429 issues

[marsalis83]

Hey. I have a 71 Mercury marquis i just bought and it only had 78,000 miles on it. I went through and replaced the little things: alternator, wires, plugs, ignition coil, etc.. The carb seems to be in good shape, it fires right up, but i have one annoying problem: Lately, when I put it in drive or reverse, It idles rough. Not horrible O-My-God- rough, but rough enough for me to worry about my engine eventually shaking itself dumb. Please help. I really love this car.

[fredjacksonsan]

How about cap and rotor? That would make it rough. You did say '71. Also, have you run an entire tank of gas through it yet? You could have something in the tank, old gas or whatever. After the cap and rotor I'd try a fill up with fuel treatment, then head out on the highway for a couple of hours, make sure everything gets good and hot.

Course, could be the idle circuit on the carburetor.

[6502dude]

Try a compression test to ensure that cyclinders are okay and you don't have leaking values, gaskets, etc. Record your measurements so you have a benchmark for future reference. I ususally do compression check each time I change my plugs.

If your lowest reading is lower than 75% of your highest, you have internal troubles. Generally readings will at most be within 10-15 PSI of each other.

Next, make sure you have spark at each plug. 8 cylinder motor will appear a little rough at low idle and okay at highway speed even with one plug not firing. This is not the case with the little 4 bangers.

If spark at each plug is okay and since you just changed wires, plugs, etc, make sure that spark plug wiring is correct. Check reference materials for plug firing order.

[marsalis83]

Hey, thanks for the reply. I did run some mid-level fuel treatment through it and I did run it on the freeway pretty open. Cleaned out all of the little bugs--but I still got that rough idle. I do, however think you may be on to something with the carburetor--where is that part located? Also, if I have a leaky head gasket, won't I have a sweet smelling white smog? or maybe the engine running hot? I don't think its internal cause the car runs beautifully--just idles like crap.

[fredjacksonsan]

Unless you know what you're doing, rebuilding the carb might not work out so well. Sounds like it's the carb, as you seem to have taken care of everything else. The carburetor sits in the center of the engine, under the air cleaner. Usually held on by 4 bolts and some linkage. Take a good look @ linkage before you take it off to make sure it goes back on right.

[6502dude]

A good spray down with carb cleaner of the trottle plates, throat of carb, and linkage may help if it is carboned up.

I would advise against any tinkering or adjustments of carb, without experience, unless you don't mind having paying someone else to fix mistakes.

[marsalis83]

Excellent. I really do appreciate all of your help. I have a set order for a new holley 2-barrel, and I can't wait to get her goin again. I rebuilt the carb in my 77' plymouth fury, and it was a long process. I don't want to even try to rebuild this holley, only because of time. I'll get it in and adjusted, and hopefully the problem will be solved. Thanks again.