If the motor doesn't run proper any more it's time to do the points. If this doesn't help to solve the problem you'd better adjust the valves. Usually it's the valves if the motor farts and spits after starting. With a little skill it's no problem to properly adjust the valves yourself. This must be done to a cold motor. So let it cool down and take your time to take off the fenders. Now you can reach the rocker caps. Before taking them off look for 2 empty cans to collect the oil. It should be large and flat enough to fit under the cylinder head and can be placed on the chassis. Both sides together hold about 0.25 l of oil.

When the caps are in position take off the nut on the cap with a 12 mm spanner. Usually the cap sticks with its seal on the head. If the person who did the valves last time has put some glue or liquid sealant on the sealing it can be hard to get the cap off. To use glue or sealant is not necessary but unfortunately many people do so. Assumed the cap is ok it doesn't need liquid seal to hold the gasket on the cap.

If the cap sticks please do not try to use a puller or a screwdriver as a lever on the edges! This will destroy the cap. Better use a rubber hammer instead to beat carefully on the cap.

After both caps are off you can start to adjust the valves. The valves are driven from the camshaft via the pushrods. When the camshaft pushes the pushrods up what makes the rocker arm to push the valve down to open it up.

In the relaxed position must be a play of 0.2 mm between the rocker arm and the valve guide. This play has to be adjusted. The reason for this play is the different extension of the different materials under heat. For the adjustment the camshaft must be in the correct position. There are 3 different ways in which position to adjust what valve. Method 1 (recommended by Citroën) and method 2 are well known for 4 or more cylinder engines. Mechanics learn those method(s) so they don't need to learn anything new for each motor. But both methods are relatively critical. And also the camshaft of the 2CV seems not to be made precise enough (like many other parts on those cars as well). So for the little 2 cylinder motor of the 2CV the 3rd method works best. But I want to tell you about all 3 ways.

Method 1: (opposite valves)

The motor must be turned until one valve on one side has completely opened. Now adjust the same valve on the other cylinder. This procedure must be done for each valve.

Method 2: (same side)

One valve has opened completely; the other valve on the same side has to be adjusted. This procedure must be done for each valve as well.

Method 3: (in balance)

On one side both valves must be in the same position. The outlet valve closes and the intake valve is just beginning to open up. When both valves are in the same height, adjust both valves on the other cylinder.

I think that that 3rd method is the safest and easiest one. On the first look it seems to be more complicated to find the position but if you know how the 4stroke engine works it's really simple.

First the theory:
On one cylinder the outlet valve is open and the piston moves up. This cylinder is doing the push out stroke. The burned gas is pushed out. Just before the piston reaches the upper dead center the outlet valve starts to close. At the same time the intake valve starts to open. While the piston moves down it will suck in fresh mixture. Exactly at the upper dead center both valves are in balance. In this moment the other cylinder has it's working stroke. Logically both valves must be completely closed there. This position is a far as possible away from the next cam on the camshaft. After one more turn of the crankshaft the next valve starts moving again.

To find the right position for the adjustment you may use the mark on the flywheel what is there to adjust the ignition. The ignition moment is 8° before the upper dead center. To get the crankshaft and also the camshaft into the correct position you need to turn the flywheel a bit more than 2 teeth after this mark. Each tooth means a bit more than 3°. So it is very easy to find the correct position for the camshaft.
How to do it:
To turn the crankshaft use the 14 mm tool you use to take off the vent. When the crankshaft is in the correct position you can check the play of the valves. The play is correct when the 0.2 mm blade of the feeler gauge just fits between the rocker arm and the end of the valve. If you release the feeler blade it must not fall down.

Picture 1 adjustment on the cylinder head

To adjust the play, loosen the locking nut of the adjustment bolt using a 10 mm ring spanner. Now adjust the distance of the bolt with a screwdriver so the feeler gauge blade can just be pulled through without any force. Try to hold the screwdriver firmly in position when you tighten the locking nut. But mostly the adjustment bolt will be turned a little bit when you tighten the nut. To compensate this, turn the adjustment bolt about ¼ turn left after the adjustment and before tightening. This is about the distance the bolt will be turned until it's tight. Now check the adjustment and redo it if necessary.

After adjusting all valves clean the cylinder heads and rocker caps (don't forget the inner sill of the cap where the new seal will be sitting) and use NEW seals. Put the new seal into the cap and hold it in place on both sides with one finger while putting the cap onto the head. So the seal should be in place without using glue or liquid sealant.

To tighten the nut holding the rocker cap I like to use a 12 mm ring spanner. The nut must not be tightened too much or you will destroy the rocker cap! If it is pressed too hard against the head with the nut in the middle the sill will bend up and it will leak oil. Once this happens the rocker cap is destroyed. So be careful!

Always handle the spanner very sensitive. I hold it as close to the nut as possible. So the lever is very short and the risk to tighten it too much is less. To check if it's oil tight I put some cleaning paper on the chassis under the cylinder head and run the engine for a few minutes. If it's leaking oil I tighten the nut a bit more. Before the next try clean up the cap from the bottom. When both sides are tight the job is done.

A few more words about the play: If it happens more often that the outlet valve has no play, the valves are gone. We discovered this effect a few times already. At least in one case this effect was caused by bad gas. The whole cylinder head was covered with a thick crust of gray "sediments". Also the spark plug was messed up with this gray stuff. This material also sat on the valve seat. In the outlet stroke the valve heats up. Because of this crust the valve can't transfer the heat to its heat sink - the cylinder head. So the valve starts to glow. Since the valves were heat treated to harden them, this hardness will go when the valve gets too hot. The valve seat is still hard, so the valve will wear out on its seat. This gives you less play. After some time the motor refuses to start, since it has almost no compression left. When you adjust the valves it will run great again but only for a limited time. With any adjustment the bolt comes out a little further until there is nothing left to adjust. In this case the valves have to be replaced.