Timing / Idle Mixture / Vacuum question

[74 C-10 Shorty]

At full throttle cable should be all the way out.

[bswilson80]

OK, so I replaced the vac advance with a non-adjustable one and I get 36 degrees at idle. The detent cable is maxed out at WOT and the vacuum modulator is getting full manifold vacuum, but when I adjust it (got up to 4 turns counter-clockwise at half turn intervals) the gear shifts don't seem any shorter or softer, so I'm guessing that this went bad during all of the turning of the vacuum advance and whatnot. The truck shifted great before all of this, so hopefully I just blew the diaphragm, I get some white smoke upon cold-start, but I didnt see any ATF in the vac mod line.

Does timing advance affect gear shifting at all? ie the more advance the later the shift? This is the only other thing that I can think is messing up my shifts, and now the clutch is really starting to smell, even after just one short run, so I need to figure this out before I have a huge problem on my hands.

[454Man]

36 degrees at idle is way to much. From your vac can all you need is 8-10 degrees this is my setup tried and true from Dave ray aka the ignition man. 375/ 41 center plates and weights from the distributor in an 84 suburban. They provide 21 degrees of mechanical plus 12 initial then 8 of vac. That is all you need. Nothing more or less.

[rich weyand]

You want 36* of advance at 3000 rpm with the vacuum advance disconnected.  For stock distributors, this should 16* at idle with the vacuum advance disconnected.

[Irish_Alley]

rich why you have to be so confusing lol i was confused about all this stuff from the get go. but why not just say 16° at idle with the Vacuum advance disconnected

[rich weyand]

Because it depends on the distributor advance setup.  What you want is 36* when the advance comes in.  The idle setting depends on the distributor.  Stock distributor, sure, then you want 16* at idle.  But if you have, say, 24* advance in the distributor, then you only want 12* at idle.  You only need to check it at rpm once to see what kind of distributor you have, then you can just set the timing at idle once you know what the subtraction is.

[Irish_Alley]

so there is no way to check what type of distributor you have unless you "zero" it out, then run to 3k then see what your advance timing is at then minus that from your "zero"

[454Man]

That's the best way because who knows what's been changed over the years:-)  better than taking a wild guess and detonating a good engine... Or just look up ignition man and follow his past threads...

[454Man]

Find the lightest advance springs and put them in (only to find out how much advance you have) then take them out. You should only have to rev up to 2k max probably Less

[bswilson80]

36 degrees at idle is way to much. From your vac can all you need is 8-10 degrees this is my setup tried and true from Dave ray aka the ignition man. 375/ 41 center plates and weights from the distributor in an 84 suburban. They provide 21 degrees of mechanical plus 12 initial then 8 of vac. That is all you need. Nothing more or less.

What size springs do you run on those weights, and is there a vac I can buy that only gives 8? Or do I have to modify one?

[bswilson80]

My apologies, I still have my good initial timing 12 at idle and 36 all in before 3000rpm. I just can't find a vac advance that gives me less than a ton of advance. Any advice on finding one would be much appreciated.

My bigger concern now is the hard & late shifts, is it just a coincidence that these started now, or is there a connection between advance and shift times?

[74 C-10 Shorty]

Here's a chart with numbers for vacuum advance canisters, maybe this will help..

http://www.lbfun.com/warehouse/tech_info/timing%20&%20vacuum%20advance/Vaacuum_Advance_Specs.pdf

[bswilson80]

Here's a chart with numbers for vacuum advance canisters, maybe this will help..

http://www.lbfun.com/warehouse/tech_info/timing%20&%20vacuum%20advance/Vaacuum_Advance_Specs.pdf

Awesome, thank you.

[454Man]

36 degrees at idle is way to much. From your vac can all you need is 8-10 degrees this is my setup tried and true from Dave ray aka the ignition man. 375/ 41 center plates and weights from the distributor in an 84 suburban. They provide 21 degrees of mechanical plus 12 initial then 8 of vac. That is all you need. Nothing more or less.

What size springs do you run on those weights, and is there a vac I can buy that only gives 8? Or do I have to modify one?

I run the medium springs in the moroso kit. I bought the accel vac can because it pulls down to 5hg.

[454Man]

Vac advance limit plate

 http://m.summitracing.com/parts/crn-99619-1