1850 poor idling

[Dunhookin]

My late 1850HL auto has always been fond of it's workshop time, needing plenty of regular TLC over the years. More recently it's been running roughly and down on compression and power. To deal with that, the head has been off for extensive valve seat work and to deal with a 5 thou warp.

It's back together and sounds fine, but it idles badly when cold, frequently cutting out. When it's idling, applying the brakes makes it cut out instantly - like flicking a switch. Can anyone please offer any advice/suggestions? Thank you

[Carledo]

Simple, the brake servo is shot!

Steve

[cleverusername]

Simple, the brake servo is shot!

Steve

Vacuum leak? Why does pressing the brake pedal open up the leak more?

[matt of the vivas]

Try blocking the vacumn line to the servo off first, isolate it before you condemn it.

[GrahamFountain]

Try blocking the vacumn line to the servo off first, isolate it before you condemn it.

I assume you mean block the vacuum line to the inlet manifold and see if that stops the rough idle. If he blocks the feed to the servo and leaves the feed to the manifold open, it may not idle well at all.

There's also that old trick (with the vacuum pipe connected) of pressing on the pedal with the engine not running, then starting the engine to see if the pedal falls as the vacuum gets to the servo. If the pedal don't move, it's a good indicator that the servo ain't working.

Graham

[Carledo]

You can check the diagnosis by clamping the servo hose off with a brake hose clamp (or a mole grip and a bit of cardboard) and see if that fixes the running fault. But i'm pretty certain i'm right!

You've almost certainly broken the bakelite ring in the back of the servo and vacuum is leaking from the car cabin, letting more air into the manifold and ruining the mixture. Pressing the pedal just makes it worse.

Even leaking like a sieve, the servo will supply a modicum of assistance, all the ones i've had fail have been the same.

Steve