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

Hi snaponbob,

to be sure i understand the problem, when you say "it hung up under full throttle" you mean it continued to run at high speed after the gas pedal was released as if the carb did not close. Is that correct ?? How fast does the engine run ?? Flat out ? 2000rpm ? 3000rpm ?

I understand you have checked the throttle blades are fully closing, that's good, that eliminates linkage problems, and tells you the carbs main systems have shut down. Main systems run on air flow, they just can not run with the blades closed.

I spoke previously about a vacuum leak, i was not referring to a external leak in vac hoses for accessories but rather a internal vac leak though one of the systems of the carburetor.

You were saying you pulled the carb today and all looked in order, it will.

There will be no obvious fault a visual inspection will reveal. Carbs are like that, i've worked on many that look perfectly fine when stripped down but wouldn't work for love nor money when put back on the engine.

I've had carbs that worked fine for years on the same engine and then played up in every way and it wasn't dirt, crud, broken gaskets, leaky needles/seats, damaged o-rings or any of the other obvious stuff that comes to mind, in the end it took complete recalibration of the carb (which goes against all reasonable logic) to bring them back to working order.

Carbys are actually a analogue mechanical computer and when you come across a difficult one like the 4300 you have you have to really get your mind around how all the circuits in the carburetor work and how they work in conjunction with each other to either find the source of the problem or do something to compensate for the way the carb is behaving.

Strip the carby down and while it is apart make a drawing in as much detail as possible of the various circuits in the carburetor including all the components and their methods of actuation and their relationship to each other.

You will need to look at:
idle jets, idle air bleeds, idle emulsion tubes and compensation system, idle screws, idle ports, transfer ports, throttle blade relationship to transfer ports, main jets, high speed air bleeds, main emulsion tubes and compensation system, fuel enrichment system (power valve), accelerator pump and check valves, hot idle compensator, secondary air valve system including damper.

As a last thought you may want to try reducing the size of the idle air bleed restriction, this will reduce the air going into the idle compensating circuit richening the idle mixture, you will then have to close the idle mixture screws slightly (maybe 1 or 2 turns depending on how much smaller you make the bleeds) and open the throttle blades slightly. This will reduce the air bleeding into the engine through that system when the throttle blades slam shut.

It sounds to me like there is air bleeding through one (or more)of the carbs circuits taking fuel with it in sufficient quantity to make the engine race on closing of the throttle. You have to find which circuit is allowing this.