Hi all. I am trying to tweak the tuning of some mild mods to a 454 MAG with a Holly 800 and MSD.

I have MSD set at 10degrees BTDC (per Merc spec) but my idle (800rpm) vacumn is 15-16 lbs with a very slight and slow pulsing of about 1lb. What is considered "normal" idle vacumn for this motor?

I read that low vacumn can be due to "retarded timing" and a slow pulse can be caused by spark gap or idle mixture. My plugs are set at .050 - per MSD but is a vacumn pulse caused by too wide a gap or too narrow? Also, should I adjust timing to max vacumn or just set by the sawtooth gauge mounted on timing cover? When they say "low vacumn at idle can be caused by retarded timing", does that mean that I should make the timing say 12 degrees or say 8 degrees BTDC?? I know there are other things that can cause this but I think it is a matter of proper tuning. If I do advance the initial timing, I am a little concerned about total advance, I am using black bushings.

Do you still have the stock cam? I don't believe you should have more than 34* total advance. How much is your distributor adding? Stock is 24*, so 10* initial is all you would want in that case.

You want to set your total advance. It should be all in by 3500 rpm. 32 - 34 BTDC is good for a 454 on premium.

Is your timing set in the MSD distributor or Merc w/ the
ignition module ??

Timing is set by MSD box - replaced all the stock ignition stuff. MSD said spark gap could be up to .060 but I wonder if the slow pulse on vacuum gauge is from plug gap. Can't recall exactly what bushing and springs I used - it is in my notes - which I don't have in front of me now. It was either black or blue with full in by 3200. I only have access to 87 unless I use a pony. What do you guys think about setting initial using a vacuum guage?

Use total timing. Let the vacuum go where it will.

Do you have an MSD distributor also????? Whats the advance curve set at???

You could always change the advance amount so you could bump up initial timing.

BTW, 15-16 inches of vacuum is fine.