1977 Carb'd Intermittent Missing

[wubie317]

Hi all you Fiat sleuths.

My '77 has been given a low-level tuneup with plugs, distributor cap, points, and fuel filter but continues to have a strange issue. It starts fine, revs well, runs smooth and warms to operating temp at usual rates. No white or black smoke out of tailpipe. Have adjusted the lean-screw towards lean, as it seemed like the plugs looked a bit black and oily, but not caked or anything and none worse than the others. I take it on shake-downs combining in-town and highway speeds, do high-revs and smooth longer runs. All seems well!

But then...

At something like 10 miles, it starts to miss...if it's any help, it seems to happen on the same flat stretch just out of town and gets worse as I climb a hill and cruise down the other side...such that I can't rev past 3000. I can always get it back to home base if I don't rev too high. And even though it's doing its weirdo thing, at idle it still seems and sounds normal.

I'm confused that it can be warm and even fan coming on, and always seems ok. And then, 10 miles from home it starts...

My small brain thinks it's something related to engine warmth and perhaps electrical (though there's not much engine-related connected to the electrical system...oh, and I've swapped coils with new-ish one and older original) and the car has very little in the way of vacuum-assisted things, so don't have a clear sense of any cracked hoses that could possibly be involved. Carb float hasn't been touched in a while, so don't see how that could be, and I've blown out the jets (mostly because I couldn't think of anything else)

Anyone got a theory to test?

Thanks

[18Fiatsandcounting]

My first guess was going to be a failing ignition coil until I got to your last paragraph. If it's not that, then my second thought is to loosen the gas cap slightly and see if that makes any difference. The vapor recovery system (and all the associated hoses and valves) are prone to plugging up, which causes a vacuum to develop in the gas tank. Makes it hard for the fuel pump to deliver enough fuel.

-Bryan

[wubie317]

My first guess was going to be a failing ignition coil until I got to your last paragraph. If it's not that, then my second thought is to loosen the gas cap slightly and see if that makes any difference. The vapor recovery system (and all the associated hoses and valves) are prone to plugging up, which causes a vacuum to develop in the gas tank. Makes it hard for the fuel pump to deliver enough fuel.

-Bryan

Personally, I love that idea...and makes some sense because it does feel 'starved' of fuel when it does its thing. If only it turns out to be that easy! I'll keep you posted.