Gravity related engine stalling

[JulioK]

I have a 1972 lwb stepside with a 307 and three speed manual transmission. Since I've had the truck it has this problem with stalling as I come to a quick stop. I have done a number of things to the truck along the way and while it runs much better than when I got it it still has the stalling issue. Here are some of the things I've done or had done along with a few more details about the truck.

Replaced the carb with a rebuilt 2bb rochester from oreillys
New air filter
New engine wiring harness
New spark plugs
New see thru fuel filter
New fuel pump last spring.
New brake master cylinder (not power brakes)
Timing is recently set (by a pro mechanic)
Truck has HEI with proper 12v hot lead
I have replaced several vacuum lines
I have sprayed carb cleaner around the carb and intake manifold and think I am free of leaks
I have replaced the valve cover gaskets
The truck burns a little oil and smokes at startup. I suspect it needs new valve seals but have not had time to try my hand at that.

Stalls are more likely to happen as I stop on a downhill.
Another symptom is that when I come to a stop I hear the rpm go way down and then after a few seconds at a stop the rpm goes back up to normal idle
The stallng happens whether I am in gear or coasting at idle with clutch in or in neutral as I stop

Sure seems gravity/momentum related. If the carb float was misaligned could this be the problem? As fuel sloshes forward in the bowl could the float be dropping down and killing the fuel flow? Truck runs great other than this one issue. My short term work around has been to leave the choke pulled out too far but the obvious side effect is the engine is idling way too high.

Thx for sharing any thoughts on the matter

[MAC71]

It is the carb fuel level. It could be the float or fuel psi.

[geezer#99]

What is your timing set at?

[JulioK]

I will try adjusting the float. I should lower the float correct? I am not sure if this is an accurate assumption on the fuel pus but I assume it's fine because I recently jumped out of the truck immediately after a stall and checked the fuel in the see thru fuel filter and it looked full.

[JulioK]

Not sure about the timing setting. I'll have to go by the shop and ask my friend that set it.

[geezer#99]

The reason I asked about timing is this.
When you close the throttle to stop you cut off the vac advance (if you're hooked to ported) and your motor only has the initial timing which can be too low to sustain a stable idle.
Your choking it to raise the rpm works because with more rpm comes more timing from the dizzy.
If your timing is set stock (around 4 to 6 degrees) you need to bump it up to 12 to 14 and likely hook your vac advance to a manifold vac port.

[JulioK]

Thx. This is good stuff for me to think and learn about. You mentioned that the throttle closing at stop cuts off the vacuum advance. The thing is I can coast (throttle closed) out of gear and the truck idles fine. It's just as I apply the brakes firmly to halt the truck that the idle drops and stalls.

[geezer#99]

Did it do the same with the old carb?
Do you have a booster?

[JulioK]

It did have the same problem with the old carb but it generally runs much better with this new carb. Pardon my ignorance but I'm not sure what a "booster" is.

[geezer#99]

Originally Posted by JulioK

It did have the same problem with the old carb but it generally runs much better with this new carb. Pardon my ignorance but I'm not sure what a "booster" is.

Brake booster!
Uses vacuum.
Sometimes a sudden vacuum change can cause the stumble you've got.
How else is your truck equipped.
Your carb change never cured it, so likely not carb problem.
You got pics of your engine bay.
We might see something askew.

[JulioK]

Thanks for all the great things to think about! I think I've solved it!! The carb float was adjusted too low. Much better now!!

[Prescott]

Stalling on braking is usually a flooding condition. In the carb, check float level, needle and seat, float leakage, and even float drop. Could also be excessive fuel pressure from the pump. It would be good if you know what your timing and idle speed are.