Vacumn leak fun and games.

[LL]

Hi all,

My mechanic friend is having problems tracking down the source of a vacumn leak on our S4 with dual webers. Just had the head rebuilt to fix the guides and the car can be tuned and run just fine with the brake servo and headlamp pods disconnected. The minute they are reconnected the mixture goes all over the place.

Suggestions on where to look next? He's apparently fixed a leak at the servo but is having trouble tracking it at the headlamps. Appreciate any sage advice!

Cheers,
Neil

[iain.hamlton]

Neil, I have had this game too.

First figure out if it is lights or the brakes by seeing which one causes the problem.

If it is the lights, try the following. At the y-piece in front of the cross-member/resevoir, connect the enginine vacuum through to the lights directly taking the resevoir out of circuit. Problem goes away? crossmember....Easy to find not easy to fix. There may be a hole inside one of the chassis uprights that support the front suspension. Trying to repair the crossmember is notorious.

Connect to the just the crossmember, lights out of circuit. Problem goes away? Lights somewhere. You then need to work through the pipes dash switch, pods etc. Not much fun, but fixable.

hope this helps,

Iain

[Elanintheforest]

Hi Neil
The usual suspect is the 'T' piece that connects the input pipe (from Manifold) to the vacuum tank (chassis) to the output pipe (to the pod or pods). If it's not the T piece try the conectors between pipes and manifold / pods. If it's not there check the pods and if it's not there....new chassis!
Your mechanic should have a vacuum pump to test these things, or they are available from good tool stores and ebay. With these, you can isolate each component, create a vacuum and see how long it lasts.
Another way is to put soapy water around a joint, and blow into the tube and see where the bubbles are! This can get messy...use a vacuum tester...and hope you find the problem before you get to the chassis tank.
Good luck...Mark

[LL]

Thanks for the info so far. The car has a Spyder chassis, does that make any difference?

[Emma-Knight]

Spyder chassis do have two drainage plugs in the crossmember undersides - they have to be closed too.

[Elanintheforest]

And most unlikely to be chassis tank.
As the head has been off, I'll bet it's just the pipe from manifold to chassis, or the 'T' piece that's been disturbed and is no longer sealing. The rubber and plastic bits get hard and if undisturbed may seal OK, but a quick fix may be to try some silicone sealent around the joints....and if it works order up some new rubbery bits.
Cheers...Mark

[steveww]

I have found that the leak is usually the haedlamp vacuum switch on the dash. The rubber in these shrinks with age and thus one leak.

[steveww]

I have found that the leak is usually the haedlamp vacuum switch on the dash. The rubber in these shrinks with age and thus one leak.

[ppnelan]

I assume you are waiting until the vacuum tank is 'evacuated' before trying to set the carbs?

Some cars have the headlight vacuum connection on the front carb manifiold, and the brake servo connection on the rear one. This makes it easier to decide which is causing the problem.

It may be worth checking the one-way valve is sealing correctly too, as this could mess things up - mainly the headlight operation.

Matthew

[LL]

Hi all,

Thanks, found the source of the leak it was the switch on the dash.

Got another problem now maybe you can help with. Car will start and run ok, but idle is slightly rough and on just off idle constant speed I'm getting popping/backfiring through the carbs. Have tried tuning with a colourtune and even though on straight idle I have a nice blue flame just off idle I'm getting occasional leaning/enrichening of the mixture - which it is I can tell but I'm opting for leaning.

I am thinking something in the Webers is not right. They've been cleaned through with brake cleaner so I'm pretty sure there is no blockage. I've read on the net that it might be related to the aux venturi springs weakening over time. Can anyone suggest anything or should I just take it to a Weber expert with a rolling road?

Cheers,
Neil