Hi,
this is pretty urgent, as I have a 3 day rally coming up on friday morning :/
After some recent changes last week, the engine has been running smoothly. Around saturday it started stuttering occasionally when at speed, maybe half a second long loss of power every time.
Today it got gradually worse. I was able to make it one way from home, but on the way back had to call for my dad to tow me to a friendly service garage. There I dismantled the whole carb and took it apart, cleaned it with carb cleaner and blew through everything with compressed air. I ran it and it seemed fine, I was able to drive out of this workshop and when I was about to drive home, the same thing happened again.
So, the symtpoms are:
The car stutters, "coughs" in the carb/intake, jerks heavily when accelerating and after this kind of "ride" even dies at idle after depressing the clutch pedal. After some "rest" it fires again and runs, until I try driving it.
Some facts:
-the ignition coil is brand new, it has only done 1500 km, it is a Ducellier coil (sold here by some company in their own brand named box) and it is clean on the outside
-HT leads new and well fitting, but I swapped those and nothing changed
-running ignition without a capacitor because I switch the coil through a transistor circuit (don't really know how to name this), which only gives low voltage and current on the points, so no capacitor required. I made 2 of these myself and my friend has been using his for the past year / 7000 km without problems, and my engine still fires, so I don't think this bit is the problem
-the carb has all stock fuel and air correction jet sizes, but apart from the famous flat spot, I don't think this is relevant?
-float levels checked and correct, float valve operating correctly
-fuel pump O.K.
-carb gaskets intact, manifold to head joints sealed with gaskets & silicone

I reset the ignition timing last week and it ran very well. The plugs are now dry and black, but this maybe due to the fact that I'm running it almost exclusively on idle with a bit of choke. But maybe something came loose (like the stationary ignition point) and the ignition has severely retarded itself? I haven't had time to check the ignition, for now.

Have you got a standard ignition set up to try ,or a 123 or similar. Or another coil ,just because it's new-ish doesn't mean it's not duff . Sounds like something electrical breaking down once it's warmed up to me anyway

Hmm, perhaps the fuel pipe from the top of the fuel tank has split?

No, I have a non-return valve fitted in the circuit after the fuel pump, and the last section of fuel line from that to carb is transparent, so I can see the fuel coming in.
I'll have a spare, old but working coil to try tomorrow.
Meanwhile I'm going to check the timing (I have no strobe, though)

Saurus,
problems with fuel delivery would match the symptoms you've described, with the misfiring and lurching gradually getting worse as you try to keep going.
Electrical/ignition problems tend to stay the same, except when the problem is a weak spark, when misfiring will be more pronounced with heavier load and/or wider throttle opening.

Have you tried driving it until it comes to a halt, then checking the level of fuel in the carburettor?

ken

But misfiring IS now more pronounced with load. It's okay idling, it runs occasionally uneven but does not stop while revving without any load, but if I try driving, it starts jerking etc.
I'm pretty sure there is fuel in the carb and the pump works, I've watched it fill the carb after this cleaning.
Timing and gap are as I set them, I checked dwell a week ago while setting, and it was 110*, I assume it still is.

Just found out that there's a similar fault developed on a car which I serviced about a month ago, with the owner describing it as 'kangaroo petrol'.

Going to go check it over tomorrow evening, hope it's the condenser, if it is I will cut the lead to the old one and fit a replacement condenser on the light bar...

ken

Are you still running advance weights? Maybe one or both are dragging

Seems it was the ignition, and of course it was so trivial it's embarassing (but it WAS hard to find).
My semi-electronic ignition unit has it's own ground connection. This wire became damaged at the point where it was fixed to the frame, but the heat shrink insulation held it in place. It started losing contact from time to time and more often yesterday. Now that I fixed this connection I was able to take a ride around my neighborhood without a hint of stutter. I hope this really was the problem, because if it fails tomorrow, I'm completely fucked.
Anyway, this device is pretty smart. My friend had to adjust his ignition only after 7000 km, when he started losing some sparks because his points clearance became almost zero (!), and the ignition did not switch the coil from time to time. The reason for that was wear of the part of points which slides on the cam, which reduced the gap since the points themselves did not burn out the slightest

I had some kangaroo petrol about a year ago, load of water in it