I’ve got a 1974 XJ12 with four Stromberg carbs.

The car sat for 30 years, amazingly original and beautiful (pics when I finish it, Fern grey with brown leather and green carpet).

I “rebuilt” the four carbs, disassembled them and replaced all gaskets, new float valves and reassembled (also had a ported vacuum source drilled by British superior on front passenger carb).

New gas tanks, new senders, new double-end stock style fuel pumps, all new rubber fuel lines and new filter.

Balanced carbs and set them up according to manual. The car starts immediately, and runs great. Will start and restart easily —until it gets hot….

if I drive it for a bit (throughly warmed) and then shut it down and come back to it an hour later, it won’t start and front passenger carb overflows.

so, I took that carb off, cleaned the bowl and replaced the brand new float valve with an original valve (cleaned and tested). Checked that float is airtight (it is) and reinstalled original float needle and checked float level (16-17 mm) AND IT’S STILL OVERFLOWING.

what am I missing? Ideas? Insights?

I used to have a 1974 XJ12L many years ago but I don't have any recent experience with Stromberg carbies.
But since you are looking for ideas I contribute the following.
Any carbie with a float bowl requires that the float bowl chamber be open to the atmosphere for the float to be able to regulate the level in the float bowl.
eg on SU carbies this is done by having what is known as a float bowl drain pipe which is actually a breather pipe. If this pipe is blocked the carbie will flood.
I suspect your carbie float bowl vent is blocked allowing it to build up pressure in the float bowl and this is causing it to flood which will mean the engine is getting flooded and contributing to the hard starting when warm.
That's my "aussie two bobs worth" ( USA twenty cents worth) of advice.
Cheers

Wanted to follow up with a solution for future searchers. In an effort to clean up the engine bay and removed (emission ) features on the carbs to make tuning them easier I deleted much of the stock emissions equipment. This resulted in all kinds of extra holes that needed to be plugged in the carbs and air cleaner housings...

Including the vent tube for the float bowls of the carbs, which connects through the air filter housing and then on to the charcoal canister. I capped this with a rubber vacuum cap--not knowing what it was.

But what tripped me up was that I had the car running and driving fine--with the air filter housings off the car, which I left off while tuning and fiddling. BUT then I installed the air filter housings, and it still drove fine, but only on short trips around the block (test and tune). Once I drove it long enough to get hot it would spit/spill fuel out of the front passenger carb--and only that carb.

After taking that carb on and off 100 times and changing the float, changing the float level, changing the the needle and various combinations of each a million times I reached out to the guy that rebuilt my distributor and added a vacuum port for a vacuum advance to one of the carbs (Rob, British Vacuum Unit) and he offered some ideas, including that the float bowl vent being plugged.

Hi Andy, do you have any pictures by chance of what you had plugged originally and unplugged? Working on a car with a similar issue right now

Maybe, lemme look and see. Be back in a day

The bols were vented. Now they are not. vent them again. to the atmosphere is OK in non emmsion test states.
When starting dormant engnes, the bowl cn be filled throught he vent. the engne will firre quicker, not havong to await the pump filling the bow.

The bowl vent goes through the air cleaner housing. The problem I had is that I was test driving my car without the air cleaners for a while before i finally installed them (freshly painted) and in cleaning up the engine bay I remove several pieces of emissions equipment, plugging holes as I went.

I don't have a picture of where the bowl is vented on the air cleaner housing but a google search of 175 stromberg bowl vent will get it. it's on the face where the air cleaner housing mates.

So start by removing the air cleaners housings entirely, if it still overflows then it's internal to the carb.

I also need to credit this post for helping solve the problem. For got to give a shout-out at the time

Thanks, Andy. Will do