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6 cyl Bristol cars Type 400 to 406 - restoration, repair, maintenance etc

400 carbs heating

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Old 27-10-10, 12:41 PM
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Default 400 carbs heating

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
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Old 27-10-10, 07:11 PM
Des Des is offline
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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.
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Old 27-10-10, 08:35 PM
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Default 400 carbs heating

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 .
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Old 27-10-10, 10:35 PM
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Default 400 carbs heating

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
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Old 28-10-10, 12:20 AM
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Default 400 carbs heating

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
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Old 28-10-10, 01:40 AM
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Default 400 carbs heating

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
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Old 28-10-10, 08:18 AM
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Default 400 carbs heating

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
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Old 28-10-10, 01:39 PM
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Default 400 carbs heating

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
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Old 29-10-10, 06:20 AM
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Default 400 carbs heating

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
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Old 29-10-10, 04:40 PM
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Default 400 carbs heating

Thank you. Yes I realize that I am the only one or one of few with this
problem....I love rarity!
Ok, a good point on the fuel... I have used under 90 octane as I figured
petrol in England was not the greatest in the 40's and also why a manual
ignition overide was fitted. I can upgrade to a higher octane and worth a
try.

When I say running smooth, I don't mean retarded. If I advance more then I
go off timing and a slight roughness comes in without the pinging that you
would get on higher compression engines, when the timing is too advanced.

What are the correct carb gaskets? Mine seem very thin and why I wanted to
put thicker ones on, unfortunately the studs and nuts are flush so I can't
increase the space much. If you give me a direct e-mail I can send you some
pictures.
Of interest, I had talked about oil leaks ( a year or so ago) and you kindly
gave me some pointers. They were things that I had already done, so other
than your kind confirmation nothing new. I did find, however, that I get oil
seepage coming up from the 2 right forward engine studs. This then messes up
the head and eventually down the side.
Possible link to the carb problem? I doubt it, but thought I should mention
it.

Cheers,
Dorien
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Old 29-10-10, 06:28 PM
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Default 400 carbs heating

Don't know if this is of any help but Burlen Fuel Systems part number for the gasket pack is GP76 their site is www.burlen.co.uk . Their contact telephone number 01722
412500.
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Old 30-10-10, 02:58 AM
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Default 400 carbs heating

Thanks Geoff,
The website is of not much use, but from it I got their e-mail so I asked
for pictures, measurements and pricing.
Cheers
Dorien
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Old 30-10-10, 06:04 AM
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Default 400 carbs heating

Dorien, I do not think you can have oil seeping from the front studs, but
may be the cross tube, anyway send me a photo and I will see if it tells me
anything. _Pegasushunter@aol.com_ (mailto:Pegasushunter@aol.com)
Regards,
Bellerophon
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Old 30-10-10, 09:10 AM
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Default 400 carbs heating

Dorien, I have just measured a new original gasket from Bristol Cars which
fits between the head and the carburetors, this is 0.060" or a 1/16" thick
new and this compresses down to approximately 0.40" or 1mm. once used.
Hope this is of help.
My regards,
Bellerophon
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Old 31-10-10, 11:15 AM
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Default 400 carbs heating

I can't follow this reasoning at all, Bellerophon.
Massive heat sinks? Glowing manifolds? (Do you mean inlet?).
If the engine is running well normally, and the car has a manual advance and
retard, I wouldn't have thought the fuel quality could be a major factor
contributing to overheating?
Fuel droplets travelling through the inlet manifold will have a cooling
effect as they evaporate.
When the engine is switched off the carburettors however obviously shouldn't
get so hot that you can't touch them.
I would have thought it much more likely to be a cooling problem.
The water temperature you see on the dashboard gauge is only the temperature
where the sensor is located.
Maybe some channels in the head are blocked (or even the head gasket is
missing some holes!) and there is no circulation at all near the
carburettors.
I have messed around with very old outboard engines, and overheating is
ALWAYS caused by blocked channels, some of which are almost impossible to
find.
Good luck!
Rubbond.
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Old 31-10-10, 05:07 PM
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Default 400 carbs heating

Well Rubbond, The 2 litre Bristol engine does not have an inlet manifold,
it does have an inlet tract where the carburetors are bolted direct onto the
cylinder head.
All fuels are not the same and some burn at a slower rate than others, so
if you have one of these in your tank you can still have burning fuel
entering your exhaust manifold, and thus end up with a very hot manifold.

Modern fuels are only intended to stay in your tank 3 or 4 weeks maximum,
after that they start breaking down into their various parts (ethanol)
which can eat away at components such as the old type of petrol pump diaphragms.
If you have one of these change it to a modern resistant one or you may
suffer the same thing I did on my way to Scotland. Although my engine kept
running a hole developed in the diaphragm which allowed petrol to fill up my
sump and resulted in an engine rebuild.
My regards,
Bellerophon
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Old 31-10-10, 05:40 PM
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Default 400 carbs heating

Correct ....and why I and others do not use car fuel in our planes.
Aviation fuel does NOT have ethanol and has preservatives for long term
storage and minimizing carburettor icing. The use of fuel with ethanol has
caused many aircraft engine failures/ problems. In Canada the only gas
station fuel without ethanol is Shell, All others have 10 to 16% ethanol.
Regards
Dorien
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Old 31-10-10, 08:30 PM
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Default 400 carbs heating

Dear Bellerophon,

Manifold, duct, tract, all the same idea I guess? One way or another, if you
have 6 cylinders and 3 carbs, you will have to split the flow somehow.
I don't think using the exhaust as a sort of afterburner however is a very
good idea. From a thermodynamic point of view it's best to burn all of the
fuel inside the cylinders.

As to the fuel pump diaphragm, neither the ethanol in a "modern fuel" as you
call it , nor all the other additives or the basic naphtha for that matter
should be eating away at it. NBR rubber was produced already before World
War II, and is pretty resistant to both gasoline and ethanol.
Or are you are refering to "sour gas", which can be formed in small amounts
in unleaded gasoline? The effect of these hydroperoxides however can be
compounded around. I hope you are not buying original 1950's stock
diaphragms?

If your fuel pump is also bolted straight onto the engine, which I suppose
it might be on a pre-war design, there are much better rubber types
available today. I have cut out diaphragms and gaskets in the past from a
piece of cured FKM sheeting and a pair of scissors. That will last forever.

A "modern fuel", or leaded fuel for that matter, certainly doesn't break
down into ethanol, although with time the most volatile fractions will
evaporate.
"Stale fuel" is just what's left after the lowest molecular weight fractions
have evaporated. This will result in delayed onset of ignition and slower
flame proagation.
It takes a very long time however for a can of petrol to burn like diesel
fuel. You can still blow your head off.
And anyway, it's too late now to put some aside thinking it will be
drinkable this Christmas.

This was all a bit tongue-in-cheek but largely based on sound facts (or was
my prof telling porkies?).

With best regards,
Rubbond.
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Old 31-10-10, 08:45 PM
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Default 400 carbs heating

I guess this then is because Ethanol is hygroscopic?
I'm going to look fairly stupid now having mentioned afterburners.....
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