XK 120 starting carb

It has been a long time ago but I seem to remember when I sold and worked on new XK120's in the fifties that you had to leave the main carburetor butterflies closed for the starting carburetor to work. If you pressed the accelerator and opened the main carburetors all the air was drawn thru them rather than the starting carburetor. I do know that it works well when everything is right. Hope this helps.

Don Young

Thanks Don, for the advice.

Now here's the full story. Any help will be much appreciated.

Having rebuilt the two SUs on my '54 120, I was unable to start the engine from cold unless I used starting fluid in the intakes. Upon examination of the starting carb, I determined it to be the non- integral free standing type adjacent to the front carb, and presented the viewer, looking down on it from above, with three app. 5/16th inch diameter circular openings letting into 3 vertical tubes cast in the body..

As I understood the workings, when the electrically operated solenoid opened the small disc valve, this allowed the engine's suction to draw in air through the jet/needle assembly via one of these vertical intake tubes, causing an enriched fuel mixture to enter the intake manifold, via the connecting tube.

It was apparent that all of the tubes were blocked as no air was observed to be sucked in and, presumably, as a consequence, no enriched petrol/air mixture entering the intake manifold to facilitate the cold starting.

Upon dis-assembly, I discovered that there was no dirt of any kind which might have blocked the flow of air. Indeed, upon closer examination, I found that both the ''side'' tubes were never drilled out in the casting, and that the ''central'' tube had been sealed off with an aluminium stop plug. All three were sealed. Neither the casting or the stop plug appeared to be any sort of modification and could have been done by the factory.

I removed the plug in the center tube but, after some thought decided against cross drilling the bottom of the casting in order to allow more air to enter.

Upon re-installing, the car started from cold quite well, and the air entering the ''central'' tube made a hell of a loud whistling noise.

Has anyone run across this event before, and if so, could they please explain how the carb would work with no ingestion of air being possible.

I await your advices with bated breath.

Thank you.

I have no knowledge of the operation of your starting carburetor. I seem to recall that the early ones were not electrical but had an air valve that was operated by a pull knob and cable. I wonder if the later ones might be intended to supply only fuel, with the air supplied from the main carburetors. That might still require that the main butterflies be nearly closed to develop adequate vacuum to pull the fuel into the manifold. If no one here has more info, you might try JagLovers forum or a service manual. I would not make any irreversible modifications to the carburetor.

Don Young

I have the same problem can you help me?