So recently I have been dealing with a puzzling problem. My completely stock 1987 monte carlo ss has been running fine from startup, no issues what so ever until after a period of time. The idle begins to become rough, the needle on the tach "bounces" and the car doesnt stay at a constant rpm. My guess is that its a problem with the distributor not advancing. This is not an issue with the timing (trust me I checked). I dont throw a check engine light so my guess is that its not an issue with the ECM. If anyone else has experienced this problem your help would be much appreciated.

 

Clean carb, intake ect, and do a tuneup. If that dont work Check to see if valves are good. on a fuel injected car I would say throttle body. If your useing a K&N filter get rid of it. It clogs everything, people swear by it but it will clog everything.

 

Maybe the springs are wore out and not pulling the weights back. Also could it be a vacuum leak.

 

Ok since its an 87 & (25 yrs old)....My best guess tells me its a V-8 with a 305/350 etc..

This issue sounds to me more like a timeing chain thats stretchn out or worn timing gears....
Rather than distributor.... HEI distributors are very very reliable....When it does fail....The car wont run at all..(usually the module fails)
Unless...someone switched it to a dual point distributor (but you said its all stock).

How many miles on it?
Does it have possibly over or around 70,000 plus miles?
How did you check the timing? with a timing light?...(not the answer)
You could also pull #1 plug...bring it to top dead center..and see where the rotors pointing...
All that tells you...is that its close. (Not dead on)
The only way to be sure is to pull the water pump off and the timing cover to be positive that the timing isnt off.(which im almost positive it is as a V-8 305/350 etc motor..)

Check the backlash in the chain....there should be 0 backlash...
If the timing marks on the cam and crank even deviate slightly...
Youll have all the signs your experiencing now....

The only resolve after that is to replace the timing chain....and possibly gears....since some model years actually used nylon on the gears to reduce noise...So the gears also maybe worn out! I dont know how long youve owned or its regular maintenance schedule,was...is....Which also attribute to wear on the chain and gear assy.....

But I would need more or confirm the info motor size, mileage etc....
Whens the last time it was tuned?....... Maybe you also got some bad gas?
With so little info from you...currently...actually the possibilitys are endless...
Could also be fuel pressure...But my first mind tells me a timing issue...(not the distributor)

Im guessing...at other than idle....It runs fine? .... under acceleration? and cruise speed...?
Maybe dirty injectors?....Maybe run a can of "DRY" gas thru it???
Is the tank on empty?(garbage in the bottom of the tank?) do you drive it daily? or store it?....

 

I would check for a vacuum leak...in particular any hoses that are routed through a themal vacumm switch that allows flow once the engine is up to temp.

 

Okay the engine is the original stock 305. I bought the car with 94,000 miles on it and the car has had a complete overhall. Brand new paint job and all the gaskets on the engine were replaced. (trust me I checked). The dealership I purchased the monte from said the owner was an older man who died letting the dealership owner (his nephew) take the car and turn a profit. The car has had absolutely no issues. The carburetor on top is a rebuilt Rochester carburetor. This is the first real issue I have had with the car. I kinda had the assumption that it was bad gas because this problem arose immediately after I filled my tank. The problem has actually improved. I drove it this morning and it would intermittently run good then bad then good and so on. I put STP fuel system cleaner in the quarter tank of gas left so hopefully by my next fill up this problem will go away.