Cutting out under heavy braking

[GT Matt]

Right then - I'm scratching my head with this one.

1.8 EX in 1990 GTII with brand new (well 1 year old and 4000 miles old) Weber carb conversion.

Idles fine, CO bang on, pulls like a train, no flat spot whatsover, cold start bob on, fast idle fine, starts within a single engine revolution, idle speed constant - all text book.

Except.......................

After driving at any speed (noticeable worse at higher speeds), if I have to stop the car suddenly, the car tries to cut out and the revs drop off. However, when it does occasionally stall, it restarts immediately and drives absolutely fine as though nothing ever happened.

Pull up steadily and it is always fine. Also, if I know I need to stop quickly, if I blip the throttle a couple of time just before I come to a standstill, it is also fine.

Work done (by previous owner):

New fuel pump, replacement fuel reservoir (fitted between pump and carb), new fuel filters (all running clear)

Work done by me:

Reset float height and cleaned out carb, checked for air leaks (none found), replaced vacuum pipes, fitted one way valve between carb and distributor, shaved off the end of the throttle cable that was fouling on the car diagphragm (keeping the butterfly off it's rest position).



I really am banging my heaqd against a brick wall. It isn't too much of a problem, it's just annoying and occasionally inconvenient.

Anyone else experienced this?
Anyone got any ideas?

Cheers

GT Matt

[Nate]

it's gonna be due to something being thrown forwards when you honk on the anchors. does it do is more when the fuel tank is leaning toward the empty side of things? and less so with a full tank?

[GT Matt]

Hi Nate

It's got to be due to something affected by weight transfer but the big question is what?

I had thought about the fuel tank but, to answer your question, the most I've ever had in the fuel tank is 2/3 full and the fault has occured at any fuel level.

Even at 10 mph, if I jam the brakes on, it will try and do it. Something is moving, but what?

Matt

[filmidget]

Laaking servo/hoses?

[GTI]

Could this be clutch related? Is it disengaging properly?

[testgerbil]

Hello all, I am the new owner of this car. In an attempt to solve this issue I've had the carb to pieces and cleaned everything up, also had the timing and mixture tuned professionally by a Dub specialist. The car is still suffering from a dodgy idle, but I'm not sure if the "stalling under breaking" is a red herring.

Around town, driving at 30mph the idle is fine and even if I have to pull up short to avoid someone/something the idle drops steadily and stays at 850revs. The problem, it seems, occurs when driving at motorway speeds.

When I'm trundling along at 70mph, I dip the clutch and the idle drops steadily to about 400rpm/sometimes even lower; and the car sounds really rough. This happens if I take the car out of gear and release the clutch too, so I don't think it's a clutch problem. This is without touching the brakes; I think the stalling issue with braking is maybe just the extra strain put on the engine with the servo assist.

THe only way I can stop the car from stalling when I'm pulling up from travelling over 50mph is if I pull the choke out when I'm slowing down. This way the revs don't drop low enough that the car stalls. If I come to a complete standstill with the choke out, the car runs very rough at low revs for a couple of seconds, then gradually the revs will pick up until it's running smoothly again at about 2000 revs (i.e. because the choke is out) If I then put the choke back in, the revs return to normal.

Has anyone got any ideas? I am wondering if something throttle related isn't returning quickly enough to idle setting, so either too much air is being allowed into the engine for a couple of seconds. Or, if there's some very large sized crap in the fuel tank which is being drawn nearer the intake when travelling at higher speeds, but I don't know if this is a fuel starvation problem, or pulling the choke out when the revs drop probably wouldn't help as much.

I'm at a loss; is there anyone in Oxfordshire who wouldn't mind helping me out on this one?

[Birdy]

Have you tried changing the gasket between the carb and the inlet manifold itself??

Sounds like it could be the carb 'rocking' under the breaking forces and if there is a slight split in the rubber gasket it will be letting in unmetered air - hence the engine revs dropping and then stalling under higher breaking loads

GSF car parts - 18310 FLANGE-CARB>MANIFOLD SC 1.8 8/83 > 19.50 (+vat of course!)

The easiest way to tell if it is the gasket is when the ending is idling, push/pull the carb in different directions and listen out for the revs dropping

I would be almost tempted to put money on this as your problem - i had the exact same thing happen with my old GTII when i ran it with a weber carb ;)

[eamon1.3]

I have the same problem with my injected one

[duggers]

It sound like air or vacuum problem. Check for gasket condition as above also check any vacuum pipe connections. the fact that it will run if richened up (choke on) suggests a weak mixture at idle. This happening when hot also points in that direction.