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.
 
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.
 
  • The potential of the exhaust being blocked (by rats or otherwise) hadn't occurred to me, I'll check that out.
  • Distributor cap carbon button: I will check that, I think I have my old cap around.
  • 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?
  • Ammeter: I have an alternator installed, and what I've observed is that with the alternator running (as in, I throttled up enough to get it to kick in), if I then reduce throttle to idle, the ammeter will oscillate between a low draw and zero. This is interesting because it should stay positive, indicating that the alternator is charging the battery, but it doesn't.
 
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.
 
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.
Click to expand...
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
 
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.
 
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.
 
"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
 
"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
 
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
 
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.
 
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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?
 
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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.
 
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5lugnuts said:
"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
Click to expand...


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.
 
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.
 
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