40 running rough. Help!

[FlatheadTed]

idle the motor then when it plays up lift the top of the carb and check the fuel level if its low then you have a fuel delivery problem .if its ok remove carb and strip it down and blow out all the air holes ..run a hot wire from bat to coil see if that changes any thing .

[Bluebell]

Ted, he has something going on that is different from one side of the motor to the other (or that's my take on it) It started suddenly and is there all the time.
As you would know, a problem in the carb (after the float chamber) effects two cylinders each side)

Wmg, O.K. now while nothing else is changed, take the four spark plugs from the side that noticeably drops, and swap them into the other side. If all four on the side that dropped off before, all do the same with the other plugs in, that will tell me that the plus are all ok (just because they are new, doesn't mean they work)

You should be able to tell if its firing on all cylinders when you short out each individual plug, so we need to know is it running on seven or eight, (or four?)
If the same four are still not showing a noticeable drop, then some thing funny is going on (intake manifold gasket, head gasket, blocked exhaust on one side)
You can get cross firing, which happens because the spark leaks from one lead into another, which makes the wrong cylinder fire at the wrong time.
If you are getting this, there is a good chance she is back firing up through the carb, or popping at the exhaust. This however doesn't fit with four cylinders in one bank performing the same.
To give you a reasonable diagnosis, we need an accurate description of the problem.
See how you go.

[Ken/Alabama]

My bet is that your jumping fire either via a cracked rotor or terminal cap. I have had the same problem twice on two different flatheads and it turned out to be a cracked rotor both times. Sometimes its almost impossible to see the crack .

[swoopNZ]

Mechanical fuel pump pushrod worn ?

[Wmgmitchell]

Here's the latest: I disconnected the terminal caps on the distribuitor and can't see any indication of cracks. I also looked at the rotor and it too looks fine. I then hooked up a hot wire from the battery to the coil. When I started the car, there was no noticeable difference in the way it ran. I then noticed that I forgot to connect the left terminal cap to the distribuitor (very stupid) but interestingly, the engine ran the same whether the left terminal was on or off.

I switched the right and left bank spark plugs and also grounded them again. Like before, the engine didn't stall nor did the revs change on the left bank, but it tended to stall out when I grounded the plugs on the right bank. I do have spark on the left bank, though.

I also put another new Napa condenser on--just to rule out that issue.

Seems to me I've got an issue with the distributor since the entire left bank seems to be affected. (As I mentioned earlier, there is spark to the left bank plugs but maybe it's weak.)

Now what?

[ken ct]

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Try swaping the inner caps side to side and or change the rotor for another one. ken ct.

[Bluebell]

So you have 4 dead cylinders on one bank. Ken is pointing you in the right direction. It pretty much has to be the rotor.

You have made gains already. You have learned stuff.

[Wmgmitchell]

You've got that right Bluebell! Changing the rotor is above my pay grade. I'm going to take the car to a shop this morning.

Thanks very might to everyone for all your help. This has been a real education and ill let you know how it turns out.

[Ken/Alabama]

Ive seen those rotors crack on the inside and short out to the shaft.

[ken ct]

Originally Posted by Wmgmitchell

You've got that right Bluebell! Changing the rotor is above my pay grade. I'm going to take the car to a shop this morning.

Thanks very might to everyone for all your help. This has been a real education and ill let you know how it turns out.

How do you know the shop knows any more than you do i would have removed the dist. and send it to BUBBA on here.Your going to have a big labor chg. Their not hard to remove and mail out to him. ken ct.

[ken ct]

Originally Posted by Ken/Alabama

Ive seen those rotors crack on the inside and short out to the shaft.

K/A thats why Bubba and myself and possibly some others put a length of shrink tubing over the shaft under the rotor,works great. ken ct.

[Ken/Alabama]

Great idea! Next time I go into my distributor I will do that.

[ken ct]

Ya can thank Bubba for that ,i got it from him a while back. ken ct.

[nospartsman]

Here is something out of left field that is easy to check. This happened to a friends 1940 pickup, it had all the symptoms of fuel starvation but it turned out to be the contact plate on the ignition switch. From a lifetime of service there were deep groves worn into the brass contacts which gave intermittent contact. All contact plates show some state of wear and should be replaced with NOS.

[fortyfords]

I would hang a new condenser on first, a cheap try.