XK 120 starting carb

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.


"Don Young" <notme.RemoveThis@nonesuch.com> wrote in message
news:13eef76opj5mj37@corp.supernews.com...
>
> "Cuddly Duddly" <engconn.RemoveThis@comcast.net> wrote in message
> news:g_GdneBuxaGYtHrbnZ2dnUVZ_jKdnZ2d@comcast.com...
>> Does anyone have any ideas as to how to make my 120 start from cold? I
>> rebuilt the carbs and it runs well and start at the touch of the button when
>> warm/hot but cold... I thought I had reconditioned the starting carb
>> properly, but I may not know the 'tricks of the trade".
>>
>> Thanks for any help.
>>
>> Cuddly Duddly
>>
> 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
>
 >> Stay informed about: XK 120 starting carb 

"Cuddly Duddly" <engconn RemoveThis @comcast.net> wrote in message
news:R5qdnZhMdqGC9XfbnZ2dnUVZ_uGknZ2d@comcast.com...
> 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.
>
>
> "Don Young" <notme RemoveThis @nonesuch.com> wrote in message
> news:13eef76opj5mj37@corp.supernews.com...
>>
>> "Cuddly Duddly" <engconn RemoveThis @comcast.net> wrote in message
>> news:g_GdneBuxaGYtHrbnZ2dnUVZ_jKdnZ2d@comcast.com...
>>> Does anyone have any ideas as to how to make my 120 start from cold? I
>>> rebuilt the carbs and it runs well and start at the touch of the button
>>> when warm/hot but cold... I thought I had reconditioned the starting
>>> carb properly, but I may not know the 'tricks of the trade".
>>>
>>> Thanks for any help.
>>>
>>> Cuddly Duddly
>>>
>> 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
>>
>
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
 >> Stay informed about: XK 120 starting carb 

">>
> 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
>Thanks for your help. You're right about the workings of the starting carb.
>That's exactly how it works but my query is from where or how does it get the
>air it needs to pass over the jet for starting purposes, if the tube is
>completely blocked? Wish some one could tell me.
 >> Stay informed about: XK 120 starting carb 

If your XK120 is the same as the later saloon and it is electric, than you
may have a bad otter switch. This is a switch mounted on the front of the
water rail that is closed when the engine is cold so it sends power to the
starting carb and send fuel to the engine. Once the engine is warm, this
switch shuts off the power to the carb and the engine operates without the
choke/carb. When the engine is hot/warm there is no need for the starting
carb so the switch remains open until the engine cools to the point the
choke is needed again. Many of us have wired a simple switch under the dash
so that we can manually send power to the carb if needed to activate the
carb if needed and the otter switch is still open.

Webserve


"Cuddly Duddly" <engconn.RemoveThis@comcast.net> wrote in message
news:g_GdneBuxaGYtHrbnZ2dnUVZ_jKdnZ2d@comcast.com...
> Does anyone have any ideas as to how to make my 120 start from cold? I
rebuilt
> the carbs and it runs well and start at the touch of the button when
warm/hot
> but cold... I thought I had reconditioned the starting carb properly, but
I may
> not know the 'tricks of the trade".
>
> Thanks for any help.
>
> Cuddly Duddly
>
>
 >> Stay informed about: XK 120 starting carb