400 carbs heating

[Geoff Dowdle]

Dorien ,

I had a similar problem some years ago with my 400. After about 18 years if
trusty service I replaced the 100D2 engine with a 85A engine complete with
reconditioned fuel pump . Cold it would start easily , stop after a drive
for more than 5 minutes it would start but only run for a few hundred
metres then run out of fuel. First thought was defective kit in fuel pump .
Change the diaphragm & valves there was no improvement . I then fitted a
fuel pressure gauge and found it had 3 - 3.5psi cold and under load it
dropped to about one psi .So on closer inspection against another pump found
the operating levers were slightly different . The one being used produced a
shorter stroke and thus less pressure . After bending the first one to
match ,it has been working well ever since at 3.5 psi ,the recommended
pressure.

In Australian summers we get air temps up to 45 deg C and have never heard
of a 6 cylinder Bristol ever having fuel vapourisation problems with either
SU or Solex carbs .

Although my experience is not the same as yours I recommend you test the
fuel pressure from your pump . Its easy to rig up , use a spare fuel bowl
banjo bolt with a barbed fitting & rubber hose to a temporary gauge in the
cockpit . I'd be very surprised if the fuel pressure is not causing the
problem .

Good luck
Geoff

If all else fails fit a 383 and you will quickly learn what fuel
vapourisation is in summer . I had terrible problems with my old 411 S1
until I replaced the cast iron inlet manifold with a alloy one and fibre
spacer under the carb. The cast iron inlet manifold got so hot it burnt the
fibre insulation spacer .

[Hydroglen]

Thanks Des and Geoff.
Ok... carb setting and fuel pressure. Presumably you imply I am little on
the lean side...but the engine runs cool. Well Re the pump...I can't get it
started because the fuel has evaporated! Still, even with more fuel
pressure, it does not explain the very hot carbs. >
I will check both things. We are heading in to winter here in Canada, so may
not get a chance to try untill the warmer temp. return.
Cheers
Dorien

[Geoff Dowdle]

Dorien,



I dont believe the carbs will get hot enough to vapourise the fuel , even
"down under " on the hottest day .
If the carbs are set too lean you will have lack of power , possible
coughing /spitting . If they are STD needles my guess is it runs too rich ,
as the jetting is set for pool petrol in the late 40's which was about 85
octane . You are probably now using 92 - 98 octane . What fuel consumption
do you get on a trip, if tuned correctly it should be between 26 - 30 mpg at
60 - 70mph. 400's on BOCA economy runs in the past have got up to 35mpg.
Even my D2 engine could acheive 28mpg.


Get the use of a mobile gas analyser and find out if it is lean?
The float level of the front starter carb has the fuel level set too low
when you follow the workshop manual .(the level is about 3/8 in lower than
the others)
I usually bend the fork up to almost straight to raise the level the same
as the centre & rear carb.
I believe the front float was designed for a side draft SU.


To open the bonnets will only make a very little difference to the temp of
the carbs or fuel .


Regards
Geoff

[Hydroglen]

Geoff,
I get about 22 to 25 on a run doing 60mph but then I am a little heavy
footed. The car runs well and is very smooth. I use 87 to 90 octane, never
higher.
No I don't have access to a gas analyzer, but going by plug colour I am ok.
Well as I said.... after sitting the carbs are so hot I can't touch them for
more than a few seconds. That would certainly boil off the fuel. Leaving one
side panel up on the bonnett solves the problem.
Cheers
Dorien

[Bellerophon]

Dorien, something does not add up here, if you say that the car runs cool
and yet after standing with the engine switched off you are unable to place
your hand on the carburetors.
I take it that you have an original water pump with no thermostat so that
the water circulation with the engine stopped reverts to thermo-syphon. If
this is the case then there is something which is preventing the water from
circulating, because no way should the carburetor become too hot to touch.
If it was my car I would do two things before anything else, first remove
the water pump and check that all is well, second remove the radiator and
flush it out. This if it does not reveal a problem, then it will eliminate
possible problems in this area. If this fails then further avenues can be
considered, but do you have the SU or Solex carburetors and is the internal
oil cooler fitted.
My regards and good luck,
Bellerophon

[Hydroglen]

Well...I would agree with things not adding up.
The carbs are ok with the engine running. It is after some 10 minutes or
more with the engine off, that you feel the heat in the carbs. So once the
water stops circulating and cooling, the fires of Hell come in to play and
it all starts perking away!
Original SU carbs. I rebuilt the water pump a couple of years ago, however,
I had the same problem with the carbs even when in the UK. When I did the
pump I also redid the rad and that was pretty much plugged up. The car had
started overheating and so because both had to come off, both were done.
After your suggestion of checking the head last year or so, I took the end
plates off (no fun there) and pressure washed the head as much as I could.
When I say the car runs cool I mean about 85 as read on the gauge, with oil
around the same although the oil does get hotter at times. Not unusual and
very much like my 6C Alfa.
What internal cooler are you referring to? I guess by my question I am
ignorant on that one.
So it would seem I have covered your points, so now the ...further
avenues....
Since by opening the bonnett I solve the problem, is the exhaust manifold an
issue? It does not appear to be hotter than say other comparable cars I have
but maybe I should do a comparison.
I have a Pyrometer and tempelsticks for measuring hot surfaces, so I could
give it a try. If it was much hotter, I would then be asking myself ...WHY?
I had checked my tappet clearence some time ago and no news there as far as
I remember, but then again I may be forgetting. Age plays a part but I know
what I had for breakfast yesterday.... LOL.
Might be worth doing again and erring on the loose side?
O course now that we are down to an outside temp. of some 8c 45F the test
may not be as significant.
What do you think?
Regards,
Dorien

[Bellerophon]

Well now Dorien, as you assure us that the cooling system is on the top
line, and I presume that you have the correct gaskets fitted between the
head and the carburetors, then there is another reason for this problem. If it
was a known fault with the 400 then other members would be reporting the
same thing, however, there is not an outcry from other members.
So, you say that you use a very low grade petrol, this could be slow
burning and building up a massive heat sink in your manifolds which then causes
heat to rise to your carburetors. I have seen certain other cars running at
night with their manifolds glowing, so there are two things to try, first
change your petrol to a higher octane one and second check that you are not
running with retarded ignition. You did say it was running very smooth
which leads one to think it could be retarded, we will wait and see.
Good luck, my regards,
Bellerophon