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.

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.

Re: Sudden rough running, out of ideas
 

Based on a couple of roadside incidents I've helped with, I've learned that if it starts to run like crap all of a sudden, the first thing to look at is the little high voltage carbon button in the center of the distributor cap that the rotor makes contact with. A couple of times we've found that to be cracked or missing, causing misfires.

Re: Sudden rough running, out of ideas
 

Re: Sudden rough running, out of ideas
 

The shutoff is OK if the leak does not happen when the shutoff is in the off position.


I assume the carb is leaking gas from the small weep hole on the bottom of the carb intake. This indicates that the float bowl needle valve and seat is not sealing/seating properly. This can be difficult to remedy. You have up to 11 gallons gas weight pressure pushing on that valve. I recommend you search on the Forum for recommendations on what to do, sometimes simple replacement does not solve the issue.

Re: Sudden rough running, out of ideas
 

Someone else will have to answer your alternator questions, I am unfamiliar with them, am running a generator.

Re: Sudden rough running, out of ideas
 

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

Re: Sudden rough running, out of ideas
 

Sounds like something I had. Timed to perfection and after running out the road it would start to backfire and run rough. After going thru the entire ignition system several times, finally found top of cam gear and the screw holding it was worn causing cam to move, throwing it out of time. Installed new cam gear & screw, problem solved in my case.

Re: Sudden rough running, out of ideas
 

Have you changed the spark plugs? Wrong plug will cause problems like you are seeing.

Just another potential cause. Look for the simple things first.

Re: Sudden rough running, out of ideas
 

"If fuel shut-off valve is left open while the car is parked, it will leak gas, slowly. That's been true for a while. Is that a carb problem?"


Sounds like you found your carb problem (too much gas). If the gas is leaking from the carb and not the line to the carb that is your issue. When the float bowl is overly full, the gas flows into the intake manifold and usually out the mouth of the carb into the filter if you have one installed. Not a problem if the engine is not running, but if the engine is idling the extra gas gets drawn into the intake manifold and cylinders causing your overly rich mixture which does not burn completely and creates black smoke which leaves soot on the spark plugs and in the exhaust manifold. Either the float valve is stuck or dirty or the fuel level in the carb bowl is too high which could cause the overflow into the intake manifold.

There should be a round gasket on the float valve between the valve and where it screws into the upper carb housing. If that is missing it could be causing a leak which is overflowing the bowl and going into the engine. If you find one gasket on the float valve it may need another one (2). The float level (gas in the bowl) is reduced by adding those float valve gaskets which lowers the float level and reduces the level of gas in the carb.

You get the gaskets with a rebuild kit. It would be a bad idea to try and bend the float. You could also have a leak in the float causing it to sink and let more gas in (probably not unless someone tried to bend the float). The float valve gasket looks like a thin reddish plastic ring between the valve and the top of the carb.

There is a measurement to check the float position with the carb apart by turning the top half over and measuring but I can't remember it now. Somebody will give that information soon. I'm just an old beginner.

Good Luck

Re: Sudden rough running, out of ideas
 

"If fuel shut-off valve is left open while the car is parked, it will leak gas, slowly. That's been true for a while. Is that a carb problem?"


Sounds like you found your carb problem (too much gas). If the gas is leaking from the carb and not the line to the carb that is your issue. When the float bowl is overly full, the gas flows into the intake manifold and usually out the mouth of the carb into the filter if you have one installed. Not a problem if the engine is not running, but if the engine is idling the extra gas gets drawn into the intake manifold and cylinders causing your overly rich mixture which does not burn completely and creates black smoke which leaves soot on the spark plugs and in the exhaust manifold. Either the float valve is stuck or dirty or the fuel level in the carb bowl is too high which could cause the overflow into the intake manifold.

There should be a round gasket on the float valve between the valve and where it screws into the upper carb housing. If that is missing it could be causing a leak which is overflowing the bowl and going into the engine. If you find one gasket on the float valve it may need another one (2). The float level (gas in the bowl) is reduced by adding those float valve gaskets which lowers the float level and reduces the level of gas in the carb.

You get the gaskets with a rebuild kit. It would be a bad idea to try and bend the float. You could also have a leak in the float causing it to sink and let more gas in (probably not unless someone tried to bend the float). The float valve gasket looks like a thin reddish plastic ring between the valve and the top of the carb.

