Idle screws may not be set correctly...start by turning them in all the way then out 11/2 turns or bad power valve,if idle screws are turned all the way in and engine doesn't die power valve is bad...jj

Last edited by jerseyjoe (5/21/2016 4:44 PM)

Really sounds like the carb is dumping fuel (probably the secondaries) when you hit the brakes.
Best
Al

Ok I turned in the idle screws and it died so I guess the power valve is good. I heard that I could spray water around the intake to check for vacuum leaks will that work? How would I check the secondaries for dumping fuel?

Ok I'm going to try this as soon as I can and try to check for vacuum leaks, I don't see or hear anything as of now but I never suspected the distributor

Well I adjusted the secondary float level down and the same thing happens. I do have power brakes, would the booster be causing this?

Tried slamming the brake pedal down while car was idling, it didn't change a thing, the rpm's stayed exactly the same. So I guess I can eliminate the booster and a vacuum leak if the engine dies when turning in the idle screw. The car has been totally rewired and I am running an MSD Digital 6A, I found no anomalies with any of the wiring. Could this be trash in the carb from fuel sitting over the winter too long?

Are you running a carb spacer?  I had a situation once where when the carb would get hot (normal driving conditions, not 100 degree days), if the fuel sloshed in the carb, it would flash/boil in the carb when the fuel hit the sides of the bowls.   Wouldn't do it sitting still or when driving but with sudden stops, bumps,  or turns it would boil over through the vent tubes dumping gas into the carb and kill the engine.  It would re-start.  It was a combination at that time of really bad winter gas in Colorado (thanks EPA) and heat soak.  I ended up with a 1" spacer and a different air cleaner to stop trapping air around the carb.  With today's gas, it doesn't take much to get a boil over.  Also, I don't think you mentioned if you have vacuum advance and where you have it connected.  If you have it connected to manifold vacuum instead of ported vacuum you will end up closing up the butterflies for curb idle due to the added advance and this may be too far closed when decelerating.  With ported vacuum, the advance comes in with airflow at higher RPM's and there is little to no advance at idle.  Therefore, you can open the butterflies further and open the transition slot for better idle performance.

check some plugs immediately after it dies.  The original mustangs, with Autolight carbs had a similar problem.  That is why they used the 'dash pot' that would let the throttle down easy.