I had to take the heads off recently and I decided to go ahead and machine 0,3mm off of both my barrels, as ECAS suggests. I read here that slightly raised compression is a common "mod" and generally works well.
The engine revs and runs beautifully, but it knocks even on idle (cold and with choke). I've never heard a pinking engine before, but I don't think this "wood knocking on wood" sound is just the exhaust note changing with the higher compression? Is this common with this slight raising of compression?
To retard the ignition, I need to rotate the points box counterclockwise, I think? The question is, how much is enough. If I don't hear knocking on idle and revving without load, is there a good chance it won't knock while driving?

I should maybe also mention that I have somewhat excessive carbon deposits in the heads, but these did not cause anything like that before. I had to change the oil control rings, because the previous ones were badly fitted and the engine burned a loooot of oil. I cleaned off what I could, figured the rest of the carbon will burn off when there's no longer oil in the combustion chamber. Again, I never heard knocking when there was 3 times the amount of this stuff on the valves, so maybe this is unrelated.

Argentinian pistons?
they don't match the heads properly and contact the heads take the heads off and look for witness marks don't run it or you'll knacker the bearings.

Nope, Perfect Cirle, I bought the 8,5:1 set but changed my mind about this recently
I turned it a few times by crank. Without plugs it was smooth and easy to turn, with plugs nice compression. I may remove the plugs again and double check. It runs very, very smooth, just knocks.

rocker shaft peg bolts tight?

damn, you're on to something... I should not work on engines this late, I tightned 2 or 3 of those only by hand, now that I think of it. I was going to check valve clearances again after this run anyway. Should i back off the stud nuts a bit an re-tighten them with a torque wrench again after those bolts are tightened?
But if this doesn't help (or eliminates half of the knocking ), what then?
And the knocking seems to have the same rhytm as the exhaust, I'll just tighten this and we'll know.

wont do any harm to back off the head stud nuts and re do them


i dont just make this stuff up if theres a way to F***k up a job on a 2cv ive done it

Did you fit the pistons so that the arrow heads on their crowns point towards the front of the engine?
If not, the pistons will knock.

Ken

I retightened those bolts, so far without moving the stud nuts. Right side ok, left totally loose. Some of the noise is gone, but not the knocking
I made a mark on the outer circular edge of the points box and the crankase. For now, I've tried moving it as far as 3 mm from this mark (counterclockwise), I didn't notice much of a difference. What the hell is making that noise?

@ken: I did. I definitely did, because I remember removing and putting back circlips on the same side of the piston, and I marked the right side barrel/piston pair so they ended up back on the same side, and also I simply remember about this and checked it while fitting pistons into barrels. I don't think pistons hitting anything is the problem, I removed the plugs and there are no unusual tight spots in the engine's rotation.

Are you sure its not the pistons hitting the Plugs?

That's a good point. Fitted the right plugs?
In my younger days fitted the wrong plug to my first outboard and the piston hit the plug and totaled the plug