[craigtempo]

Ok the first 2 cylinders on my sierra lwb aren't fireing on idle ... once revving all good ...I've checked plugs ... leads .... cap and rotor button all seem fine ... oil clear as is coolant!going for tune up tomorrow hopefully they can sought out the problem... any ideas ?

[Brenno]

Pull the plugs and see if it has spark at idle? Check spark plug gap and re-gap if necessary.

You could also check this using a timing light over the suspected plug leads at idle. If there is no spark to those leads at idle then trace the system back (plug, lead, dizzy, coil etc).

[bumstein]

If there's spark there make sure there isn't a vac leak. I had the same problem perplexing me for ages before I realised that a plug had fallen off a vac port on the manifold