I am still having a problem with a rough idle on my 292.The motor is stock and last year I added the "B" manifold and later distributor as well as a 390 CFM Holley(newer 8007 model).Plugs are good and wires are new.Plugs not showing any abnormal colors.The motor runs great at any speed-except idle.Carb is propely adjusted for mixture and turning screws in cuts off fuel as it should when working correctly.I have readjusted timing backing down a bit from maximum vacuum using the gauge instead of a timing light.I have done a check for vacuum leak and found nothing.If I use a compression gauge what am I looking for as far as possible burnt valves?This motor was rebuilt many years ago and its possible hardened seats were NOT installed at the time and I am thinking maybe valves are starting to go.Any other suggestions?The rough idle has only started a couple of months ago-was not there when I changed manifold etc.

  

Where is the idle speed set?

Are you running ported vacuum to the advance?

 
 Steve Metzger       Tucson, Arizona

Steve,Idle is set just above stall speed in drive.Enough that there is a "clunk" when shifting into drive and car "creeps" ahead with brake released.I don't think its too slow idle speed.If I turn idle up the ported vacuum advance opening is exposed.The advance is hooked into the ported oulet so no  vacuum advance at idle speed.I tried unported and made no difference.I do have a 1 inch phenolic spacer under carb and heat crossover is blocked.The problem is worse at full operating temperature and its not above 75 ambient here now.Thanks

  

Hi, i am wondering if you should go over the valve adjustment, just in case theres some seat or lobe wear. let us know if you work it out, best regards bill.

  AussieBill            YYYY    Forever Y Block     YYYY

 Down Under, Australia

Thanks for the input Bill.I have redone the adjustment 3 times and I am confident that is not the problem.I have seen this type question posted every so often.Its much more noticeable at full operating temperature.As far as lobe wear or valve seat thats kind of my question.How can I diagnose that condition without teardown.I seem to recall compression readings can give an idea of valve condition.

  

Usually valves not seating show a much lower compression reading than rings, would you know what vacuum reading the engine shows when missing? it can be most accurate guide to engines condition. regards bill.

  AussieBill            YYYY    Forever Y Block     YYYY

 Down Under, Australia

Bill,At idle out of gear I have 19 inch vacuum with only a slight varience of less than 1/2 inch as it runs.This is NOT an indication of valve problems or cam lobe wear according to the diagrams and info supplied with the gauge that I purchased about 30 years ago.So where do I look for the cause?

  

Are you running a Pertronix ignitor or on the points now? I've been thinking that maybe its just not getting 100% fired.....the odd missing spark here and there. Nothing sounds too much amiss with the carb and valve settings.

How about the transition as you accelerate from stop? Smooth or hesitation in any way? The bird tranny always starts in second and it stumbled a bit for a while until I got the primary float level to where it liked it. I just had it a little too far on the lean side - which will cause the odd misfire too.

 
 Steve Metzger       Tucson, Arizona

Hello Steve! I have an accel unit in it.As far as stumble etc I have none once warmed up.As far as your thoughts about misfiring thats what it feels like.Doing a visual on the plugs doesn't show anything.Why would it only be a problem at idle and not under load when driving?Any thoughts on what to check out?As I stated prior the vacuum gauge shows only a slight variance in reading at idle.Thanks

  

Do you have a dwell / tach you use for tuning? What kind of dwell does the Accel chip apply to charge the coil at idle? You hook up the test lead just as though the coil was being operated by points.

I've got an Accel "HEI" four post operating module that I've done some tests on. HEI units don't use a ballast resistor - they lower the dwell time at low RPM to lower the heat load on the coil (which runs on full voltage).  The chip in the control module just keeps raising the dwell time as the rev's rise - Jeff Morgan (my favorite local mechanic) told me he has seen some of the HEI chips do 35° at higher revs - and as low as 12°-15° at idle. 

I wouldn't be surprised for Accel to have engineered their points replacement unit off the HEI chip (or similar). What sort of total coil / ballast resistance was recommended with your unit? Maybe the dwell time it is allowing (at idle) is too small compared to the resistance in the coil primary circuit (on the edge of too weak a spark) - but then as the rev's (and the programed dwell) go up, things begin to work better.  

 
 Steve Metzger       Tucson, Arizona