Just spent a lot of time trying to tweak my 3.4 MK1, it runs
quite well how ever I am having a problem after the initial
run restarting the car
after I take it out for a run in the morning and park it for
10 minutes when I return to the car it will not start it
will simply crank over and over and over.
I spoke to my friend who has an S type 3.4 and he told me
that he is does the same and ‘‘they all do it’’ his
suggestion
was to put my foot flat to the floor and just crank the
engine until it starts!
I am sure the XK engineers did not design the motor to do
this!
Am I losing fuel or is the spark just not enough?
Cheers Mick–
mmcinnes
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In reply to a message from norm larsen sent Wed 27 Aug 2014:
I agree, it is getting heat soak into the carbs. Touch the
carbs when this happens; do they feel super hot? Do you have
the 1/4 inch thick insulating spacers in between the carbs
and manifold? These make the carbs noticeably cooler than
the manifold.–
XK120 FHC, Mark V saloon, XJ12L Series II, S-Type 3.0
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In reply to a message from aussiemick sent Tue 26 Aug 2014:
If this is happening on very hot days, then I agree it is likely a
heat soak problem affecting the fuel in the float bowls. What
could be happening is the fuel is expanding from engine heat and
pushing past the needle and seats in the lids of the float bowls.
This is causing a flooding problem and may explain why it takes
full throttle and a lot of cranking to restart the engine.
My XK120 came from Arizona and was equipped with a small switch
under the dash to shut-off the electric fuel pump. On very hot
days, I switch off the fuel pump about 30 seconds before shutting-
off the engine. This drains down the float bowls, which allows
room for the fuel to expand without flooding the engine.
Before restarting the engne, I flip on the fuel pump and refill the
float bowls. The engine starts instantly no matter the ambient
temperature.–
The original message included these comments:
Just spent a lot of time trying to tweak my 3.4 MK1, it runs
quite well how ever I am having a problem after the initial
run restarting the car
after I take it out for a run in the morning and park it for
10 minutes when I return to the car it will not start it
will simply crank over and over and over.
I spoke to my friend who has an S type 3.4 and he told me
that he is does the same and ‘‘they all do it’’ his
suggestion
was to put my foot flat to the floor and just crank the
engine until it starts!
Ditto. I put an override switch in to disable the auto starting carb on the
3.4. Didn’t need it when the engine was hot, but with it enabled, it would
often flood.
Prior to this (believe it or not) I kept a screw driver and (8mm?) drill
bit. I’d remove the fuel hose and block it off (with the shank of the drill
bit) while cranking to clear the engine. Then it’d start when reconnected!
As I recall, if it didn’t start first time when cold, I needed to clear the
engine to remove the excess fuel.
No doubt this was not fixing the root cause, but it worked.
(That engine had insulators on the carbs)-----Original Message-----
My XK120 came from Arizona and was equipped with a small switch
under the dash to shut-off the electric fuel pump. On very hot
days, I switch off the fuel pump about 30 seconds before shutting-
off the engine. This drains down the float bowls, which allows
room for the fuel to expand without flooding the engine.
In reply to a message from Mike Balch sent Wed 27 Aug 2014:
Close, but no cigar. The fuel would not push past the float
valves (those would be more forcefully closed by the raised
fuel level) and if it did it wouldn’t flood the engine,
just re-enter the fuel supply plumbing.
If there is overflow into the engine from expansion it goes
through the normal open jets into the carb throat and
floods the engine from there.
I like the MkX/420G triple carb heat isolators, which I bet
do a superior job to the Bakelite insulators. The big
sedans used cast alloy double o-ring spacers similar in
principle to Weber DCOE mounts. They work for both heat and
vibration and are held together by spring tension on
longer carb fixing studs. I transplanted three onto my
E-type and although they moved the air filter snorkel a
fraction further out, they worked well. I used them because
the usuals were selling 1/8’’ carb insulators instead of the
original 1/4’’ and also because I already had the MkX bits
handy.
If your heat soak continues to give trouble you might try
something similar, and make sure your float level isn’t
borderline high in the first place.
Pete–
The original message included these comments:
could be happening is the fuel is expanding from engine heat and
pushing past the needle and seats in the lids of the float bowls.
This is causing a flooding problem and may explain why it takes
I understand the purpose of the insulators which would tend to allow the
carburetors to stay cooler than the engine while the engine is generating
heat and the carburetors are evaporating gasoline. However, I can’t imagine
they can make much difference once the engine is shut off. The engine block
will still be hot but the carbs will have nothing to cool them. Since they
are both under the bonnet a steady-state equilibrium will soon be reached
where both the engine and the carbs will be at the same temperature. I have
the flooding/restart problem with the V12 and expect to install a fuel pump
switch to drain the bowls before shutoff.
