S Series Vacuum Pipe

[Simon]

At present my 1.6 S series Maestro (unleaded spec) runs like a dog, it's all over the place. When I pull off the vacuum pipe from the inlet manifold to the plenum chamber vac switch, the car runs superbly, but cold start fast idles at 1750 rpm until warmed up, and when idling "normally" the ECU starts to govern the idling speed at about 900rpm and I can't adjust it down on the carb because the screw runs out of travel.

Any ideas?

[e692wtt]

Undo the banjo for the servo, and plug up the hole in the manifold. That will eliminate any air leaks from the servo into the inlet manifold (and also any excess leaks via the thermac valve). Then see how the engine runs.

I don't think a vacuum leak in one of the small bore pipes would cause the revs to be raised so high...

[G Force]

Hi Simon, sounds like the engine is running too rich.

Pulling the vacuum pipe off does two things, it allows extra air in to weaken the mixture and also disconnects the fuel supply from a possible failed vac switch.

Pull off the vacuum switch from the side of the carb and suck on the vacuum connection, if you can suck air through it, the vac switch needs renewing

Once you get a new switch then reset the idle and fast idle.

Cheers gary :)

[Terry]

I had a simuler thing on my Monty. The garage traced the problem to a faulty vacuum switch on the side of the carb. This was changed and the problem went away.

[Simon]

I'll check the vac switch, the car is certaily running rich. The carb was brand new, fitted earlier this year, but it didn't have a vac switch on it, so I had to reuse my old one.

Thanks for the replies, everyone:)

[Simon]

Difficult to suck through (although just possible I think), I can certainly blow through it however!

With the manifold vac pipe disconnected, the car drives as if it's got a turbo under the bonnet!

[G Force]

Hi sounds like the culprit, try get hold of an LZX 2292 as this is the modified valve.

Incidently if you plug the vacuum pipe up that connects to the vacuum switch with something suitable the car should drive normally once the engine is warmed up.

Gary :)

[e692wtt]

... manifold vac pipe ...

Is this the 'thin' (leads from the carb to the vacuum switch and the ignition ECU) vacuum piping or the 'thick' (leads to the thermac switch on the plenum chamber) vacuum piping?

A 'shot' vacuum switch causes rough running (rich mixture - the car needs more throttle for a given 'go', it's pretty obvious that something is amiss), and regular stalling at idle (great for winding the kids up when you leave them in the car with the engine running - then it stalls and you ask them what they did :laugh: ), but I cannot recall a raised idle speed from this. If you can 'blow' through your vacuum switch, that would indicate that it is 'shot' though.

[Simon]

Getting one now, thanks to Maria! I thought about a temporary cure of using a U tube and blocking the small bore vac tube tube, however I'll wait until I've popped on the new vac switch and can then re-tune the engine, resetting everything to sensible parameters.

Rich, the raised idling speed (especially fast idle) comes about when the manifold pipe is removed, weakening the mixture to near normal levels. I set everything up with a dodgy vac switch so my car ran very rich.