1949 Olds 88 with new fuel pump, clean tank (visually inspected with camera), new fuel filter and fresh carb rebuild- the Olds runs PERFECT with half a tank of fuel, but 10 minutes after a fill up the Olds begins to hesitate and if I really get on it, it will backfire which tells me its running rich. I suspected some issue with the vent in the tank so I tried running down the road with the gas cap removed. No improvement. Timing is spot on, ignition is spot on, plugs are in great shape. I cant stress enough just how perfect it runs with less than a full tank of fuel. Half a tank is the sweet spot!! At half tank it idles, accelerates and even travels highway speeds with no hesitation. But fill it up and the issues come quick. Thoughts? I've tried various octanes and fuel brands. Im confident the choke is set corectly. I've tried to go lean or rich but that only brings issues when the car is at half tank. Im missing something for sure! Thought Id check here for some collective wisdom. Thanks friends and happy summer!

 

I don’t have anything helpful other than to question your assumption that it’s running rich, and to think of other possibilities.

 

Running down the road with gas cap off doesn't confirm the tank vent isn't clogged. Filler necks on most gas tanks empty into the lower half of the tank. There could still be an air space above the fuel level that is un-vented.
A backfire could also be a lean fuel/air condition. During cold weather a car will backfire if not enough fuel to air. So, using a closed choke richens the mixture to start and run.
......Just my two cents worth.

 

love the feedback. Thanks everyone. I used a camera down the fill neck to check things out. There is definitely air space between the filler neck and the fuel. The tank isn’t filled to the brim. So I’d think it impossible for the tank to build pressure with the cap off. It’s curious for sure.
I hear what you are saying about backfires and lean conditions but the weather is 80+ degrees. I really thing the backfire is excess fuel in the exhaust that eventually “pops”. But I’ll try to enrich the mixture and see what change it makes.
the owners manual says the tank vents through the filler neck. Definitely a puzzle. Keep the ideas coming! Id appreciate it. I’m definitely missing something

 

When did this issue develop? IOW, what was the last thing done to the car prior to development of this current issue?

 

Is the backfire coming from the carburetor or the exhaust pipe ? Carb backfires are usually a lean condition. Exhaust pipe backfires are usually excess un-burned fuel igniting.

 

My guess would be the carb float needle is leaking. When the tank is down, less head pressure. Full tank, higher fuel level, leaking needle will overfill the float.