S3 Stalling on deceleration - Help!

Hi all, would be very grateful for some advice: S3 2.25 petrol. Was running fine last week. Left for a few days then drove yesterday. On slowing to stop at junctions, truck is stalling. Will restart instantly on the key.

I changed the fan belt late last week, and ran with it for approx 20 miles. I’ve checked tension, and it seems ok - certainly not slipping. (Is it possible to have it too tight, causing binding on the starter/alternator?)

Other possible symptom - getting some exhaust popping on decel. Took the top off the carb last night (Weber), cleaned float bowl, the two brass fittings at the bottom of the float bowl and idle jet. Float bowl had a lot of sediment, which I removed.

Thoughts very much appreciated, this is very frustrating!

Cheers

Have you knocked the tick over stop?

does it tick over normally after choke has been pushed in?

Popping in the exhaust is fine. It’s just unburnt fuel getting hot in the pipe

Thanks Phill. I assume the tickover stop is the part on the throttle linkage/choke that is set with the brass screw on the side of the carb?
It starts fine from cold without choke, idles ok. I’ve turned the idle up a bit this morning to try and stop the revs dropping so much on decel and then stalling.
So don’t think it’s the tickover.

Firstly, you need a decent fuel filter before the carb. Secondly, it sounds like an air/fuel mixture problem. Mines a zenith so easy to adjust the air mixture screw. It's been a long time since I had a Webber so I don't know how to adjust those, Google or YouTube should give pointers though. I doubt if the stalling problem is anything to do with the alternator if it starts easily after stalling.

Col

sounds like it could be a fuelling issue.. webers are fairly simple to set up and always worth doing at the hint of a problem then you've ruled it out if your problem persists

Colthebrummie said:

Firstly, you need a decent fuel filter before the carb. Secondly, it sounds like an air/fuel mixture problem. Mines a zenith so easy to adjust the air mixture screw. It's been a long time since I had a Webber so I don't know how to adjust those, Google or YouTube should give pointers though. I doubt if the stalling problem is anything to do with the alternator if it starts easily after stalling.

Col

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Thanks Col. I recently changed the fuel filter so that should be doing it’s job.
I’ve just dug up the Weber instructions do have the info on how to set the mixture and idle. I’ll definitely try that.
I’m wondering if I might have a membrane gone in the carb - I can hear a sucking/hissing if I run the engine at idle and then open the throttle by hand while standing by the engine.
Should fuel drip from the vacuum advance port if the tube is uncoupled? I would have thought not.

Speedy6111 said:

Thanks Col. I recently changed the fuel filter so that should be doing it’s job.
I’ve just dug up the Weber instructions do have the info on how to set the mixture and idle. I’ll definitely try that.
I’m wondering if I might have a membrane gone in the carb - I can hear a sucking/hissing if I run the engine at idle and then open the throttle by hand while standing by the engine.
Should fuel drip from the vacuum advance port if the tube is uncoupled? I would have thought not.

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A sucking noise could be the air filter isn't attached properly of a poor gasket between the carb and manifold. I don't think fuel should be dripping from the vac advance pipe, there should be negative pressure there. One of the reasons I don't li,e webbers is that they seem overly complicated to me, many disagree. What's it like on tickover, does it idle ok?

Col

i've had a weber make odd noises, drip etc.. it needed a really good clean and new seals

Ok. Had the carb body off - nuts to manifold were barely finger tight. Also undid and checked the pump spring/ diagphram.
Reassembled. It’s about 50% better on test driving. Still feel like it wants to stumble and stall on decel from 5-0 mph, but doesn’t always do so.
Interesting new finding. Standing on the brake pedal at idle I can hear the revs drop and the engine begin to run less well. There is a vacuum line from top of manifold to the brake booster, so I suppose this could be leaking, weakening the mixture? Replacing that hose is easy enough.
@kermit_rr - thanks for the input. I’ve found a service kit online so perhaps should just do that as well anyway.

That sounds to me like it's running quite lean

Is there any chance there's a vacuum leak in the brake servo?

kermit_rr said:

Is there any chance there's a vacuum leak in the brake servo?

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How would I diagnose that? It’s possible. Very occasionally I find I need a second pump on the brake pedal after which it bites sooner.

Running lean - perhaps. I did richen the mixture a bit but perhaps I’ll do it a bit more and then back off the idle.

on a weber, you set the mixture by backing off the idle completely or until it almost stalls, then adjust the mixture for best running, then back off slightly. this must be between 1-2 turns out. Then set your revs.
you should probably do this with the brake hose disconnected and the nipple blocked off on the carb - to rule out any servo leak
you should normally get 3 presses of the brake pedal from a healthy servo and it should hold vacuum for a decent amount of time