miss at idle

[TrollFromDownBelow]

I've got a slight intermittant (i.e. not on a regular rythmic basis) miss at idle...seems to have gotten worse over the past month?

stats:
- just reset dwell, at 43 degrees (was at 50, thought that this may have been the source)
- timing at 29-30 all in with vacuum off
- 7 solid lifters adjusted at .006, one hydraulic set at one turn in from zero lash. I've been checking maniacally since Colin and I dropped in a 'slightly used engine' and valves have always been at the same setting..no changes in valve lash over the last 2,000 miles or so.
- I do have a hole in on the exhaust tube that leads into the muffler... but this doesn't sound like an exhaust leak.

back in the day, this was a classic sign that a car needed a valve job..that is my fear..put a lot of time making it road worthy, not looking forward to pulling the engine and replacing the heads.

What is everyone elses opinion? If this is a sign that it needs a valve job, if so, can I run it as-is without doing damage to the engine?

[hambone]

You can't assume a slight miss means a valve job...could be a million things, from a bad sparkplug onward.
A compression test will tell you. What about leakdown? Tho I've never done one of those.

[TrollFromDownBelow]

Alas - the most suspect cylinder ... #2 I can't do a compression or a leak down test - it has a plug insert, and the plug is married to it.

Plugs are all good - just put them in when we swapped engines.

But I was wondering ... I remember Colin posted once that the pushrods for hydraulic lifters are made out of different metal than solids (IIR steel for hydraulic, aluminum for solid). All mine are aluminum. Seems to be missing more since a changed the valve lash on the hydraulic from 1/2 turn past no clearance, to one full turn. I wonder if the expansion rates are different enough (assuming aluminum expands more than steel) that I should have a looser tolerance...anyone else have thoughts on this?

[vwlover77]

The hydraulic lifter should deal with the expansion no problem. But, maybe set it back to 1/2 turn to eliminate that variable. My question is, what cam is in your engine? One designed for solid lifters or hydraulic?

My engine tends to miss at idle if the mixture is too lean. Just for fun, try turning the mixture adjustment screw on the AFM slowly clockwise (richer)while listening to the idle. Keep track of how many turns you make so you can set it back where it was if needed.

[TrollFromDownBelow]

My assumption is that the cam was designed for solids... it's a '76 block, all of the other lifters are solid, and it doesn't look like the case has ever been split. Me thinks the one hydraulic is a 'hippie fix' ... also the one hydraulic lifter is in the same cylinder that has the insert.

I will give the AFM adjustment a shot ... if anything it may be too rich - Colin set it up originally when we dropped the engine in last summer, and I've found and fixed a few vaccuum leaks since then.