Reply to "Exciting -------------- when the throttle hangs up !!"

Once shut off and re-started, it's a 50/50 that it will idle properly.

4300's can have a percolation problem in the float bowl after being shut down which can lead to excess fuel in the manifold on start up and thus hard starting and poor idle for a while after start up.

It can be remedied somewhat by float level ajustment but if lowered too much that in turn can lead to a excessively lean condition.


If shut off again, and waiting 30-60 seconds, it's more likely to idle okay until hard pedal application.

I am wondering if the vacuum system for the secondaries is hanging up. Only happens when the throttle is pushed well past half throttle.

It's worth a look at the piston and spring for the secondary vac plates (Air valve damper piston) to be sure they are free and working correctly but in themselves the secondary vac plates wont make the engine run uncontrolled if they are stuck open (they will only make throttle response terrible)

The throttle blades themselves must be open for uncontrolled engine speed, if the blades fully close all the wizardry in the carburettor that is above them ceases to function and the engine will slow down, so if the engine speed is uncontrolled (seems like the carb is hanging up) primary or secondary blades must not be returning to fully closed.

That doesn't need to be very much when the car is in neutral, just a fraction open (1/16" inch or even less) will make the engine race.

There could be a internal vacuum leak somewhere that is only taking place after the carb has been opened up and the engine revved up.

If that leak continues and there is enough fuel available it will act like the carb as not fully closed back to the stops.

You may want to check the 4300s power valve (auxiliary valve)for proper functioning and possible vacuum leaks into that part of the system.

Also check out the "hot idle compensator" if it has one. It's the bimetal spring gadget at the back of the carb in between the secondary barrels.