engine runs rough
- Thread starter freak
- Start date
Purdy Swoft
Well-known member
With sticking or slow sticky valve action, you will lose power, big time. If more than one valve is sticky, you will hardly have enough power to take off without stalling . I had this problem once with my roadster. Ethanol adultrated gas will do it if it stays in the tank for a month or more. I drained and added fresh alky-rub free gas . I connected a temporary vacume line to the intake and stuck the other end in a bottle of marvel mystery oil. With the engine running fast enough not to stall, vacume will draw the marvel oil into the engine. It will smoke like a tar kiln but the valves will usually free up and the engine will again run smooth with power. I don't know if sticky valves are your problem but if they are this will usually do the trick.
If you are running an original distributor, the modern style cap may be the problem. The air gaps can't be adjusted with the modern style cap. Some of these caps have so little or no gap on some of the cylinders that the effected cylinders will have weak or no spark at all. In some such cases you can see the distributer cap jiggle as the engine runs. As a test, if you've got a good used spare rotor, grind a few thousants off the rotor tip and see how it runs. This won't give equal air gaps but will at least give some air gap to the effected cylinders.This may tell you if the air gaps in the cap is the problem. More air gap at the rotor tip gives hotter spark. The smaller the gap, the weaker the spark will be. Just a couple of thoughts that you can try.
If you are running an original distributor, the modern style cap may be the problem. The air gaps can't be adjusted with the modern style cap. Some of these caps have so little or no gap on some of the cylinders that the effected cylinders will have weak or no spark at all. In some such cases you can see the distributer cap jiggle as the engine runs. As a test, if you've got a good used spare rotor, grind a few thousants off the rotor tip and see how it runs. This won't give equal air gaps but will at least give some air gap to the effected cylinders.This may tell you if the air gaps in the cap is the problem. More air gap at the rotor tip gives hotter spark. The smaller the gap, the weaker the spark will be. Just a couple of thoughts that you can try.
tbirdtbird
BANNED
we can't offer concrete suggestions that have any meaning until you give us a full compression test set of readings.
you want ALL plugs out and throttle full open. Spin engine sufficiently on each cyl. until needle stops climbing, usually 3 compression strokes or 6 revs of the crank. Much easier with 2 ppl, one in car activating starter.
You said this was sudden onset so I am not sure how that is carb all of a sudden or even stuck valves all of a sudden. You must go thru a proper diagnostic paradigm
what kind of work has recently been done to this engine
while plugs are out can you take a pic of each and post them (closeups) so we can see the burn color of the porcelain, check for carbon, etc. You may have bridging of a plug gap by a foreign particle. With the plug in place, it will still show a spark when you hold the wire away, but the spark never jumps the gap in this scenario (because there is no gap)
If a modern type dizzy with modern cap you could have carbon tracking which will give spark crossover and take out whichever cyls are being crossed.
Take closeup pic of inside of dizzy cap so we can try to look but this will be tough to see in a pic; hard enough to see it in person.
The point is a lot more info is needed.
Tom, the propane torch idea is just *superb* less messy than using WD40 and not as dangerous as using ether. dave
you want ALL plugs out and throttle full open. Spin engine sufficiently on each cyl. until needle stops climbing, usually 3 compression strokes or 6 revs of the crank. Much easier with 2 ppl, one in car activating starter.
You said this was sudden onset so I am not sure how that is carb all of a sudden or even stuck valves all of a sudden. You must go thru a proper diagnostic paradigm
what kind of work has recently been done to this engine
while plugs are out can you take a pic of each and post them (closeups) so we can see the burn color of the porcelain, check for carbon, etc. You may have bridging of a plug gap by a foreign particle. With the plug in place, it will still show a spark when you hold the wire away, but the spark never jumps the gap in this scenario (because there is no gap)
If a modern type dizzy with modern cap you could have carbon tracking which will give spark crossover and take out whichever cyls are being crossed.
Take closeup pic of inside of dizzy cap so we can try to look but this will be tough to see in a pic; hard enough to see it in person.
The point is a lot more info is needed.
Tom, the propane torch idea is just *superb* less messy than using WD40 and not as dangerous as using ether. dave
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Ed Saniewski
Well-known member
Freak, did you test for spark between the spark plug tip, and the spark plug cable? You said you got spark when you grounded the plug, but try testing the plug itself. Ed Saniewski
Tom Wesenberg
Well-known member
You can check the spark plugs by swapping 1 & 2 with 3 & 4 to see if the miss follows the plugs.
Growley bear
Well-known member
No compression with stuck valve. If any valves are stuck, they will be open. If any valve is not opening there is a larger problem. Did this problem begin suddenly or gradually?
Chet
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freak
Senior Member
I'm going to run a full compression test tonight. Nothing has been done to this motor recently. I bought the car in September and haven't touched it.
freak
Senior Member
freak
Senior Member
It was sudden. I am wondering now about a head gasket.
Growley bear
Well-known member
Just because a spark plug shows a spark out in the atmosphere doesn't mean that it will fire when under compression.
Chet
Growley bear
Well-known member
If a head gasket is blown, compression will be 0 or almost. A gasket blown between two cylinders will also have an effect on the other two due to the nature of the induction system.
