Technical It ran fine before! Idles super rough when vacuum reconnected

Hello again. Long time car lover, newb mechanic, learning the hard way.

1955 Ford with a transplanted 302/C6 out of an 80's F150, converted to Holley 1850 and 1965 single point distributor. New cap, good rotor, new coil, new properly gapped plugs, good wires (resistance check), good compression 130+/- 5 on all 8, no vacuum leaks that I can find. Still, rough cold idle, but OK idle when warmed up. At speed, no bog, stays cool, runs great....until....

Still working for perfection, installed new condenser and set point gap to .021 (I have a dwell meter on order). Set initial timing to 8 BTDC with timing light set to 0, and vacuum hose removed and plugged.....just like you're supposed to. Idles fine, until....

...Reconnect the carb vacuum hose to the dizzy vacuum advance, and NOW it idles SUPER rough, timing light won't even fire!

Checked the vacuum advance canister with a vacuum tester, still advances like it always has. Checked the vacuum line by blowing through it, can hear my own breath coming into the carb, so it's not plugged. Any guesses? Should I put the old condenser back in? I mean come on, right?!

Thanks in advance.

 

Bad wire inside the distributor. When the point plate moves with the advance the wire inside the distributor shorts out or is broken inside and looses contact.

 

Sounds like a vacuum leak. Dist. hose OK? Sounds like it’s cracked/split not plugged.

 

If my memory serves right that is rather common with that distributor. I can't remember if it is a bad wire or bad positioning of the wire.

 

And it only leaks when he connects it to the vacuum canister on the distributor?

I'd suspect that, as @BJR says, there is a marginal connection inside the distributor, possibly shorting when the plate is pulled by vacuum. Does the idle settle right back to smooth if you pull the hose off while it's running?

 

Venturi vacuum should leave the distributor at the same advance it had without the vacuum. I’m thinking the vacuum can is bad. Stick a hose on the can, **** on it, stick your tongue on the hose and see if it holds.

 

Could the additional advance (connecting the dist. hose)cause a faulty condenser to screw up? They always say to check the last thing you did.
Posted same time. See above, answered my question.

 

I'll check that tomorrow...

 

I'll put the "old" one back in tomorrow....process of elimination...

 

Well.....yes. The hose being the leak. The carb is plugged running fine.......

 

I'll check that tomorrow, although it's a new can, only been on a couple months, and it checks good with a vacuum pump tester.

 

Worn bushing/shaft will make the points gap/dwell go away.

Check for side play.

 

Could also be bad ground from point plate to ground.

 

That's what I would normally suspect, but it's a new hose, only a couple months old, and again....it idled fine (warm) before I replaced the condenser, both with and without the hose connected. It's screwy, the kind of thing that seems to happen only to ME!

 

I'll check that tomorrow by disconnecting the carb vacuum hose, and connecting my vacuum pump tester....get it idling at 8 BTDC, then pump up some vacuum, see if it reproduces. But again, it idled and ran fine before I changed the condenser and regapped the points....maybe I jimmy'd something below the advance plate when I did all that....

 

Will do, thanks.

 

Could be.....seems unlikely, but I may have jimmy'd something when I changed the condenser and regapped the points, will check it all over tomorrow. Thanks.

 

Check the dwell when you connect the vacuum advance. It probably does ballistic. Common problem, worn pivot on the breaker plate. I bet when I sold parts at Ford dealers we would go thru 5-10 a week.

 

Yes, I've replaced a ton of worn breaker plates in Ford distributors. A dwell meter really helps find these. With a worn breaker plate the dwell changes a lot when you apply vacuum to the advance.

 

What ever you do, only change one thing at a time and then test. Otherwise you will never know what the problem was.

 

To check Pull cap have points closed have switch on pull vacuum test light on point plate if you get a light with vacuum you have a bad ground. Next block points open you should have power at the points pull vacuum at any time the light goes out during the pull if light goes out you have a bad wire.

 

OK, here's how it sorted out. Removed points and condenser, cleaned breaker plate, checked vacuum canister movement with a vacuum pump tester....breaker plate moves fine, canister holds vacuum without leaking down. Installed NEW points, gapped to .016; Installed OLD condenser. Set initial timing to 8 BTDC, nice idle; used vacuum pump to pull vacuum advance, idle speed increases, no roughness like yesterday, steady timing light. Reattaced carb vacuum hose, now it's back to idling like it did before....not great, but not super rough like yesterday with the NEW CONDENSER. But, BJR, as you say, I changed more than one thing, so still not sure what was going on yesterday, but...I'm happy.
So, thanks everyone!

 

At speed on the road, the engine runs great. Idle is still not great, so I'm suspecting carb. Plan to replace the Holley 1850 with a Edelbrock 1403 AVS2, see what that gets me. Rebuild the 1850, why not.
NEW QUESTION: this 302 is post-78, has non-adjustable fulcrum style rockers. Thinking about replacing them with the Crane or Comp Cams stud-type adjustable rockers, so I can tweak the valve adjustment....which I've never done before, but doing it old-school with the engine running looks like fun. Good plan?

 

Thanks for the tip, when I get the dwell meter I'll check that. I had the dizzy apart 6 months ago, and the nylon/teflon/plastic glide inserts seemed good, breaker plate seemed to move smoothly, but this is my first dizzy teardown so I don't really know what's "good" vs "bad".

 

Thanks. When you say "worn pivot plate", is the part that wears the "pivot" that greybeard360 refers to? The 3 plastic/nylon/teflon glide inserts seem to be good, and I put a little grease on them, but I didn't really look at the pivot.

 

Thanks, sunbeam. I'm from Hutch, well Sterling really. Navy brought me out to San Diego, married a local, been trying to get her back to Kansas but she's not really interested.

 

Yes, the pivot where the two pieces of the plate hinge together. The nylon slides have to be in good shape too, but you've already checked those.

 

Those distributers are real bad to loose the top bushing too. Perhaps when the point plate gets pulled around it lets the points cam wobble. easy to check, pop the top and wiggle the points cam.