Peculiar idle problem

[claptonfan]

Sometimes while driving, under no particular conditions, my car will start to idle very poorly. If i put it in park for maybe 10-15 seconds, it will slowly return to normal. Where should I start looking to fix this? The car in question is a '65 mustang w/ stock 200 and C4.

 

[CobraSix]

hmmm, my first instinct is that you have a plug that is getting fouled due to a misfire while underload at low RPMs since you have an automatic and it clears up when you put it into park after 10-15 minutes. Question, if you were to turn the car off immediately upon the bad idle and start it back up, does it go away or does it still need 10-15 minutes of non-loaded idle to clear up?

Second thing that crosses my mind is that your alternator or battery may not be up to snuff and putting out enough voltage to keep the ignition system running under full power. Test your voltage output on your alternator and across your battery while in park and in drive (DON'T USE YOUR PARKING BRAKE FOR THIS, get a friend to hold it in drive and STAND TO THE SIDE!

Slade

 

[Anonymous]

Other possible sources are
1--old plug wires-look under the hood at night for any arcing(highly overlooked item)
2--worn ign. parts like points, rotor, plugs and cap etc. (High maintainence items)
3--timing just a little to late
4--Dirty air cleaner
A lot of times we go looking for major problems when a normal maintainence item needs attention. Like all the people who rebuild motorcycle suspensions when a tire needed air(this happens literally 1,000's of times a year across the country)). We have gotten used to maintainence free cars, these cars are not in that genre. Carbs get blamed for a lot more then they are responsible for. Worn ign is most noticeable at low rpm's(lower voltage output unable to overcome higher resistance)and hi rpm's (worn parts can't keep up)