engine runs rough

freak

freak

Senior Member
My engine has been running a little rough. It lopes as if it has a spark miss. The fuel system is very good all the way through the carb. I pulled wires 3 and 4 today (both at the same time) and it sits there and runs as it was with the wires on. I pulled the plugs and they look ok. I ran it with the plug connected and grounded and spark was present. It has a high compression head, autolite 3923 plugs and what I would call a modern style distributor cap. I have no idea where to get a cap for it since is isn't the original. I ran a compression test on #3 and it showed steady 100psi. I'm worried that a valve could be stuck. Any other ideas?
 
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My experience has been that a Model A engine will lope if you have the GAV valve open too far. The Zenith on my Victoria was rebuilt by me with all the jets flow tested and re-sized. If I open the GAV too far the engine will lope. A Zenith with properly sized jets provides for actual GAV control. Many improperly rebuilt Zeniths have the comp jet too large and it ports so much fuel into the secondary well that the GAV valve no longer has any control. This is the same as having the GAV valve open too far and it will cause the engine to lope at idle or low speed.

Tom Endy
 
What is the compression on the other cylinders ? We should probably know that before going much farther.
But, a lope is generally from a too rich a fuel mixture. Where are the fuel settings, both of them ?
 
I ran out of time tonight. Will run more compression test tomorrow. What do you make of the fact that I can pull #3 and #4 wires with no effect? Pulling #1 or #2 kills the engine.
 
My GAV was set about 1/2 to 5/8 which is pretty much where I always run it unless cold. I haven't touched the mix screw since the carb was tuned by the carb builder. The rough running developed today as I was driving. It was loping a bit a few days ago and got worse today. Lost some power as this happened.
 
I keep reading about "lope" could someone care do describe what that exactly is... or sounds like to a knewb?... Thanks in advance! :)
 
I've got to agree with Tom Endy in post number 2. If the engine is running rich it will lope at idle and sometimes emit small puffs of black smoke at the exhaust . Loping sorta sounds like a race cam . When cold I crank my engines with the GAV opened 1/2 to one full turn open in the counter clockwise direction. In a short time when the engine warms it will start to lope and I reach over and close the GAV down in the clockwise direction to about 1/4 turn off the seat for model A Zenith carbs and close the GAV completely on B carbs after warm up. The GAV does have an effect on idle
 
Trailwanderer said:
I keep reading about "lope" could someone care do describe what that exactly is... or sounds like to a knewb?... Thanks in advance! :)
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I hear it all the time when my neighbor runs his lawn mower, which is running too rich. He needs to clean the air filter or adjust the mixture needles.

It's always better to short plugs to ground rather than pull wires. The high voltage tries to reach ground, even if it has to short through the coil windings or tower.:eek:
 
Tom Wesenberg said:
I hear it all the time when my neighbor runs his lawn mower, which is running too rich. He needs to clean the air filter or adjust the mixture needles.

It's always better to short plugs to ground rather than pull wires. The high voltage tries to reach ground, even if it has to short through the coil windings or tower.:eek:
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It's a nice jolt when it reaches ground through your hand!!
 
Tom Wesenberg said:
I hear it all the time when my neighbor runs his lawn mower, which is running too rich. He needs to clean the air filter or adjust the mixture needles.

It's always better to short plugs to ground rather than pull wires. The high voltage tries to reach ground, even if it has to short through the coil windings or tower.:eek:
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Ok everyone thinks GAV and I will work with it but does a rich condition explain why I can pull 3 and 4 with no change?
 
If you have a severe air leak in the rear intake runner, then you will have no work from the last 2 pistons. A short in the distributor cap will also mess up #3 and 4, and I've seen this on two A's in the past few years.

To check for an air leak I use an unlit propane torch. When the propane gets sucked into the opening the engine will smooth out and speed up.
 
Loping or "Hunting" as the Brits say is when the engine surges and then slows down all by it's self. It sounds like verrrum, verrrum, verrrum. Although you're not touching the gas.
However, I'd like to say that my engine was running rough recently, too. Always remember, when troubleshooting, try the simplest and cheapest possible solution first. So I poured some MMO (Marvel's Mystery Oil) into the tank. It runs much smoother now. The valves seemed to be sticking.
Terry
 
freak said:
Does anyone know how to diagnose a sticking valve?
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Use a vacuum gauge connected to the intake manifold. A slow to close or stuck valve will show up as a flicker on the needle.

A good ear at the intake of the carb or at the tail pipe might also help.
 
i also say check for a vacuum leak at the intake.. if 3-4 are not firing when you interupt the spark find out why. i am thinking if you had a sticking valve your compression would have showed up lower....

running on 2 cylinders yes it will lope

put an inline spark tester on and compare the spark intensity to the other known good cylinders..
 
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freak said:
Thanks but I'm scared to even run that test. What is the typical fix?
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Scared to run which test? The propane is safer than crossing the street.

A good fix is a can of Sea-Foam in the gas, and another can in the oil.

Remove the spark plugs and spray penetrating oil on the valve stems.

If they are really stuck then you might have to remove the manifolds and spray the valve stems.
 
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