Rough running, stall on idle (mostly solved)
 

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.

Re: Sudden rough running, out of ideas
 

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

Re: Sudden rough running, out of ideas
 

Condenser?

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

Joe K

Re: Sudden rough running, out of ideas
 

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.

Re: Sudden rough running, out of ideas
 

(Modern distributor plates installed last month, older coil)


Did you use the wireless upper and lower plates??


Bob

Re: Sudden rough running, out of ideas
 

Fouled plugs? Try cleaning them.

Re: Sudden rough running, out of ideas
 

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 .

Re: Sudden rough running, out of ideas
 

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.

Re: Sudden rough running, out of ideas
 

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?

Re: Sudden rough running, out of ideas
 

I agree- check your gas flow from tank.

Re: Sudden rough running, out of ideas
 

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

Re: Sudden rough running, out of ideas
 

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

Re: Sudden rough running, out of ideas
 

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.

Re: Sudden rough running, out of ideas
 

Timing gear??
Paul in CT

Re: Sudden rough running, out of ideas
 

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.

Re: Sudden rough running, out of ideas
 

Sound’s like some one turn the GAV out to much.

Re: Sudden rough running, out of ideas
 

Hi Patrick.


Just PMd you for clarification on your post.

Re: Sudden rough running, out of ideas
 

• GAV: Not the GAV, I tested all sorts of settings
• Timing gear: Probably not? I've been setting the timing using the timing gear, so I know it is rotating properly.
• Condenser: It's practically new and it's the burn-proof type, so I'm hesitant to blame the condenser without a test that would point to that as the source
• Upper plate: I don't know which upper plate it is. It's from MAC's, so I guess that makes it more likely that it's cheap. The lower wireless plate is from Bratton's. So maybe using a different upper plate would help, but again, it's not clear why that would be the source of the problem if all my spark tests are good.
• Compression test: This sounds worth doing, I'll get the tools for that.

That does seem to be what's going on, but as you said it's not clear why. Also it doesn't explain why the ammeter would be wiggling when the throttle is at idle.

Re: Sudden rough running, out of ideas
 

My opinions, opinions may vary.


In addition to the GAV, the needles/passages also control mixture when above idle. I know this because recently I had a motor rebuilt, and when run the plugs were slightly black. The Shop pulled the Carb and discovered the Jets were not quite sized correctly.


So I am still of the opinion you may need to pull the Carb and do a thorough disassembly/cleaning/blowout including the Jets and passages.


I am not looking at electrical issue because of your info about what you have done for testing. But you never know - hindsight once fixed is 100%. If there is an electrical find the running rich issue 1st.


Ammeter, guesses - If it is wiggling only slightly. If running really rough possible the generator is varying speed enough to cause the needle to vary? If running slow enough/uneven seeing the ignition/coil on/off draw? Or possibly the ignition cutting out?






The other issue not mentioned so far is smoke - If running rich and you have an exhaust leak. You should also smell it. Usually if it is a exhaust manifold to block leak, you can also hear it. If a leak at the manifold/exhaust connection, you may or may not hear it. If it is black perhaps wait until the other issue is resolved.


Compression- even if it turn out good, is nice to have the #s, write the reading down, and periodically rerun the test to see how your motor is doing.

Re: Sudden rough running, out of ideas
 

Fact -- Plugs show signs of too much fuel or not enough air. You saw black smoke from back of engine compartment.

If rodents are in the exhaust air can't escape so cylinders run rich. To check disconnect or just loosen exhaust pipe from manifold.

If gas is spilling into carb, the float valve might be leaking. If gas tank fuel shut off is left open for a few hours does carb get wet or drip gas? As already said, gav might not be set properly. Gently close gav, then open 1/2 to 3/4 turn.

Is choke plate operating properly? Inspect now without removing carb. Will engine run better at higher than idle rpm while parked? If yes, the issue might be gas related. If not, recheck ignition.