Backfiring ?bad msd

[LFogelson]

I have an '82 SC. A few months ago my Bosch CDI unit went out and I replaced it with an MSD 6420, high vibration coil, and regapped the plugs. The car had been running better than ever, with the o2 sensor hooked I was getting 18-20mph (I drive heavy footed).

I was coming off the highway 2 days ago and the idle started to bottom out and the car was backfiring (thank goodness for the pop off valve). I was able to limp home by unplugging the o2 sensor and using the co mixture screw to richen the mixture and the idle adjustment. I thought maybe I had a huge air leak. However after playing around a bit I couldn't find a big leak and when I unscrewed the oil cap the rpms dropped. Now I am wondering if my timing is way off and if my MSD is blown. When I cold started today the car Backfired like crazy... the rpm went up like the WUR is operating. Adusting the CO mixture again I was able to keep the car running but the fumes were so bad from the exhaust my eyes started burning. Further I cannot make any adjustments to keep the car idling under 1500 rpm.

I plugged in the old blown CDI and the car ran ok for a couple of minutes then started running irregular (why I switched to the MSD). I wonder about how my MSD is working because I have a 6000rpm rev limit resistor in it and it seemed to be revving beyond that when the car was running well.

I checked my cap and rotor and they are ok. My next plan is to look at the plugs and see if they are fouled. Then maybe spray misted water to look for an vacuum leak. Then maybe I will check my control (fuel) pressures. Anybody have suggestions. I would really like to switch out another MSD or Bosch CDI and see if the car runs.

Thanks in advance

Larry

[LFogelson]

Update:

Plugs clean, normal fuel pressures. Checked again for a vacuum leak and found the Intake manifold sleeve on cylinder 4 (first on right) blew its back side out. For diagnostic purposes I removed the clamps, wrapped the sleeve in electrical tape, replaced the clamps and started the car. Almost back to normal....

I ordered 2 new sleeves and manifold gaskets from our host. I am going to try and change the sleeves on cylinders 1 & 4 without dropping the engine or removing the CIS. The other sleeves look good and I might save those for a future engine drop.

[flipper]

From what I understand, the MSD works or doesnt work, no in between. But, I could be wrong.

[RANDY P]

Frequency valve operating? Is your domelight working?

[LFogelson]

I think the problem is the manifold sleeve vacuum leak. (dome light works, unplugged freq valve relay and the engine stalled) Not sure why the rpm would still drop with opening the oil cap with an existing large vacuum leak?

[2.7RACER]

Try your old coil. The new one may not match your MSD.

[LFogelson]

Update:

Replaced cylinder 1 and clylinder 4 manifold sleeves the car runs really smooth and strong.
I wonder why the rpm still dropped with removal of the oil cap with such a large unmetered air leak in sleeve #4?
Easy project, did without dropping the engine. Eventually I will need to change the other sleeves but 1&4 were the worst looking. Thanks for the help.

[Early_S_Man]

I don't know why people keep perpetuating the myth that electronic units either work or not, because that is patently NOT TRUE. JWW has reported several times on this board that MSD units can be intermittent, and that is pretty solid evidence if you ask me!