Re: Carb problem need help

["NICK CLARK" <nick.clark@domain.elided>]

What I have found on my Rover V8 Stag is the following.

When the engine was hot and turned off for a short period it would start
without problems, if it was left for 20mins - 30 mins, it would not start
easily.

Also the when the engine was hot the tick over was lumpy.

After overhauling the carburettors the problem remained.

After fitting some 12mm Aluminium spacers the same shape as the gaskets
between the Carburettor's and the inlet manifold, and purging air into the
manifold via a 5mm hole drilled into the spacer. (to control the amount of
air a 5mm piece of air pipe and restrictor was added), this sorted out the
tick over problem, however the problem with staring remained.

The next thing that I did was to replace the Aluminium spacer with a PTFE
one (Without air purge), this cured the tick over and so far also the
starting problem.

This would indicate that problem in my case was due to petrol boiling in the
carburettor.
1) At tick over the fuel was heating up and forcing too much petrol out
between the needle and jet.
2) The petrol was boiling away when it stood still with the engine hot.

I hope that this is some help, it worked for me!

-----Original Message-----
From: Mike Wattam <MikeWattam@domain.elided>
To: Ralph Vrana <rvrana@domain.elided>
Cc: TSR Mailing List <stag-digest@domain.elided>
Date: Saturday, April 10, 1999 12:27 AM
Subject: Carb problem need help


Hi Ralph

This hot starting menace is a common problem with Stags (and some other
cars) in hot weather, and popular opinion says that it is due to fuel
vaporisation.  While I think this is perfectly possible, I do not think it
is the only possible reason.

1.  However, let's assume it is and that if things under the bonnet were a
bit cooler, then it would not happen.  If you have aircon, can I suggest
wiring your fans top be constantly live.  This would allow the fans to
continue running for a few minutes after your engine is switched off to
divert some of the latent heat away from the engine.  Similar set-ups have
recently been used on new cars with high performance engines and downdraft
type carburettors, just prior to the almost universal adoption of
injection.

2.  Others (this includes me) say it is fuel leaking past the carburettor
float chamber and flooding the engine, so you have fuel laying in the
bottom of the manifold and a plug foul the next time you try to start.  I
believe this, because the effect seems to happen very easily with a newly
filled petrol tank.  This is because petrol is kept below ground in cold
conditions, and on a hot day if left to warm up in the car, will heat up
and expand so much it can dribble out of the fuel tank vent - if blocked
(common) the only way it can go is into the carbs and engine.  Similarly,
if the air in the tank is cold and the car is left in the sun.  Hot exhaust
tailpipes will make the effect worse.

3.  Another thing to be considered, is that Lumenition and similar
electronic ignitions tend to stop working when overheated.  While the car
is moving, the air flow should keep things on an even keel but when parked
after a hot run, the top of the bulkhead at the back of the engine gets the
worst heat soak.

A simple first step is to take out a spark plug when it fails to start, to
see if it is petrol-soaked.  This would at least eliminate cause 1.  then
look for pressurising from the fuel tank, by removing the filler cap.  Is
pressure released?  This would tend to eliminate cause 2.

Please let the digest know the result of your investigation.

IMHO - Mike Wattam
Chairman - Triumph Stag Register