Vacuum Advance Question
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From: Wyoming
Vacuum Advance Question
When timing an engine, the hose to the vacuum advance should be removed and plugged off. When you are done timing the engine, you put the hose back on and the timing advances somewhat.
My question is this - I don't have any vacuum on the hose at idle. I timed my '81 460 last night, and it was 10 degrees with or without the hose on. I put a vacuum gauge on the hose, and it maybe moved up to 1".
Most of the bench racers at work say I should have alot more vacuum, but one of them says "no, the distributor advance should be about zero until you rev the engine." I've got the right-hand carb port hooked up to the advance canister.
I think it should have more vacuum at idle - what's the answer?
My question is this - I don't have any vacuum on the hose at idle. I timed my '81 460 last night, and it was 10 degrees with or without the hose on. I put a vacuum gauge on the hose, and it maybe moved up to 1".
Most of the bench racers at work say I should have alot more vacuum, but one of them says "no, the distributor advance should be about zero until you rev the engine." I've got the right-hand carb port hooked up to the advance canister.
I think it should have more vacuum at idle - what's the answer?
Senior User
Joined: Sep 2002
Posts: 356
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From: Southwest VA
well the vacuum that controls the vacuum advance is supposed to be ported vacuum (ont eh carb/throttleplate)which will not have vacuum at idle..you'll only get vacuum when you open up your throttle
On the other hand you will have vacuum from the manifold (will be a "tree" with a bunch of vacuum lines on it) that's where you would measure your vacuum at idle.
John
On the other hand you will have vacuum from the manifold (will be a "tree" with a bunch of vacuum lines on it) that's where you would measure your vacuum at idle.
John
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Joined: Sep 2003
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From: lino lakes, mn
its an emmisions thing no advance at idle reduces hydro carbon emissions at idle the two types of vacum are maifold a vacum sourcetaken before the throttle plates and ported /venturi vacum thaken after the throttle plate check your timing at a hig idle setting say 1500 rpm with and without the vacum line attached most engine emission stickers (the one on the valve cover will give a rpm setting for checking timing thisw is usually when all the mechanical advance is"in" and it is usually at a point when you would be ported vacum
More Turbo
Joined: Mar 2002
Posts: 508
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From: Easthampton, Ma.
You have 2 types of timing advance, mechanical (centrifugal) and vacuum.
When the motor is idleing, no advance is necessary other than the 10 degrees static timing you set with a light.
Think of why you are setting this.
You are allowing the distributor to send a spark to the plugs at 10 degrees of crankshaft rotation BEFORE the compressed air/fuel mixture on top of the piston reaches top dead center (TDC)
This allows the spark to get to the plug and ignite the fuel mixture precisely at TDC
As the motor goes faster, the spark needs to get there quicker, thus the mechanical and vacuum advance.
Your friend that said "no vacuum at idle" is correct and you do have the correct port on the carburetor.
Mechanical advance will advance the timing by the engine speed.
Vacuum advance will advance and retard the timing depending on engine vacuum (engine vacuum varies by engine load-throttle position)
Good luck!
When the motor is idleing, no advance is necessary other than the 10 degrees static timing you set with a light.
Think of why you are setting this.
You are allowing the distributor to send a spark to the plugs at 10 degrees of crankshaft rotation BEFORE the compressed air/fuel mixture on top of the piston reaches top dead center (TDC)
This allows the spark to get to the plug and ignite the fuel mixture precisely at TDC
As the motor goes faster, the spark needs to get there quicker, thus the mechanical and vacuum advance.
Your friend that said "no vacuum at idle" is correct and you do have the correct port on the carburetor.
Mechanical advance will advance the timing by the engine speed.
Vacuum advance will advance and retard the timing depending on engine vacuum (engine vacuum varies by engine load-throttle position)
Good luck!
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