There is a measurement to check the float position with the carb apart by turning the top half over and measuring but I can't remember it now. Somebody will give that information soon. I'm just an old beginner.

Good Luck

Re: Sudden rough running, out of ideas
 

Two suggestions:
1. Inspect the carburetor and flow test the jets.
2. Check for any exhaust leaks at the exhaust pipe clamp. Any leakage would go into the carburetor making a lean mixture.
Good luck, Ed

Re: Sudden rough running, out of ideas
 

Thanks for everyone's suggestions. I'm headed out of town for a few days, but when I get back I'll swap out the condenser & dist. cap, test the compression, and check the exhaust for obstructions/leaks. I'm holding off on messing with the carb because I could easily make things worse.

Will report back in a week or so.

Re: Sudden rough running, out of ideas
 

OK, here's the results so far. I haven't changed out the condenser yet, but I did check the exhaust and the compression.

To test the exhaust, I connected a vacuum cleaner to the end of the exhaust pipe and then cranked the engine by hand with all spark plugs removed. I could feel the suction on each cylinder in turn; no cylinder was open when it wasn't supposed to be. So this seems to indicate that the exhaust pipe is clear and the exhaust valves are more or less working.

To test the compression, I connected a testing gauge and, with spark plugs removed and throttle fully open, cranked the engine using the starter. All cylinders showed 60-65 psi except for cylinder 2, which showed 40 psi on two separate tests.

I performed a wet compression test on cylinder 2 by squirting some oil into the cylinder with an oil can (I made sure to get the oil in the cylinder itself). Rechecked compression and it wasn't substantially improved, maybe 45 psi.

So I guess I have two questions:

1. Everyone think this is most likely a head gasket?
2. Is this likely to be the problem that's causing the rough running or is it more likely to be a long-standing problem that I just hadn't detected before?

Re: Sudden rough running, out of ideas
 

Ignition

Re: Sudden rough running, out of ideas
 

Retorque your head, but I think it should run good with the 45 in one cyl.

Re: Sudden rough running, out of ideas
 

Could be a jet loosened up in the carb.

Re: Sudden rough running, out of ideas
 

You seem to have two issues
Gas out the carb - Caused by leaky fuel shutoff, and Carb Float Seat/Valve leaking. Replace fuel shutoff with a Brattons shutoff (guaranteed for 2 years, USA made). Most other venders shutoffs may fail out of box with same issue. Replace the Carb Seat/valve with a Viton tipped one, then run Marvel Mystery Oil , 4 oz.. per tank, helps upper end lubrication and keeps the Viton Tip good over time.


Bad compression reading -
I agree with Jackson, then compression test again.
Look for water in your oil - bad head gasket
Block test - Tests for exhaust gasses in the coolant. A Block Test Kit is available from NAPA for about $50. Or maybe a local mechanic can perform. This is a super simple easy test. - Bad head gasket if fails.
Do you have any seepage or white powdery substance where the Block/Head mate? Indicates Head gasket sealing issue.
Remove the Valve Access Cover - have some one hand crank or use the starter to observe the valves operate, look for sticky valves and inspect for valve seating. Possible sticky valve, or maybe carbon on a seat/valve causing it not to close properly. Others will have to comment on how to solve this issue,, I have not had this issue.


If not resolved, will have to pull the head.

Re: Sudden rough running, out of ideas
 

Alternate method of measuring float level.
You can also purchase a gauge to check the float level while it is in the
car. Depending on who does it, I have seen 5/8 in, 3/4 inch, and 15/16 inch. Purchase Gauge from Renners Corner. A lot of the other Vendor ones use a small diameter tube, and can cause faulty reads. The gauge simply screws into the hole for the Carb drain bolt. And you measure the amount of air in the tube between where the carb halves mate to the level of the gas in the tube, easy peasy.

Re: Sudden rough running, out of ideas
 

From the responses, it sounds like this is an unrelated issue that I should fix at some point, but not a prerequisite for the rough running issue. I'll move on to the ignition.

Re: Sudden rough running, out of ideas
 

Have you tried replacing your carburetor with another working carburetor?