I like the MkX/420G triple carb heat isolators, which I bet
do a superior job to the Bakelite insulators. The big
sedans used cast alloy double o-ring spacers similar in
principle to Weber DCOE mounts. They work for both heat and
vibration and are held together by spring tension on
longer carb fixing studs. I transplanted three onto my
E-type and although they moved the air filter snorkel a
fraction further out, they worked well. I used them because
the usuals were selling 1/8’’ carb insulators instead of the
original 1/4’’ and also because I already had the MkX bits
handy.
In reply to a message from Mike Eck sent Thu 28 Aug 2014:
Good points but I think the time element is missing. Yes,
eventually everything is steady state but the slope of the
temp curve against time can be moved by reducing the heat
conductance pathway, which I think two o-rings and an air
gap around them do better than a solid Bakelite spacer
over the full flange area. By slowing down the heat
transfer into the non-functioning carbs you presumably
delay the equilibrium. That would therefore lower the
temperature at which steady state occurred, because the
engine would have cooled down more in the interim. I
suspect it’s small potatoes overall but it would be nice
to know what the rule is regarding insulation properties
and spacer thickness for the 1/8’’ v 1/4’’ Bakelites. I
assume it’s more like halving of heat flow rather than a
square rule applying?
Pete–
The original message included these comments:
they can make much difference once the engine is shut off. The engine block
will still be hot but the carbs will have nothing to cool them. Since they
are both under the bonnet a steady-state equilibrium will soon be reached
where both the engine and the carbs will be at the same temperature. I have
I’m not concerned so much about conduction through the bakelite but rather
conduction through the air that is trapped under the bonnet. That is
especially problematic with the V12 where the carbs are mounted above the
cast iron exhaust manifolds. Besides, even if the delayed steady-state
temperature is slightly lower than boiling it will still be warm enough to
evaporate all the fuel in the float bowls, thus flooding the engine. We
know that occurs because every time we start one of these engines we hear
the fuel pump tick quickly to fill the empty float bowls. If the engine has
been off for a day the excess fuel has time to dissipate back through the
air cleaner but if it has only been off for a half hour the fuel will still
be in the intake manifold and need to be burned, thus making the engine hard
to start when warm.
Here’s a test: Run your car hard and then park it. Come back in 10 minutes,
open the bonnet and see if you can touch the carbs without burning yourself.
EVERYTHING under there will be about 200 deg. F.
Good points but I think the time element is missing. Yes,
eventually everything is steady state but the slope of the
temp curve against time can be moved by reducing the heat
conductance pathway, which I think two o-rings and an air
gap around them do better than a solid Bakelite spacer
over the full flange area. By slowing down the heat
transfer into the non-functioning carbs you presumably
delay the equilibrium. That would therefore lower the
temperature at which steady state occurred, because the
engine would have cooled down more in the interim. I
suspect it’s small potatoes overall but it would be nice
to know what the rule is regarding insulation properties
and spacer thickness for the 1/8’’ v 1/4’’ Bakelites. I
assume it’s more like halving of heat flow rather than a
square rule applying?
Pete
The original message included these comments:
they can make much difference once the engine is shut off. The engine
block
will still be hot but the carbs will have nothing to cool them. Since
they
are both under the bonnet a steady-state equilibrium will soon be
reached
where both the engine and the carbs will be at the same temperature. I
have
In reply to a message from aussiemick sent Tue 26 Aug 2014:
I have had the same hot no start but cranks fne
issue…yep,heat to the carbs/floats. Let it cool…starts
right up…sometimes if I am where I can open the bonnet up
and hang out that will speed it up…I now have a fuel pump
cut off switch below the dash.
also…a hot coil that is on its last hurrah…can cause a
similar problem…the coil sits even closer to the hot
engine.
Similar…but different…is the heat from exhaust to the
master brake cyl.especially the tandem,thus the after market
insulator shield that is available. And similar but
different on the xk is the heat from the engine to the
driver!! That is real heat soak !! Thus the dynomat heat
shield material.
Nick–
Nick53XK120S
Spokane WA, United States
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In reply to a message from Nick53XK120S sent Fri 29 Aug 2014:
Wow thank you all very much for your direction. I really do
appreciate all of your input I will be using the car in about
a week so I can attempt these temp related problems and I will
check out the spacer materials while Im there.
Cheers to all Mick–
mmcinnes
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In reply to a message from Nick53XK120S sent Fri 29 Aug 2014:
Wow thank you all very much for your direction. I really do
appreciate all of your input I will be using the car in about
a week so I can attempt these temp related problems and I will
check out the spacer materials while Im there.
Cheers to all Mick–
mmcinnes
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