79 Spider with twin 40's running rough - "Putt Putt Putt"

[SpyderJoe]

Hi all,
My spider started running rough just last week. I popped the hood and noticed that the spark plug wire from cylinder 3 was pulled out out of the distributor cap, (the metal actually broke off from inside the dist. cap). I replaced the cap, all new wires and started the engine. It is still putt putting away. I then replaced all the plugs, (figuring maybe it was a spark plug issue), and it runs the same.
I fogged cylinder 3 thinking maybe I lost compression in the cylinder, (per advice from my mechanic that I go to when I cant figure it out). The putting is more pronounced under load or when driving vs. sitting in my driveway and tapping the gas pedal. the Gas is new. I changed the fuel filter and still have the issue.

I have a couple of videos but I don't know how to post them. Someone with a lot more experience than me may be able to hear the engine running and have an idea of what to check next. Thanks.

UPDATE:
I pulled the new plugs to check their condition and I found that the plug from cylinder 3 is carbon fouled while the other 3 are clean. Does that mean that there is an issue with the cylinder? I am guessing with all that I replaced from the electrical system, I should now focus on # 3 cylinder? Engine was rebuilt ~4 yrs ago with upgrades to pistons, cams and added the dual webers. It ran great until the spark plug wire broke...

Spyderjoe

[DieselSpider]

How long did you drive it with the misfire on 3? If the cap has been slowly failing for a while the plug could be fouled from the weak spark however it its been fouling for a long time then excessive wear to that cylinder could have occurred so a compression test could be in order.

You also may have a bearing going in the distributor if the break was caused by the rotor striking the #3 tower in the distributor cap. There is also a chance of a burnt valve it this has been been gradually progressing to failure as ignition timing to that cylinder could have been off for a while.

[seabeelt]

Need to clean the carb for that cylinder/pair

[SpyderJoe]

Seabelt,
You were right. I pulled the carbs yesterday and cleaned all of the jets, main body, fuel filter on each carb. Cylinder 3 was acting up and the fuel filter was just clogged with red particulates in that particular carb . The fuel bowl had a lot of these red particulates on the walls and bottom. Cleaned it all up, installed, synced and tuned today. Running 90%. I need to fine tune which I will do tomorrow. Thank you.

[hayesbd]

Not entirely related, but in the future if someone has some rough running issues, it's gonna always be a good idea to check the grounds. Make sure the engine is nicely grounded to the chassis and make sure the battery connection to the chassis has been cleaned even if it looks okay. It's an easy-to-do job that would at least eliminate that possibility...

The reason I bring this up is that I have been fiddling around with my '73 for quite a while with similar issues. I thought it was fuel related at first, then I thought it had to be ignition related, but it turns out that merely cleaning the grounds has restored it to good running condition. The clock now even runs accurately! It's odd that the car would turn over pretty well with the starter, but had a degraded enough ground to affect the spark (and clock)!

Cheers