Slant 6 engines made before 1981 have mechanical lifters and therefore the valve lash needs to be adjusted periodically. If you are experiencing rough idle, misfires, poor runnability, or gas mileage you may be overdue for a valve adjustment. Misadjusted valves if left for too long without being addressed could cause more issues such as burning your exhaust valves.
Adjusting the valves on these engines is a bit different than some engines as you have to adjust them while the engine is warm and preferably, while running with the valve cover taken off. To some, this might seem a bit intimidating but it's really not that difficult at all and less messy than you think.
Tools Needed
Screwdriver
7/16 socket
3/8 socket and wrench
3/8 locking extension
small 3/8 breaker bar or ratchet (breaker bar is preferred but a ratchet will work)
feeler gauges (.010 intake, .020 exhaust)
First step in the whole process is to make sure you have the engine warmed up (taking the vehicle for a fairly long drive helps). After you have the engine up to operating temperature try and turn the engine off. Next, do a quick clean around the valve cover and remove it, then place some cardboard underneath the vehicle to protect the floor from any oil that comes off of the head. Now start the engine and set the engine to idle as low as possible without dying and have a quick inspection of your rockers arms and valvetrain while its running (make sure oil is getting to all of the rocker arms, making sure all of the pushrods are turning, etc.). This part might seem a bit scary but its not all that bad, Now use your feeler gauges and measure the clearance of the valves by sticking the feeler gauges between the rocker arm tip and the valve stem (while the engine is running!) and moving the feeler gauge back forth, don't try to force the gauges between to rocker arm and valve if it doesn't fit then the valve is too tight.
Valve Clearance Specifications
Most Slant 6's use these specifications but always double check your service manual or another reliable source just be sure
Intake clearance .010
Exhaust clearance .020
Valve Order
Back of vehicle EIEIEIIEIEIE Front of Vehicle
E=Exhaust
I=Intake
The feeler gauge needs to have a little bit of drag but, not be so loose that it easily could moved in and out of between the rocker arm and valve stem and not so tight that it requires a lot of effort to move the feeler gauge. Adjust the rockers with either a 3/8 size socket on a locking extension (to prevent the socket from falling inside the engine) or a 3/8 wrench. The tool will bounce around in the process of adjusting it but you will get used to it after a few adjustments. When adjusting the rockers the adjuster should feel a little bit stiff to turn if not and it turns very easily then you might need a new rocker or adjuster bolt however, sometimes you can install a jamb nut on the adjuster to tighten down the adjustment once your done or clean the threads enough to use green Loctite on the adjuster if you can't get your hands on a new bolt or rocker. Make sure to check all of the valves more than once just be sure your adjustment is as accurate as possible. Once all of the valves have been adjusted the engine should be running a lot smoother. Shut off the engine, reinstall the valve cover with a new valve cover gasket, readjust the carburetor as needed.