Diagnose this...

[51Deluxe]

I've had my 51 Kaiser for about a month and in general it runs great. Starts in the first half revolution. Once it gets warmed up, though, it occasionally will, when pulling out into traffic, stumble and miss, with no power, for as much as 10 or 15 seconds. If I back off the gas, it will generally smooth out and is OK until the next time it happens (unpredictable, except never happens when cold). I don't think it is vapor lock as I've checked the accuracy of the temp gauge and when this happens the coolant temp is only around 190 degrees. Also my gas lines are not routed particularly close to the engine. I'm fairly sure it is a fuel problem instead of ignition though. Any ideas?

[joefrazer]

Check the carb float setting. Also, if it hasn't been rebuilt, you may need an accelerator pump. They are leather faced and don't last long with today's fuel.

A low float setting will cause a stumble due to fuel starvation.

[retired wrench]

 Also look at heat riser and vac. advance on dist. as well as any vac leaks.

[51Deluxe]

Carb is supposed to have been rebuilt. Despite that I suspect the float level or accelerator pump (it seems to work but is not particularly responsive) Heat riser works perfectly. I'm sure vacuum lines and distrib were done during the rebuild- it seems to me that problems with them would be a bit more consistent than a fuel issue- or is that a wrong assumption? Gas tank was cleaning and coated but I'm not sure I trust that process in general.

So which of the fixes mentioned by other posters corresponds with my other issue, "running out of gas" when there should have been as much as 5 gallons in the tank? Maybe there is a connection.

[BigDave LM6174]

I had similar issues back in 1990's with my 51 Deluxe.  I did everything mentioned with no help.  It came down to vaporlock.  I installed 6v delco inline pump above left rear wheel on frame.  Only flip switch if I feel vaporlock.  In our Southern California heat, does not happen all time but it does happen.  Flipping on the switch to turn in pump, vaporlock is instantly eliminated.  I could leave on all time but I don't. 
Look where fuel pump is, right under the manifold!!

[Fid]

I don't think it is vapor lock as I've checked the accuracy of the temp gauge and when this happens the coolant temp is only around 190 degrees.

Vapor lock really has to do with how hot the manifold is and how much heat is coming off it and heating the fuel pump. Engine coolant temperature probably will not matter.  What matters is how hot is the fuel line/fuel pump as that is what will contribute to percolation. The car sits still and the heavy steel fuel line is hot and boils the gas. Outside temp and manifold temp will contribute to it more than anything else.

[51Deluxe]

I don't mind installing an electric pump for vapor lock, just don't particularly want to crawl under the car, cut the gas line and install it on the frame, then have to run a wire 10 feet. In the 70's my 41 Ford did great with a pump installed on the firewall. Wouldn't that work just as well today?

[Corsairdeluxe]

If you install the pump below the tank fuel level ,it will last longer and and will stay primed.Yes it will probably work on the firewall or inner fender,but if you are going to depend on it ,I would not put them there.

[51Deluxe]

I think I'm going to get some flexible  thermal insulation made for fuel lines and give that a try first.