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LOST!?!? changed Distributor cap and rotor Idles fine but rough over 2000rpm?

I have a 1964 327 with a 1977 HEI distributor. replaced. Cap and rotor it Idles GREAT but when it hits 2000rpm it runs extremely rough.

Is the new cap identical to the old one? Even if it looks identical, find the index key on the old one and compare it to the new one. The index key is a little protrusion on the base that mates to the distributor body so all the plug wires line up where they are supposed to.

Check the offset of that key to the nearest spark plug wire tower. If the towers don’t line up, then it is possible that as the distributor mechanically advances the spark, the spark could be jumping to the wrong tower causing a misfire.

Check the new rotor against the old to make sure the tip of the rotor is pointing exactly in the same direction as the old one. A few degrees difference could cause a misfire as the timing advances.

Lastly, the weights for the mechanical advance are right below the rotor so check the underside of the new rotor to the old one and make sure there is nothing there that could be blocking the advance mechanism.

One way to check this is to put the old rotor on and then grip the rotor and turn it. There should be some spring tension but it should move about 20 degrees in one direction and spring back when released. Repeat with the new rotor and see if it is the same. If the mechanical advance is not working, it will cause the symptoms you are experiencing.


Did it do this before you changed the cap/rotor? Does it do it when you put back on the old cap/rotor?

One other possibility is a loose wire, either at the cap or plug end. Or a bad wire can do this.

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I went back checked everything you suggested The Cap is exact except color. Index key is sameRotor is the same and it does move about 20 degrees in one direction and spring back just like the old one.

It was running ruff but under acceleration (any rpm) checked the Rotor and Cap yesturday and the points and rotor was badly worn (i’m not sure how old the old ones are)

How old are the wires? My '95 Suburban ran rough at speed because of bad wires.

Sorry I am getting confused, do you have the HEI, or points? +1 to @texases question. Changes in Dwell angle via point wear can affect timing, Start with new points? Lack of cam grease for points can cause point chatter. Might be float level in the carb or failing fuel pump diaphram also. Sticking mechanical advance? So many options.

No Idea they were on it when i bought it My guess would be probably age of the cap when it was put on. have plans to get and change wires next weekend. migt be a good idea to put the old parts back on and do it all then.

when they rebuilt the car 12 to 13 years ago they converted it to a 1977 HEI distirbutor
the Carb was replaced 8 months ago.

What points do you mean?

Contacts in the cap that the rotor connects with

How about plugs?

the Advance mechanism is really dirty. do you think that may cause this problem?

plugs are old they seem to be working fine. (planning on new plugs and wires next weekend. )
I replied to Kieth above " the Advance mechanism is really dirty." what do you think will that cause this problem?

finally got hold of a friend of mine (i thought was out of town) he brought some of his testing equipment over I turns out my Vacuum advance has gone bad.

That would do it! Thanks for closing the loop.

The engine should run well with the vacuum advance disconnected, if the ignition system is still breaking up and the engine won’t run above 2000 RPMs with the vacuum advance disconnected you have a different problem. There is no vacuum at full throttle, the engine should perform well without the advance.

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I would clean and lubricate the mechanical advance springs and weights so they work smoothly. A light machine oil like sewing machine oil or 3 in 1 oil should work.

The vacuum advance leaking causes two problems. It affects the fuel/air ratio but mostly at idle and it throws off the timing curve.

I was going to suggest that you check your spark plug wiring order just in case you mixed up a couple of wires but that would lead to a rough idle.

Did you do anything else like try to adjust the timing or fiddle with the carburetor adjustments? Fess up if you did.