400 carbs heating

[Hydroglen]

Some time ago I asked for help in solving a vapour lock problem and I received a number of suggestions. In addition, Bellerophon came up with the fact that the head may be clogged with sediment.
All is OK and the problem persists.
This is what happens:
Car starts well when cold and drives well under ALL temp conditions, even on the hottest days and in traffic.
I stop for fuel and it starts right up again. A fuel stop being some 5 minutes?
If I stop for much longer or go in to a shop/restaurant it is a real issue to get going again.
The SU carbs are so hot that I can't hold on to them for long.

The only way I solve the problem is by raising the hood/bonnet and then all is well.

I have removed the inner fender panel on the exhaust side, but no difference.
Previous suggestions included fuel pump issues. For those not familiar with a400 the fuel is gravity fed to the pump and the pump is on the cool side of the engine so no problem there.

I was going to use spacers under the carbs but there is not much room. The studs / nuts are flush and even if I replaced them with longer ones there is a bit of a height issue with the hood. Also I would have to redo all the copper lines. Can do...but since I seem to be the only one with this issue, what are the suggestions??
Thanks
Dorien Berteletti

[Des]

I bought a car from a friend some years ago, with identical symptoms to yours, my friend accepted that he had to open the bonnet for a while, waft it even, before starting from hot. I assumed this being down to modern petrol being a more volatile, I thought about electric fuel pump with a return feed to provide cool fuel at startup, electric fan on radiator to over run to prevent heat soak and / or blast the hot air from under bonnet prior to startup, but did neither when adjusting the carb mixture cured it, it was miles out.

[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