Hi,
I have a late 26/35 carb with 18/26 venturi ('87 car). I now have all stock jets (fuel jets 102.5 primary, 87.5 secondary), but it has the famous flat spot, and no matter what it won't exceed 100 km/h. I blame the carb for this as the engine and ignition are really good right now. I recently drove some 350 km on a 3 day rally and while the car performed excellent in general, I could never get it to reach a higher speed.
My 2CV has very nice low rev pickup, then comes the flat spot, then on the second choke it goes really well again, until it stops accelerating way too early.
I know size 107 is recommended for the primary jet. Any thoughts on the secondary? Knowing that I'll be swapping both jets, should it be enough to upgrade the secondary jet to size 90 after the first has been changed to 107?

110 primary max unless your local petrol is more than 5% 'save the world' ethanol.
Don't touch secondary (we even made it smaller for racing).
100km max not down to jetting - are you getting full throttle travel?
Ignition timing, check it.

Check the secondary air corrector is in place (will cause drastic top speed loss if not there) - you see it from the top of the carb in the middle (not submerged in fuel).

Saurus,
increasing the primary jet size to 107 is usually all that's needed, as the enrichment of the primary barrel carries over to when both barrels are open.

In any case, the usual rule is to make only one change at a time, so that the effect of each can be gauged.
I 'think' I recall Dave finding that his fuel consumption decreased after the primary jet was increased in size, since it was possible to cruise on a smaller throttle opening...

ken

I've never actually seen E10 on any gas station here. I use regular leadfree 95 octane.
I thought about the throttle opening. I only checked if the pedal moves the lever all the way back, I'll take the top off the carb and check again.
ken: I'm hoping it improves my MPG's, there is a weird effect now, where I can drive at 40 km/h in 4th at minimal throttle and it's smooth, then at my regular everyday speed which is 50-55 km/h it gets a bit weird, noisier gearbox and diff (as if power is not quite constant) and at 60 km/h it gets very quiet and smooth again. Maybe the mixture will sort that out, or maybe some elements are just wobbling on their resonant frequencies at that speed and it cannot be helped
It's almost impossible to get Solex jets here. I'll have to make them to order or buy from ecas, I would just take some and have them drilled out but I don't have any spares and can't risk screwing it up.

PM your address to me?

ken

I'm definitely getting full throttle opening at both. Air correction jets are 1F2 primary, 2AA secondary.
Ignition timing set 2 weeks ago, is a tiny bit more than 8* static. Dwell is a little over 110*. I have not checked timing with a strobe, but as acceleration is good up to mid-high revs, I think advance weights are working fine.
Carb totally cleaned a week ago. All gaskets and joints are ok.
Valve clearances set 2-3 weeks ago (0,2 mm inlet, exhaust between 0,2-0,25 mm).
Vacuum in crankcase isn't bad - last time I hooked up a tube in place of the oil dipstick I got more than 30 cm of water height sucked in the tube.
So why won't it go faster?

Saurus,
there's one, very important, step still to go...
If there's insufficient centrifugal advance, the engine will not perform as well as it should at high rpm.

ken

Oh yes,it certanly did.And that flat spot has gone. Thanks for the heads up Ken

I checked advance with a strobe and it progresses quite smoothly through the revs, full 7 teeth from the static mark. There is around 1 tooth equivalent of scatter. While I checked the static point, I corrected it a bit and it is now a perfect 8* static advance (was a little bit too much, idle is now better). So if it's not the ignition dvance, what's going on?

is the tension device on the throttle cable set correctly

do you have rubber mats/carpet? insulation material on the lower bulkhead stopping the pedal and limiting the throttle opening

have someone press the pedal fully and see whats goingh on with the butterfly in the choke and look at what the linkage does.

does it go over center there is a tab on the spindle that hits a bump on the carb body to stop the butterfly going too far the bump wears and the lever can get bent.

fuel pump weak and fuel lines drawing air

coil is another limiting factor, a seemingly ok but old coil can struggle at high revs. A decent new coil or resin cast one with a primary resistance of 3.5-5ohms will make a noticable differance.