I have a 72 stock 350. Original 4 barrel Quadra Jet and factory distributor. Car starts a little hard cold. Starts easy warm. Issue is a rough idle but smooth operations off idle. Revs right up with no hesittation an performance is good off idle. Just seems to be a little rich at idle. Idles around 1000 when warned up. Idle gets a little better warm.

Could be a number of things but i have read Quadra Jets might have some common problems in this area. Receipts show a rebuild kit was installed in 2009.

Please point me towards the most common areas I should check for this condition.

 

The very first place to begin is to perform a tuneup based upon specifications outlined/established in the CSM: Dwell, Timing & A/F mixture. Can you establish each of the specifications outlined in the CSM for a proper tune up?

 

How long have you owned the car ?
Do you press the accelerator to the floor once before you start the car when cold ?
Do you have the tools to check points dwell, timing, RPM's and engine vacuum ?
Did you check the screws around the top of the carburetor to make sure they are snug ? Don't over tighten them.
Is there a readable placard under the hood with tune up specs ?

 

Car is new to me. I can check points gap and timing. I'll have to check if there are any stickers with specs. If not where can I find the specs for timing, dwell and vacuum?

 

Tune-up specifications are located in the CSM (Post #2). After 50+ years the original OEM placard/sticker located on the fan shroud has often disappeared/disintegrated. Many of these tune-up placards/stickers are available via on-line sources (such as the one below). It's located on the fan shroud if you have one affixed to your fan shroud. Specifications for each engine are contained in the CSM (Post #2).

Tune-up is always in this order:
(1) Set Dwell;
(2) Set Timing (be mindful to follow directions in the CSM regarding vacuum advance hoses);
(3) Adjust A/F mixture screws (performed w/ a vacuum gauge) to highest achievable vacuum. Adjust each screw one at a time until the highest vacuum is achieved for each A/F mixture screw. A stock 350 Oldsmobile engine should achieve a vacuum of ~17"Hg.

NOTE: When performing a proper tune-up, it is generally assumed you have ignition parts in satisfactory working conditions i.e. distributor contact points, distributor cap, distributor rotor, spark plugs, ignition coil wire & spark plugs wires. In many situations these will often be satisfactory; however, if you can't perform a tune-up to the specifications as outlined in the CSM you'd next begin to diagnose a faulty state - e.g. perhaps plugs are bad, coil wire bad, distributor cap faulty, rotor faulty, points faulty, etc. Most often the ignition coil itself may not be bad. But these items are what comprises the basis to perform a proper engine tune-up.

If you can achieve the desired specifications of a proper tune-up & the vehicle still stumbles at idle, it could be choke adjustment, fast idle adjustment or slow idle adjustment. Follow the tune-up specs in the CSM.

 

I agree with checking the vacuum hoses coming off of your carb. Look in the CSM for proper placement and replace them if needed. Mine were dry rotted out when I bought my Olds. After putting new ones on, and rebuilding the Quadra-Jet, the engine really smoothed out.

If the Q-Jet was last worked on in '09, it might be time to do so again, or it might be just gummed up and needs a cleaning out. I wouldn't worry about it though. Rebuild kits are cheap, and it's not a hard job to do yourself.

 

My 72 Olds 350 developed a rough idle and the cause was that the original base gasket was deteriorating and plugged the idle transfer slots at the base of the carb. It idles baby smooth with a new gasket.

Rick

 

^^Good advice^^

Maybe I should have (also) highlighted the importance of measuring vacuum because you do, in fact, bring up a valid point e.g. deteriorated base gasket can rob vacuum. Vacuum hoses are inexpensive and critical to supplying and MAINTAINING the proper A/F mixture (ratio) throughout the vehicle's drive cycle as well e.g. idle, cruise, acceleration & WOT. Cracked/Deteriorating vacuum hoses can rob vacuum, as well. When reading the vacuum gauge ensure you have a "steady" reading (~17"Hg).

 

Pulled off the air cleaner and found this vacuum hose connected to the manifold. The hose is held on to the bottom of the air cleaner base but is wide open. Can't see anywhere it should be plugged in. What is this for and should it be plugged.

I also manually opened the secondary flaps and they don't snap back. Pretty gummy I guess. Can I spray some areas or this require it to be pulled apart and cleaned.

 

That is basically a open vacuum leak. Plug it until you find out what it should be hooked up to. Report back on your findings.

 

That particular vacuum hose connects to the air cleaner base. There is a temperature activated vacuum valve that controls the flap in the air cleaner snorkel.

Rick

 

You can spray carb cleaner, but I am guessing you have no tension or insufficient tension on the "Secondary Air Valve" spring. There is probably the procedure for setting the spring tension. You'll need a small screwdriver and a 3/32" Allen wrench.

 

That was it. Purrs like a kitten now. That was easy.

 

Checking timing and need some help. Unplugged and blocked off vacuum advance in the distributor. Someone painted over the timing marks so want to verify. Top point on the timing marks is 12 degrees, correct? At idle it reads above the top mark by a small amount. Maybe 14. When I throttle up though the timing advances. With vacuum advanced unplugged it shouldn't move, correct?

 

It will move due to the mechanical advance mechanism. 14 is where I liked when my car was mostly stock with a points distributor.

 

You did read and review the tune-up label I posted? It specifically states a 350 4 bbl. Post #5 What does the timing tab demonstrate? Additionally, the timing tab is located in the CSM. Additionally, as stated in the CSM, there is a difference in idle adjustment depending on whether the car is either equipped w/ A/C or not equipped w/ A/C. While it's to your benefit you found a hose disconnected leading to the air filter housing, it isn't going to hurt anything to evaluate your A/F mixture w/ a vacuum gauge to establish the correct A/F mixture. My 350 4 bbl runs perfectly @ 12° BTC.