Help with rough running

[TLXregnar07]

I have a 70 F100, 360/auto, 3.25 gears, dead stock. I’m having trouble with it running poorly at idle,in drive. Seems to happen when warm.
Situation is I drove to work, 60 miles, no real issues. Headed home, up expressway, ran fine in creeping traffic, and then great at highway speeds, 70 and up. I stopped on the way home, and once I got started, at every traffic light it would start to run rough and almost stall. Once going, it ran fine. Really great again up to 70mph and more.
I have the vacuum retard diaphragm disconnected, and the timing is right at 10 before. I disconnected the retard, in hopes of getting more advance, to allow best mileage. It looks like it dropped from 11-12 to 10-11, on the same trip cycle. By the way, I have seen as high as 14, and fairly regular 13 mpg. I was getting some of this rough running before disconnecting the rear diaphragm but was hoping the higher idle without it would cure it.
I have not checked dwell/point gap in about 400 miles, it was dead on then. Coil is 2 years old, maybe 1000 miles. Plugs, I’ve never changed them, 14k miles and 6 years.
I have a pertronix unit to upgrade the ignition, but I think the underlying issue needs to be solved first.
What’s the best procedure to check?

I was thinking, check gap first,then see what initial timing is, set if needed, maybe reconnecting the rear diaphragm. See what happens.
Inspect plugs, maybe change them.
Then what?

Any ideas what is causing this?

[Manny]

Have you put a vacuum gauge on the old girl yet?? Sounds like maybe a vacuum plug line blown somewhere. Ignition normally breaks up under load and once you get pulling going or is always missing popping. The just crappy at idle to me often winds up more carb/vacuum issues. Timing a 10 BTDC is not bad at idle, and wouldn't be an issue sounds like. Anything specific or was it stopped shut it down fired up and it fell crappy. Be curious what the vacuum is at idle.

[basketcase0302]

2nd vote for a vacuum leak. I love using a can of starting fluid for checking for vacuum leaks, (with the engine running taking care to not spray any hot exhaust parts or electrical parts). You'll instantly hear the engine RPM change when your spray finds the leak source. Spray intake valley gasket mating surfaces / carb itself / carb base gasket / all vacuum lines (look for a cracked dry rotted vacuum line also).
Also check the small short piece of tubing that attaches the steel vacuum line to the transmission itself.
And the modulator valve itself is a known cause to fail / leak-it's part #7A377 in the below image.

Stop by the welcome mat when you get a chance-that mileage is really decent for a 360 in the Georgia hills!

[TLXregnar07]

Thanks.
I’ll have to see about getting a vacuum guage and checking it out. Maybe also some vacuum plug assortments. It’s not a daily driver, so it may have to wait for a warmer day.
I’m not expecting perfectly smooth like a new car, just need it to not quit at traffic lights and minimal spit on acceleration.
The annoying thing is I could drive it all day at speed with no problem, just a pain at traffic lights.

[Ranchero50]

Bump the timing to 15 degrees. Pull the vacuum hose off the carb and suck on it. If it won't hold a vacuum the advance diaphragm is blown. Very common on dual advance units. Do the same for the auto. If you taste trans fluid on the tube, your modulator diaphragm is leaking.

Engine should idle Rolls Royce smooth. Big chunk of iron with low compression and mild cam should just purr at idle. If it doesn't, your tune isn't correct. Cup your hands over the exhaust and pull it up to your nose. Sniff test tells how it's burning. Most tune problems are related to the auto choke not working correctly. PITA to sort out but pays dividends once it is. Choke stove on the manifolds must work and feed warm air to the choke for it to work correctly. Idle mixture is only adjusted at full engine temp. Cold idle etc is done by the air flap and cold idle RPM steps. Again, PITA to adjust and takes a bunch of heat cycles to verify it's correct.

Even Southern boys should understand their chokes.

[Manny]

Bump the timing to 15 degrees. Pull the vacuum hose off the carb and suck on it. If it won't hold a vacuum the advance diaphragm is blown. Very common on dual advance units. Do the same for the auto. If you taste trans fluid on the tube, your modulator diaphragm is leaking.

Engine should idle Rolls Royce smooth. Big chunk of iron with low compression and mild cam should just purr at idle. If it doesn't, your tune isn't correct. Cup your hands over the exhaust and pull it up to your nose. Sniff test tells how it's burning. Most tune problems are related to the auto choke not working correctly. PITA to sort out but pays dividends once it is. Choke stove on the manifolds must work and feed warm air to the choke for it to work correctly. Idle mixture is only adjusted at full engine temp. Cold idle etc is done by the air flap and cold idle RPM steps. Again, PITA to adjust and takes a bunch of heat cycles to verify it's correct.

Even Southern boys should understand their chokes.

No reason these because they are old the should idle rough. My opinion they should run better and smoother than the new ones. Dial her in, and sort it they should be butter cup.

[briansbronco]

YUP one more for you got yourself a vaccum leak. At Idle I can't hear my trucks running until you walk around the back. SHMOOOOOOVE... Vaccum guage or carb cleaner shot through a stray around the carb and hoses should tell the tale. Shoot stuff under the hood with the carb cleaner and if your engine revs a little, whatever you shot is your leak....

[TLXregnar07]

Ok, so this weekend I had a few minutes to look at the truck.
Got the following readings
Vacuum gauge on the rear diaphragm line, I know that’s not the best as it comes off a switch of some kind, but tried for initial tests. Had vacuum at idle, around 17-18 steady at first test.
Ported vacuum from front diaphragm, 0” at idle like it should be, does increase as RPM goes up. Manual vacuum on this port does increase timing, and slight RPM increase.
Hooked up dwell meter: dwell reading around 27, in spec.
RPM on meter at idle in park, advance plugged: around 680, drops to 650 or so in drive. I think this meter responds slowly, so these are a bit suspect.
Timing is at 10 BTC, best I could tell. My light was weak and I need to mark the balancer for easier reading.
At this point it looks like it should run just like it did when it rolled off the lot.
One last test, rigged gauge to the manifold vacuum, rear advance line again, and an put it where I could watch it while in drive or reverse, holding the brakes. Eventually it started the rough running. When it did, the vacuum would drop 2-3” then recover.
So what does this mean? Misfire? Weak spark voltage at low idle? Bad plug wire? Needs new plugs? Most of the references I’ve read seem to say misfire causes that vacuum symptom, or valve issues.
Or something worse? It runs fine at speed, or maybe just evens out.

[ValdezF100]

Someone had posted this in another thread and thought it might help a bit.

http://www.secondchancegarage.com/public/186.cfm