What are possible causes of a car randomly stalling at idle

[sujitroy]

I need to get back to trying to sort out a random stalling issue at idle. I have an auto (BW35) with ZS carbs. When I come to a stop, randomly it will stall. I have checked the following over the years, but the problem persists. I have Lumenition with a new Bosch red coil. I have looked and adjusted the temp. compensators, The vent valves have been checked to be operating. The dash pot oil is OK. Car is air balanced. Timing is OK. I'm not aware of an air leak. ( Removing oil filer cap causes engine to run rough. ) I check float valve heights. Diaphragms are OK. Air filter is clean.
The spark plugs are brownish but I can smell fuel on the odd occasion. Fuel consumption has been marginal (short trips 13 US miles per gallon)

Here are some observations. Compared to Stag no.2 (manual,) i have to keep the chole out, but choke is only on fast idle. When I start the car for the first time in the day I need to pull the choke out more than fast idle, but can smell unburnt fuel from exhaust.
I do have a hole in one silencer which is getting worse I need to take care off. Idle speed on my auto is about 800, but when I engage in drive, engine speed dropped to about 700 ish or slightly lower.
Stag no. 2 (manual) has ZS carbs as well. I overhauled these the same way. The only difference is that this Stag has Pertronix. This car runs very well. It idles well at around 500 rpm.

I changed the coil as it took care of a missing issue at speed. The connector from the Lumenition box to optical senor is OK. Is there something I have missed or overlooked?


Sujit

[StagJonno]

Air leaks on the butterfly spindles?

[sujitroy]

Wouldn't an air leak in the spindle says issues cause issues al the time?
Sujit

[jbuckl]

possible Erratic Fuel pump electrical supply or earth.
clean contact inside inertia switch.

[DMT]

I believe Lumenition units are prone to periodic rebooting.

[StagJonno]

Wouldn't an air leak in the spindle says issues cause issues al the time?
Sujit

I'd reckon this would be more noticeable at idle than with the butterfly open - just considering the proportional effect .

[jbuckl]

imho Spindle air leaks mean just when you think you set up the carbs nicely, you realise they aren’t any more. (Anecdotally)
very easy to check with a volatile spray though.

[trunt]

Sujit,

Pull those temperature compensators again and make sure that when they are closed the “bullet” actually seals. Mine were both iffy, since I fettled them the idle has been much more stable.

I tested them by sucking on the ends, a good one doesn’t leak at all.

[sujitroy]

I will investigate and report back. The carsb are from a UK spec Stag and don't have the by-pass valves. I have a spare set of crabs from a Fed. '71. It has the retard port as well as advance port. It has the by-pass valves. Not sure what thy do, but can I block them off?

Sujit

[trunt]

I was referring to the temperature compensators they are the long things with the plastic covers.

The bypass valves leak a little air past when the throttle plates are closed, this helps to burn the very weak fuel mixture that occurs, cleans up the emissions and has a slight effect on popping. They can be blocked off by fitting a solid gasket or by adjusting them fully closed (fully counter-clockwise) as long as the diaphragms are OK. If they are floating or leaking they will cause the idle speed to be too high.

[staggering]

. I have a spare set of crabs from a Fed. '71.

Sujit
[/QUOTE]

I wouldn't fit those they'll make the car go sideways

[Stag About Town]

I had a similar problem once but cured it by fitting fuel line hose clips instead of push fits. Only takes a tiny air leak to cause such problems.

[grahamd]

i am trying to solve the same problem i went out saturday drove for 20 minutes got stuck in traffic car stalled would not restart called the breakdown they took me home trying to work how to get it in driveway i just told the driver to try and start it and it started it has done this the last 3 times i have been out the other times i just waited and it started again. i have replaced coil and tightened my fuel line jubilee clips as the fuel filter (glass filter king) was very low on fuel sometimes . not taken it out since so dont know if its cured but i have the optical luminition fitted ,looking at the luminition technical bulletin it states .there is a possibility that the optical switch is failing .if it is there will be a very definite pattern to the intermittency which will be as follows.the ignition system works perfectly when the engine is cold but misfires or stalls as the optical switch is heated by the rising engine temperature. if the engineis stopped and left to cool for a few minutes(also cooling the optical switch) the engine will start an run perfectly but fail a few minutes later when the distributor re-heats the optical switch so on.dont know if this is any help to you.

[DJT]

Check the carb piston(s) are not sticking. If not dropping back cleanly the engine will stall and my not start again until they have dropped. Remove the air filter assembly and drive around. If it stalls, get out and look to see if one or both of the pistons is raised.

[Stag About Town]

Check the carb piston(s) are not sticking. If not dropping back cleanly the engine will stall and my not start again until they have dropped. Remove the air filter assembly and drive around. If it stalls, get out and look to see if one or both of the pistons is raised.

This is true. I also suffered with that following a service. If sticking and not dropping down it can cause flooding which stalls the engine. Liting the dash pot pistons a tiny bit and then screwing the caps back down allows the engine to re-start. I was told that the dash pots have to go back exactly as when removed. Same side and same hole positions. A real pain because the problem only arose when the car stopped at junctions etc. No problem if you could keep going.