OK well I didn’t break it but last Saturday while out for a cruise, the cat started idling rough and I could not get above 35-40 MPH.
I haven’t changed anything except battery cables so this is totally out of the blue. Today, I’ve checked timing and replaced rotor, cap, and coil and wires.
Here is the deal, the engine idles beautifully when in park, but as soon as I put it in gear the idle drops and the engine starts to shake violently. So what do you engine experts think?
Vacuum leak?
Mine had a small crack in the hose going to the FMX tranny, Took me ages to find it.
Did you try a wiggle test ?
I’m no expert, all though I have been called one.
My first guess, vacuum leak.
Or points shot, but yes, vacuum leak too.
Pertronix…No points. Could that be bad or is it a works or don’t work thing?
wiggle test???
I think he means to grab the vacuum lines and give them a wiggle to see if any are loose.
Vacuum leaks can be a fairly quick fix. At least if the vacuum hose are still in pretty good shape. Go to your vacuum line connections and clip each line about a 1/4 inch with a razor. This will take care of any cracks in the lines from drying out. Only do this if you have enough length in the line to reconnect them. Good luck and I hope this helps.
Steven
Vacuum leak- Easy to test for and find. Easy to fix. No worries.
Should be easy, but I have used almost a whole can of carb cleaner and have not found it yet! I’M ON A MISSION THOUGH 
Similar problem I had a while back. It wasn’t a vacuum leak but the vacuum advance went out on my distributor. GOt a new one and problem solved.
Ok so I replaced the brake booster vacuum line and thought I had it licked but went for a drive and 3 miles down the road, started acting up again.
BTW, vacuum gauge at idle is at a solid 20 does move a bit.
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Do you have a check valve in your brake booster line?
Also, check the vacuum advance problem mentioned above by pulling the hose to the advance off the carb and applying pressure or suction. Shouldn’t be able to do a lot of either. If it’s torn, you’ll know pretty quickly. Could be. It’s getting old.
Shaman…I do have a check valve and I tested it last night. It only works one way and brakes work fine so i’ll eliminate that. I’ll try to locate my mightyvac and test advance. I guess if that is bad it will only show up under load?
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The idles smooth in park but rough in gear sounds like a leaky brake booster but that would not cause the not being able to get over 35-40 MPH.
Bill
I don’t know anything about anything but if it’s okay in park but not in gear, how does that suggest vacuum but not something in the drivetrain? Would a vacuum problem not be there except under load? My first thought would be something like the torque convertor but then there’s the first six words of this post. 
Well I am still looking at vacuum as putting the car in gear does put a load on the engine…but it’s been a long time since I’ve owned a car that you can actually diagnose problems on without a computer so I’m a little rusty!! 
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The subject of the brake booster has come up a couple of times. To settle that, disconnect the vacuum line for the brake booster and plug it. You will not have much for brakes, but if you can take it out safely and see if that makes a difference in how it runs, it will be a yes or no to that part of the system.
Next, disconnect the vacuum advance no the distributor and try taking it out again to see if that makes a difference.
You said that you have a Pertronix, have you had the car running before this with no problems in that unit? I know with points, if there not opening up enough, it will make the car run like crap and it will idle but as soon as you go to take off, it will sputter and cary on and no hard to even get up to speed. This could be a bad pertronix, check and do a search on the net to see of there are any symptoms for a bad or faulty Pertronix.
I have had vacuum leaks like that, and I pulled all the hoses off their fittings, and plugged the fittings, if the car runs better, then I start putting the hoses back on one at a time, until I find the bad one. Doesn’t take to long, if it’s a vac. leak
OK everyone…Here is the resolution. The problem ended up being a bad Pertronix Ignitor. I replaced it with an Accel point eliminator kit and problem solved!!! The Accel wasn’t my 1st choice but it was the only thing any local parts stores had in stock and I was fed up and wanted this problem fixed!
So to those that say the Pertronix is electronic so it works or it doesn’t, I say bullsh*t. The unit worked, but made the car totally undrivable.