Finally got my Fiat out for a good run. Got it up to 100km/h.

I noticed when accelerating at higher revs the engine occasionally loses power (like no gas is getting through) for a few hundredths of a second, which is enough time to cause the car to lurch, which makes accelerating not very smooth.
Any ideas why this is happening? Other than that, it starts up good, idles good.

I have put new spark plugs in the engine, changed the oil, new air filter, rebuilt the starter, new flame trap, new thermostat (though the vent doesn't close all the way even with the new thermostat). It has an electric fuel pump.

Hi Chris, I am wondering whether the fact that originally it was running without an air filter connected could be a contributing factor? It maybe that the jets in the carb are blocked up with dirt and it only becomes a factor when the fuel air mixture demand is greater. i.e when accelerating. The fact that it is fine when idling and in general ok apart from that short hiccup would sort of suggest that the ignition side of thinks is ok.

It maybe a case of removing, cleaning and rebuilding the carb, it's a straightforward enough job and the rebuild kits aren't too expensive. Its difficult to be 100% sure from afar but given the history it sounds like a sensible place to start.

Tony

Ok will see how cleaning / rebuilding the carb turns out and report back.

Thanks for the advice.

Chris, If you have trouble, I wrote a carby rebuild thread a little while ago, with pictures. Chris

Removed the carburetor, cleaned it up as best I could as per the guide Chris made. It runs significantly better now and starts easier too.

Still get the odd lurch, but its much better than before. It actually looked pretty clean to my untrained eye, but I guess those small pipes and jets really need to be extra clean.