Timing with a Vac Gauge
Hotshot
Joined: Dec 2003
Posts: 18,423
Likes: 9
From: iowa
only problem with all this is everyone is forgetting to add the vac advance in when figuring where to set base timing to get total timing. I am going out on a limb here and say most of you are running 42+ total running down the road. it's not optimal for power or economy when your that far over what the combustion chambers like for total timing.
Proper way is figure out how much vac advance your dist has, how much mechanical the bushings are set for, add them together and subtract THAT number from your total desirable.
So if you have a 21deg mech, and a 8deg vac and you ONLY subtract the mech from 36 you end up at 15 base. BUT in reality your total timing is now 44 deg which is WAY to much.
now if your engine doesn't like to idle with 7 deg base timing (the amount you would be at to add 8 and 21 total advance to get 36) then you need to reduce your advances. Drop the mech down to a 14 and leave the 8 vac and you end up at needing 13 deg base timing.
you just can't totally leave out one source of advance in the calculations.
Proper way is figure out how much vac advance your dist has, how much mechanical the bushings are set for, add them together and subtract THAT number from your total desirable.
So if you have a 21deg mech, and a 8deg vac and you ONLY subtract the mech from 36 you end up at 15 base. BUT in reality your total timing is now 44 deg which is WAY to much.
now if your engine doesn't like to idle with 7 deg base timing (the amount you would be at to add 8 and 21 total advance to get 36) then you need to reduce your advances. Drop the mech down to a 14 and leave the 8 vac and you end up at needing 13 deg base timing.
you just can't totally leave out one source of advance in the calculations.
Post Fiend
Joined: Apr 2008
Posts: 8,261
Likes: 15
From: Big Sky Country
Vacuum advance isn't factored into total advance, total advance is initial advance + mechanical advance, as the vacuum signal at both idle and WOT is zero, so it's not part of the equation when setting total advance.
Vacuum advance merely improves part-throttle driveability, its vacuum signal is strongest just off-idle, weakest at WOT, think of it as an on/off 'switch', ON when you need it, OFF when you don't.
During light load, part-throttle conditions vacuum advance kicks in an additional 8...10...12 degrees of advance, exactly when the engine needs it, as it's not seeing enough rpm's to reach total mechanical advance.
When you get into the throttle more the rpm's increase and the vacuum signal drops off, the 'switch' is turned OFF, if you will.
Vacuum advance should have no bearing on where you set initial/mechanical advance.
Vacuum advance merely improves part-throttle driveability, its vacuum signal is strongest just off-idle, weakest at WOT, think of it as an on/off 'switch', ON when you need it, OFF when you don't.
During light load, part-throttle conditions vacuum advance kicks in an additional 8...10...12 degrees of advance, exactly when the engine needs it, as it's not seeing enough rpm's to reach total mechanical advance.
When you get into the throttle more the rpm's increase and the vacuum signal drops off, the 'switch' is turned OFF, if you will.
Vacuum advance should have no bearing on where you set initial/mechanical advance.
Laughing Gas
Joined: Jan 2013
Posts: 1,015
Likes: 4
From: Gulf Coast, MS
If you ask 20 people about timing you will more than likely get 20 different answers. One thing that I found consistent was you do want the 34-36 total timing. What I did was hook up the timing light and set my timing to 10 degrees (nice even number) then revved it to see how much mechanical advance I had. I saw that I had 20degrees so I set my timing to 34 which left me with a base of 14. Runs awesome now. I ended up with about 16in of vacumn. If I set the timing using the guage to get 20-21in (highest reading) that resulted in a base of 20 something degrees and over 40 total.. Was not good.
Senior User
Joined: Mar 2004
Posts: 280
Likes: 2
From: Alabama
Well I had my wife rev it up, but yeah, I set the total timing (at 34*) while the mechanical advance was all in. So when I set it at 10* base, revved it up until all mechanical was in (10+20=30*) then advanced it to 34* while using a timing light.
Laughing Gas
Joined: Jan 2013
Posts: 1,015
Likes: 4
From: Gulf Coast, MS
LOL yep still confused but I am tired. Been working on the new motor for mine so I am wiped out. I am gonna go to that link that someone posted and read up on that and see what I can learn from there. I guess I am getting lost between the mechanical advance and then the vacuum advance and how it ties all together
Cargo Master
Joined: May 2009
Posts: 2,587
Likes: 47
From: Eagle River, AK
The vacuum advance is to assist in the advancement of the timing during the time between idle and the higher RPM's when the mechanical advance kicks in.
It took me a while to wrap my head around it too.
It took me a while to wrap my head around it too.
Post Fiend
Joined: Apr 2008
Posts: 8,261
Likes: 15
From: Big Sky Country
After that, vacuum advance can be dialed in for fine tuning, assuming you have an adjustable v/a canister, most oem replacements don't, they're fixed at a set 8-16 degree advance.
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