Tuning 1968 classic 500 Loud 'pop' then lost power, barely made it home

[Nts550]

I was driving in town, heard a 'pop' (not too loud, more quiet than a back-fire) and lost power. I had to have my family get out of the car so I could make it up a slight incline; I was able to limp back home in 1st gear (approx 1 mile). What could've happened? Head gasket? There is a small puddle of fuel beneath the reservoir on the Webber carb. The car idles/runs louder and rougher. I thought that the exhaust manifold may have broken, but it still looks good.

I replaced the entire fuel system last year: new tank, new lines, rebuilt carb, rebuilt fuel pump and cleaned the fuel sender (tube in the tank). Since I replaced the fuel system, my 500 has started and has run perfectly. I don't know where to start in trouble shooting the problem. I am stumped...

 

[smart51]

I'm still a bit of a novice so I can't think of anything that would cause what you describe. So I'd start a methodical examination. First off, it still runs, so nothing has seized and no rods have been thrown.

First, I'd inspect every surface of the engine and gearbox, using a mirror to see the hidden sides. Are there any holes or cracks. A condenser can go with a bang. The coil too. My exhaust went recently. There was a loud pop but then it was really loud afterwards.

Next, I'd take off and inspect the carb, look inside the distributor, take off the rocker cover and look inside. The oil filter cover too. Anything that is easy to take off in situ. The sump if you can jack up the back of the car.

After that, I'd worry that it was the valves, the head gasket or a hole in a piston, which sounds like a head off job.

 

[fiat500]

I was driving in town, heard a 'pop' (not too loud, more quiet than a back-fire) and lost power. I had to have my family get out of the car so I could make it up a slight incline; I was able to limp back home in 1st gear (approx 1 mile). What could've happened? Head gasket? There is a small puddle of fuel beneath the reservoir on the Webber carb. The car idles/runs louder and rougher. I thought that the exhaust manifold may have broken, but it still looks good.

I replaced the entire fuel system last year: new tank, new lines, rebuilt carb, rebuilt fuel pump and cleaned the fuel sender (tube in the tank). Since I replaced the fuel system, my 500 has started and has run perfectly. I don't know where to start in trouble shooting the problem. I am stumped...

Limping home as slowly as that suggests maybe firing on one cylinder. If you're lucky, you might just find one plug-lead fallen off. They can end up tangled on the exhaust if not secured properly or if they are over-long.

 

[Bleeding Knuckles]

Hi Nts, I'd agree with @fiat500 and check the plug leads are secure. If they are start up and then remove one plug lead (use insulated grips) and note if there is any change in the engine note, replace the lead and try the other - this will isolate which cylinder is off.
Ian.