Mono jet won't start without priming

[retro4now]

Hi
I'm new to this forum and am finding it very useful.
I just bought a 1971 GMC step side with a later model 250 six.
It had been sitting quite awhile, so I had the tank professionally flushed and cleaned, changed the fuel lines, new sending unit, new fuel filters, and rebuilt the carb.
It's a one barrel MonoJet. It runs fine except for one thing.... When it sits over night it won't start without priming with some gas, no matter how much I crank it or pump it. Then it starts right up and every time after that until the next day.

Any ideas where I should start looking. I haven't had a chance to pull the top off yet first thing and check the float bowl. Seems like it might be leaking down from somewhere, but I don't know where that might be.

Thanks
Gerry
Albuquerque, NM
'71 GMC C1500

[special-K]

It does sound like leak down, but it should strat once fuel is pumped. I wonder if your pump is weak or possibly partially gunked up. I didn't see you mention dealing with the pump at all. I'd buy a new one anyway to complete the fuel system refresh, they're pretty cheap. If you end up needing to rebuilt the carb, you'll have a completely new system

[57taskforce]

Pump. That's where I'd start. My k/20 does the same thing and I know for a fact it's the pump.

[geezer#99]

Choke!!
Is your choke working?

[retro4now]

Thanks for the replies.
I did forget to mention that I did install a new pump as well. I rebuilt the carb and I used to rebuild carbs in my sleep, but I got to admit I haven't touched on in over a decade, so I very well could have missed something.
The choke seems to be working, I step on the pedal when it's cold and the butterfly closes.
I definitely think it's a fuel leak down problem, but where. No leaks externally, and I have a clear fuel filter installed above and in front of the valve cover and it's always full of fuel.
So I'm thinking the fuel must be leaking down into the intake from the carb. somewhere.
thanks again
Gerry

[Wrenchbender Ret]

How about the accelerator pump. If you look down the throat & work the throttle you should see a squirt of gas going in the throat.

[retro4now]

Update: First start of the day, I pulled the air cleaner cover and looked down inside and gave the throttle a couple of blips. Sure enough, it's squirting fuel. However, the choke plate didn't close. When I manually closed it, the truck started right up without priming. So, looks like I'll be checking the choke adjustment and/or replacing the bi-metallic spring.

thanks for all the suggestions.

Gerry
Albuquerque, NM
1971 GMC, 250 six

[special-K]

Glad you figured it out. I hate gremlins

[retro4now]

Put a manual choke cable on it yesterday. Works great and starts right up.
Thanks to everyone for the advice.
Gerry