

[30 Closed Cab PU]

Hi Patrick.


Just PMd you for clarification on your post.

Originally Posted by Patrick L.

Good info above. One thing not mentioned was a vacuum leak.
so,
compression test.

condensor test, or, replacement with one of the new burn-proof ones.

I don't have an issue with the modern upper plate as long as its not a wireless unit and it has the higher price quality points.

Improper fuel flow can be a real issue, but, it seems like you have that checked. Running with fuel cap loose is an easy way to check the cap for venting.

Spray the intake area to check for a leak and check to make sure the bolts are snug.

[Joe K]

Condenser: Could be the condenser, but then why does the spark test good every time I check it? I checked it with the engine off and on. I guess I can't be certain it's working 100% of the time.

Timing is EVERYTHING. Without the condenser, the spark is formed, but the timing is irregular. Unless you have hyper-consciousness you see the spark (energy has to go SOMEWHERE from the coil) but have no sense of the timing.

I've never used a timing light on a Model A. There are those that do. I wonder what a timing light would show with a bad condenser? It might be a way to actually see the timing? Or its lack thereof?

Joe K