Broken Monobloc Throttle Stop

[Mick D]

Hi

I have an Amal Monobloc carburettor with the throttle stop screw broken, (the PO has even filed it flush with the carb body ). The screw appears to be some form of brass. The carb is possibly late fifties in origin and has not been run since the mid nineties so everything is a bit 'gluey'.
My initial thoughts are a good session in the ultrasonic cleaner filled with brake cleaner then drill and solder in a 'key' but I'm open to other suggestions.

Has anyone come across this sort of thing in the past and if so, how did you get the broken screw out?

Regards Mick

[Rob Harknett]

Not as easy as you may think. Note it is a special screw that has a smaller dia. peg on the end, than the screw thread dia. Try penetrating oil on the inside, drill a short depth, find a small flat blade screw driver you can tap in the hole, cutting into the screw both sides a bit. With luck you will be able to unscrew it. Its worked for me on studs etc. if I have not had a screw extractor tool that would fit. The latter of cause my first option to unscrew

Ashampoo_Snap_27 December 2018_19h52m50s_001_.jpg

It is pilot jets I have found snapped flush, perhaps by over tightening.

[g5wqian]

try a left hand drill , the heat generated when drilling may loosen the thread and because its left hand rotation it may undo itself at same time .
pop a tiny drill in there first and then increase hole size so you dont go out into the threads in the carb body .

sometimes just heat from a blow lamp and then dropping the carb body into cold water will make the screw loose enough to undo without trying to drill into the broken screw etc .

[Mick D]

Hi

Sorted it

The screw refused to budge with the normal extractor despite the prolonged application of heat and penetrating fluid, (ATF and acetone mixed 50/50).

Got a replacement screw from which to determine dimensions and thread details.

Carefully drilled axially down the broken screw until the reduced portion could be withdrawn from the carb body via the throttle slide bore.

Cleaned out the thread with a suitable tap, trial fitted the new screw - job's a good un

For info: the thread is 2BA, a tapping drill for which is 4mm. The reduced portion of the screw is 1/8" diameter.

Before

Throttle Stop.png

After

Throttle Stop Screw.png

Regards Mick

[alanengineer]

thanks for sending us your solution to the problem

[cbranni]

That's a good result Mick to an awkward job, well done.

Colin