Chet
Patrick L.
Well-known member
You are removing one plug wire at a time, right ? I'm thinking that if #3 has 100# compression then there is nothing wrong with the head gasket and valves are not sticky.
If you still feel you have some sticky valves, then fill it with non-ethanol and a heavy dose of Sea-foam.
I'm agreeing with the fellas that there is an intake manifold or gasket problem, that is, if #3 and #4 have good spark.
If you still feel you have some sticky valves, then fill it with non-ethanol and a heavy dose of Sea-foam.
I'm agreeing with the fellas that there is an intake manifold or gasket problem, that is, if #3 and #4 have good spark.
Growley bear
Well-known member
Stock Model A intake manifold? Vacuum wiper motor? Tillotson? Zenith? The flow of fuel vapor and negative pressure in an induction system will follow the path of least resistance. The Model A intake manifold runners are all connected at the junction of the carburetor mounting pad. Any leak on either end albeit an intake or head gasket leak will limit the negative pressure at the upper side of the carburetor thus limiting the proper flow of fuel vapor. 1&2 affect 3&4 and vice versa. There needs to be a proper intake manifold seal on 3&4 or 1&2 will function although only well enough to allow the engine to run. A flow of propane from an unlit torch as Tom suggested is the best way to find any induction leak to the atmosphere. Remember that engine (suction) does not (pull) the mixture into the engine. Atmospheric pressure pushes it in. Therefore if there is a leak in the induction system that is the path of least resistance and the point at which atmospheric pressure is induced into the system. A very lean condition is the result.
Chet
Chet
tbirdtbird
BANNED
if this happened while you were out driving then the more 'catastrophic' problems i doubt. i am still back to an ignition problem. do you know what carbon tracking is? I do not know how we all of a sudden jumped to a head gasket. slow down and get rid of the doomsday attitude!! (Smiley Face)
Purdy Swoft
Well-known member
I mostly agree with Tbird. The problem probably isn't anything Major. Loping is mostly a result of running rich. Model A's can sometimes have multiple problems, all at the same time. I've had experience with a few of the modern distributor caps some bad but not all. I wouldn't be too quick to mess with the head unless I was sure that the head gasket was the problem. I've only had one head gasket failure in 54 years . The repro steel clad gasket burned in two in the narrow area between cylinders 3 & 4 and did not involve the water jacket. This type failure can happen between cylinders 1 & 2 or 3 & 4 but not likely between 2 & 3 because the gasket is wider between 2 & 3 cylinders. When this type failure happens you will loose all compression between the adjoining cylinders but have NO effect on the water jacket . In other words, coolant won't enter the combustion chambers or the crankcase. No need for a compression gauge, a simple thumb over the spark plug hole while the engine turns will reveal whether you have any compression or not. If all compression is lost between the mentioned cylinders, my next move would be to remove the valve cover. If I didn't find a stuck valve in the open position in both adjoining cylinders my guess would be a burned in two gasket between the cylinders. If the problem is the gasket it would cause much worse running than just loping.
When I had problems with sticky valves, I still has some compression. The slow valve action caused by ethanol and sticky valves weakened compression to the point that the engine had no power under even the slightest load.
Your problem is probably as simple as incorrect GAV adjustment, 5/8 turn open after warm up is enough to cause loping. Running too rich is sometimes enough to cause the spark plugs to become so sooty that they don't function at optimum level. I would also take a good look at the modern distributor cap.
When I had problems with sticky valves, I still has some compression. The slow valve action caused by ethanol and sticky valves weakened compression to the point that the engine had no power under even the slightest load.
Your problem is probably as simple as incorrect GAV adjustment, 5/8 turn open after warm up is enough to cause loping. Running too rich is sometimes enough to cause the spark plugs to become so sooty that they don't function at optimum level. I would also take a good look at the modern distributor cap.
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Growley bear
Well-known member
Carbon tracking in the distributor cap if tracked between the center and one wire lug will fire that one every time a spark is fired. If tracked between two plug wire lugs inside the cap, there will be backfiring almost as if the distributor was installed out of time.
Chet
Chet
Growley bear
Well-known member
As a side note; the Model A engine has a certain rhythm while it turns over before starting. With spark plugs installed and ignition OFF, turn the engine over with the starter and if a cylinder is low on compression you will hear a change in the rhythm of the engine.
As Purdy said earlier, pull the plugs and place your thumb over the hole if there is compression you will know it. At this point all you are interested in is if there is compression. The first method that I mentioned is very quick, easy, and doesn't require any tools. If the sound is erratic, then further checking will be necessary. Start with the easy and work toward the more complicated. If all cylinders have decent compression, (will blow your thumb off the hole), you could rule out a blown head gasket between two cylinders.
Chet
As Purdy said earlier, pull the plugs and place your thumb over the hole if there is compression you will know it. At this point all you are interested in is if there is compression. The first method that I mentioned is very quick, easy, and doesn't require any tools. If the sound is erratic, then further checking will be necessary. Start with the easy and work toward the more complicated. If all cylinders have decent compression, (will blow your thumb off the hole), you could rule out a blown head gasket between two cylinders.
Chet
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