johnnyhawthorn

I have a 1970 Cutlass Supreme with the L-74 350 olds engine. The drivetrain is stock with a 4bbl Quadrajet and 3-speed hydromatic 350. A couple of weeks ago while driving on the highway, about 20 miles into the trip, the car started to run rough. I turned around and headed home. Didn't quite make it. The car died, couldn't get it started, it was misfiring out of both the carb and the tailpipe. Had it towed home in case it was the timing chain. Two weeks later... car starts right up, runs like it has a miss, but it runs. Compression is fine. Fuel pump seems to be in good working order. No problems with the timing that I can tell. Car was tuned up (plugs, wires, cap, rotor, points) three months ago, timing and dwell were still set to factory specs. It seems to be a firing issue. What else could be the problem? Could the coil be failing, given that the problem happened when the car was hot? Ideas, thoughts? In 17 years of owning this car it is the first time I've been stumped.

MDchanic

Bad coil or bad condenser are first on the list for me.

Carry a spare of each and swap 'em when it does it again to see which one it really is.

- Eric

M-14

Make sure the distributor shaft isn't sloppy loose.

joe_padavano

Assuming this is a stock points-style ignition, Eric is correct. Those are the leading candidates. My vote is the coil, as it will get worse with heat then run better when it cools down. Of course, an electronic ignition module will also do that if that's what you have.

rjohnson442

In all of that tune up I see nothing about the carb, when was the last time it was checked? Possibly something as easy as a stuck needle an flooding out.

johnnyhawthorn

Being stock it has a points ignition system. The distributor shaft is not loose, checked that early on and as the timing was correct it was not adjusted during tune-up. Carb was cleaned back in December when the choke was replaced. Will check the shop manual for the coil diagnostics; I am leaning towards that now, as heat would cause a failing electrical component to fail even more, and it is 44 years old.

joe_padavano

I know you said it was stock, but please keep in mind that we get a lot of posts asking about troubleshooting a "stock" vehicle, only to find out later that, well, it's "stock" except for the Holley... oh, and the Chinesium HEI... etc, etc. Sometimes offering help is like twenty questions.

JohnnyBs68S

This sounds like it may be flooding. You might check to see if the carb float has taken on some fuel and is allowing the fuel level in the bowl to get too high over time and start dumping fuel down onto the throttle plates resulting in it running too rich and misfiring. Letting it sit for awhile allows it to "dry out" some and run good for a while. I JUST got through a very similar situation w/ my 2-bbl, it was flooding after a rebuild, replaced the float which fixed the flooding..........for awhile. A few weeks after that it started flooding again (running rough / rich / stalling & spewing fuel out the top of the carb and bowl vent right down into the throat of the intake). Pulled the NEW brass float out and sure enough it had fuel in it. Put a ANOTHER NEW brass float in it last night and it fixed the flooding issue. Keeping fingers crossed that this new float isn't a leaker.

I think the brass floats take on fuel after a few hot/cold cycles (cooling draws fuel into the tank), while the plastic floats just gradually get heavy.

johnnyhawthorn

I've got three simple things to check now. I'm just glad it isn't the timing chain. Will look forward to other suggestions, but from here on out it should be a simple repair.

johnnyhawthorn

Post Script: we're all wrong. It was the vacuum advance.

oldcutlass

Was it leaking, stuck,...?

johnnyhawthorn

It was leaking. The vacuum diaphragm was worn out.

MDchanic

Vacuum valve?

- Eric

classicmuscle442

Never heard of tune up without setting timing/