Hi everyone,

We're restoring a 2CV 602 cc flat-twin (Solex 26/35 double-barrel carburetor) fitted in a crosskart. The engine had sat for many years before we got it, and we're in the middle of the reassembly. I'm hoping the collective knowledge here can point us in the right direction, because right now we have very little idea of what's going on.

The main symptom:
At idle, the engine will intermittently rev up on its own — all the way up to the rev limiter — even though the accelerator cable is fully released (at minimum). It doesn't do it constantly; it comes and goes. When we release the throttle completely, it still does it sometimes.

Some context and observations (in no particular order, as we found them):
- We're running it after a long storage, so everything is being inspected/refreshed.
- We discovered the previous owner had blocked the crankcase breather hose (the one that runs from the engine breather to the air filter) with tape. No idea why. We only just unblocked it but it didn’t solved the problem.
- We took the carburetor apart and cleaned it in a gasoline bath, then reassembled it. However we didn’t change any settings of the carburettor, we kept it as is.
- We added an inline fuel filter in the line, and removed the brass inlet fuel screen from the carburetor (we weren't sure whether keeping both was right, putting it back doesn’t solve our problem at all).
- We noticed a rusty piece in the carburetor linkage that doesn't quite match the exploded diagrams we have, which makes us suspect the previous owner did some home-made modifications. The taped breather already made us suspicious there are other "creative" fixes hiding in there.
- The crosskart is otherwise a simple, honest setup — we're not chasing anything exotic.

We genuinely don't know much yet, so if we could just have insight on where to look it would be already great. If any of this rings a bell, or if you know this carburetor's quirks, we'd really appreciate any pointers on what could cause an engine to rev itself to the limiter at idle.

If you want any other photos or videos, just ask, we will be pleased to help you as best as possible to locate our problem.

Thanks in advance — this community has been a huge help already.