Help - Hunting Idle after spark plug change?

[B-Reight]

I've just stuck in a new set of B7ECS plugs and the engine now hunts at idle from 700 to 1000ish rpm. Previously it was solid at 950rpm.

The gaps are the same at 0.8mm/32thou and i haven't swapped out anything else. I stuck the vac gauge on the manifold, previously it held a solid 15 "Hg with a sway of half an inch either way. Now the vac rythmically swings from 10 to 15 "Hg corresponding with the idle change.

The idle speed and vacuum will stabilise if i set the advance to 20 BTDC from the previous 16 but 20degrees sounds like far too much advance to me.

[jeff m]

Spray rubber hoses with carb cleaner, WD40 (anything) to check for air leak.

ETA the vac gauge is only telling you something you already know (by the revs)

Edited by jeff m on Sunday 6th February 14:25

[Wedg1e]

Does sound like you've pulled something electrical off (throttle pot wires?) or an air leak. Vac advance pipe off the distributor connected to the plenum OK? Non-return valve the right way round (if you have one)...?

[mikesp]

In the past I've found obvious things like leads on wrong or internal problem on lead/cap when they were pulled off, a faulty new plug. lead routed differently and earthing out, plus others already mentioned like vacuum pipe etc are the place to start. Seems unlikely that it would be something bigger after such an apparently simple job.

[Campbell]

Is your car a hot wire or a flaper..
20 BTDC sounds way to much timming, if the car start can you get the revs held at say 2000 rpm and see what the timming is with out the vac pipe connected, from memory i think these engines dont run much more than 36 degees at 4000 rpm, try setting the car up with 16 BTDC and check all the air pipes around the plenum and to the brake servo.
I would take out all the plugs and refit them just incase they are not seated correctly or cross threaded also the WD40 may help too.
Good luck

Cambelt