I've been dissatisfied with the quality of my idle--it seems a little rougher than it ought to be to me. Tried various things, then hooked up a vacuum guage this afternoon. At 700rpm idle, the needle fluctuates very quickly between 18" and 20". When I increase the rpms it steadies out.

I'm thinking worn valve guides, stuck valve or some kind of ignition miss. I've checked the plugs and wires, and they seem OK (wires are brand new GM).

Whatchall think? What's the best way to figure out what I'm dealing with? And if it's worn guides, is that something I need to get taken care of or can I just leave it til when I get the engine overhauled in another 20 or 30K miles?

JB

 

How ‘bout misfires JB. Old Smokey Yunick said ½” separation minimum. The modern high voltage ignitions are capable of causing induced currents in adjacent wires. He mentioned that many piston failures were blamed for something else when the actual cause was a high voltage induced misfire. With a 18436572 firing order extra concern is needed for #5 and #7. Use separate routing and cross at 90* if needed.
BTW headers can burn up a new set of wires in no time. You may need to replace and try some wire shielding – insulating socks. :rolleyes:

 

JB, did you put those braided wire covers on your car? If you have, it might reduce your chance of crossfire. Rapid fluctuations in vacuum at idle usually do indicate one or more worn valve guides. My old 350 did exactly the same thing, and when I pulled it down I found pieces of the old stem seals under the valve covers and in the pan.
Replacing the stem seals probably won't fix the problem but it might help just a little bit.
The only way to really find out is to run a leak down test on each cylinder.
I know your car is a very original BB car, and probably has never been overhauled, it might be time for one, if you want it to run like a new one.
The leak down test will tell you if it is just a valve guide and if there is a problem with worn rings. If it is just valve guides, you can probably resurrect it with a valve job. Do you know of a good machine shop in the San Marcos/San Antonio/Austin area?

 

Cardo--no headers, and they are the braided stainless covered wires, and they're all routed through the stock guides, so I think probably I'm OK there.

Steve--I think I will see about getting a leak down test done. It's never been overhauled that I know of, and while it's only got 51K miles on it, everything's 33 years old and I don't know how hard it's been driven in the past. There are a number of local machine shops, and I've used one of them a few times but not for anything major. I know Will Shepard (Flynhi) has a machine shop he uses and is happy with in Austin, so I may talk to him about it. If it is valve guides, am I damaging anything by continuing to drive in without a valve job, or is it just a matter of living with the bumpy idle?

JB

 

The BB seals are "umbrella" type... not o-rings.

The seals go bad over time for no other reason than age.

Worn valves/guides will cause new seals to fail prematurely due to th added lateral stress.

Hardened seats are an "over-sell" but do not cause any problems unless the machinist cuts into the water jacket to install them... it happens.... been doing heads professionally for 30 years and never saw a cause & effect need to install them... I consider the unleaded fuel/hardened seats a "witch-hunt".. based on direct personal experience in the business. It was used as an argument against unleaded fuel when the industry switched over in the early seventies.
Chewed up seats & valves are due to lack of proper engine care... cheap fuel, dirt, etc. Occasionally I have run across damage done directly by a machine shop that did sloppy work... e.g.- sunken valves

3 angle valve job...

The valve face is cut at one angle... say 45*. The seat in the head is also cut 45* to match the valve face. (I don't cut 45/46 interference angles... consider them Bubba)

Just below that, to allow a good transition into the port, a 60* angle can be cut in the head. On the top side, a 15* cut can be made to allow a smootrher transition of fuel flow as well.

The angles can vary, depending on appliction... some use a 30* primary angle instead of a 45.

To take it all one step further, some people will blend the angles into one sweeping, smooth port while retaining the 45* interface between the valve & head.