Technical Misfiring engine under load

[Stxox50]

I have been investigating a problem with my Fiat 500L (1971).

When the motor gets warm goes under load it is misfiring.

We have replaced the fuel, carburetor, points, condenser, coil, leads, and even tried another distributor but to no avail, so this is leading us to think there is something wrong inside the engine. We have also adjusted timing, tappets etc but the mystery remains.

Does anyone have any idea what might be going on or where I could look next?

 

[bugsymike]

I have been investigating a problem with my Fiat 500L (1971).

When the motor gets warm goes under load it is misfiring.

We have replaced the fuel, carburetor, points, condenser, coil, leads, and even tried another distributor but to no avail, so this is leading us to think there is something wrong inside the engine. We have also adjusted timing, tappets etc but the mystery remains.

Does anyone have any idea what might be going on or where I could look next?

Lots of things can cause it, a sticking engine valve can possibly, but usually spitting back through carb and getting worse are symptoms.
If only in hot weather then things like fuel vapourisation due to no insulator block between carb and manifold or after market fuel filter too close to heat from engine.
However there are several here on Forum who can give more specific advice.