Technical A very specific miss

[Charlie82]

I've been having a miss on my '82 124 that seems to come and go. After I studied it for a while, I finally realized that it only seems to miss in a certain spot... between 2700 and 3300 RPM's (give or take 100). If I keep the RPM's at 2500 or lower, then no miss. If I keep them at 3500 or higher... no miss. But as soon as I get "into the zone" of roughly 2700 to 3300, then it starts missing and sputtering.

It doesn't seem to matter how fast I am going or what gear I'm in... it's only depends on that specific RPM range.

Any ideas?

 

[Charlie82]

Okay so I was not completely accurate earlier. I noticed earlier this evening that the engine also has a noticeable miss once you get up to 4000 RPMs. I didn't really drive the car that fast when trying to figure it out yesterday... but now i'm feeling it.


So on take off, you definitely feel it missing as you pass through the 3000 rpm zone thru each gear... and in 5th gear, it's noticeable again around 4k. Driving with 3500 rpm's seem just fine.


So far, I've changed the plugs and fuel filter. Had no effect.

 

[Charlie82]

UPDATE

Okay, first, thanks to all that chimed in with suggestions. Huge help!
I'm noticing that this board has a whole lotta questions and not a lot of answers.


Anyway, changed the fuel filter, plugs, wires, rotor and cap so far. The cap and wires looked pretty bad. The cap looked as if someone threw it out the window at 70 mph! It may have had a hairline in it as well.


After that, I could tell it was zippier, but still starts missing as the engine winds up around 3k.


I inspected the centrifugal advance mechanism in the distributor... seems to move freely as it should.


Then I pulled the vacuum hose from the vacuum advance and put a gauge on it and gave it some revs.... gauge shows good healthy vacuum on that line. BUT, never noticed any movement on the advance. Then I put a hose on there and blew into it... it went right thru it! Could hear it just fine... the diaphragm in the advance is toast!


Well then... that makes sense... not only is the vacuum advance not operating.. I also have a vacuum leak since it's diaphragm is ruptured!


If nothing else, at least all of this is starting to make some sense! I was beginning to get annoyed with trying to trouble shoot this!


So, question for those that have played with this before: Is it possible to change the vacuum advance without removing the distributor? Or will it be necessary to pull it out?