Stumble/miss at 2100 RPM Options

[Rrrockhound]

I've got a pretty persistent stumble or miss around 2100 RPM. She purrs like a cat above and below that, but at that RPM, I get stumbling if I rev in neutral (no load) or a miss if accelerating from a stop. I tried disconnecting the throttle switch to see if it would go away, and it actually got a little worse. I cleaned the throttle switch tracks anyway, but no dice. What say you? Trigger points, maybe? It's a 2.0 D-jet, with new plugs/wires/cap/vacuum lines and Pertronix unit.

[bob174]

May be trigger points. Don't know if you read my post from earlier tonight. I changed those trigger points out as a last effort because everything I had read said they were rarely a problem. It fixed a couple problems on my car. They're expensive though. But I look at it like this--everything on my car was old--difficult to narrow down problem areas that way. I started upgrading/replacing stuff with new things and it helps the diagnostic process. Can you narrow it down to a particular cylinder by pulling one injector wire plug at a time, running it to 2100 and seeing if the stumble is still there? It's going to run like crap with one cylinder not firing, but if it's still has a stumble you'll know that the cylinder you pulled was not the problem. Then it's fuel delivery or ignition on the one that's the problem child. Check the fuel pressure--maybe a bad injector?

[Rrrockhound]

That's a good idea. I'll try it. I'm hoping it's not an injector, tho. It's more of a driveability issue than a real problem, so I guess I can live with it if I can't figure it out.

[r_towle]

Try a simple and cheap fix first.

The old distributor grease makes it so the advance plates get stuck.

Remove the distributor, remove the points and the condensor
remove the vacuum advance.

Under all that, and held in with several screws from outside the body of the distributor, are two plates that slide against each other to provide advance...
I have found that if you take those out, you will find some "grease" that is more like gum at this age...
len them up pretty, and regrease with a good wheel bearing grease..(that is what I use..it works fine)
This makes it so the plates move freely again...no sticking...

It takes about 30 min to do, and cost is 0.05 cents.

Rich

[jk76.914]

I've had similar problems over the years....

First, I found the the PO had removed the resistor that is in series with the cylinder head temp sensor. Put one in and it solved it. I seem to think it was only on acceleration, though- under load. Don't recall trying it no-load. Mine is a '76, and I couldn't find any reference to the resistor in any of my manuals, so I didn't know it was supposed to be there until someone asked me about it.... This was about 20 years ago..

Second time was last month. This time, my Pertronix module was interfering with the distributor cap! It's a 20 year old unit, and never gave any problems, until I replaced the cap as regular maintenance. My previous cap was aftermarket- black- and must have had better clearance. The other symptom was that the idle wouldn't settle down to 900, instead hovering around 1400. First thought that my vacuum retard was toast, but it seems the dist cap was skewing the module, and the breaker plate below it over towards the cam. The top of the module was machining grooves in the shaft of the rotor, and the twist on the breaker plate was causing it to stick in the advance position, increasing the idle..... Fix was grinding down the module- just enought but not too much- and even then, I had to relieve a spot inside my distributor cap with a drum sander in my Dremel..... I'll have to do that from now on I guess when I change the cap.

I think I have a Rev zero Pertronix- It's really old, and pictures of the new ones look like they're physically smaller.

Just my thoughts.....

[Rrrockhound]

Thanks for the good ideas, guys. I'll start with the simplest and work up. I wondered what I was going to do this weekend. Shoulda known the 914 would help me out.