Timing question...hope this is the right place

[SouthernBrandt75]

So everything Ive read says set timing at 14 degrees with advance disconnected. Then it goes on to say something to the effect of after connecting advance line then it should be at 34 degrees? Im either confused or something isnt right lol. nothing changes when I connect my advance....please help. Thank you in advance.

Trey

[VileZambonie]

You haven't provided any information on what you are working on.

As far as base timing, is this a stock application? For no change with the vacuum advance connected, are you connected to full vacuum or ported vacuum? Not sure where you are getting your specs from but I think you are referring to off idle increased RPM.  and not simply connecting the vacuum hose. Nevertheless, we need to know what you are working on and if it is stock.

[75gmck25]

There are three aspects to the timing on your engine.

Base timing - the 14 degrees BTDC you measured with the vacuum port plugged
Mechanical advance - this timing advances as engine RPM advances.  For most stock engines, the distributor will advance the timing about 18-20 degrees by about 2500-3000 RPM.
Vacuum advance - when engine vacuum is applied to the port on the vacuum can on the distributor, it will advance the vacuum about 18-20 degrees.  Vacuum advance is mostly used to advance timing at cruise to improve fuel economy.  Your engine will run if vacuum advance does not work, but fuel mileage will drop off significantly.

Your carburetor has two vacuum ports. One is below the throttle plates and is connected to engine vacuum; the other port (ported vacuum) is above the plates, so that it only draws vacuum as the throttle plates open and are above idle.  If vacuum does not advance beyond 14 degrees when you reconnect the canister, then the line is connected to ported vacuum, or the canister is bad.  You need to manually apply vacuum with a vacuum pump, or connect to a full vacuum port before you measure.

Your total timing should be base+mechanical+vacuum, or no more than about 14+20+20 = 54.  However, you can't easily measure them all at once, since if RPM is high enough for mechanical to kick in, then vacuum will often be too low to bring in vacuum advance.   The only time you get them all working together is at a steady highway cruise.
Measure base timing- 14 degrees BTDC
At idle, pull vacuum on canister and measure base + vacuum - 32-34 degrees
Plug vacuum line and measure base + mechanical at 3000 RPM - 32-34 degrees

Bruce

[SouthernBrandt75]

You haven't provided any information on what you are working on.

As far as base timing, is this a stock application? For no change with the vacuum advance connected, are you connected to full vacuum or ported vacuum? Not sure where you are getting your specs from but I think you are referring to off idle increased RPM.  and not simply connecting the vacuum hose. Nevertheless, we need to know what you are working on and if it is stock.

Dude im an idiot lol. Its a 1986 C30 with the 454. Only thing not stock is holley carb. truck has about 57000 miles.

[SouthernBrandt75]

There are three aspects to the timing on your engine.

Base timing - the 14 degrees BTDC you measured with the vacuum port plugged
Mechanical advance - this timing advances as engine RPM advances.  For most stock engines, the distributor will advance the timing about 18-20 degrees by about 2500-3000 RPM.
Vacuum advance - when engine vacuum is applied to the port on the vacuum can on the distributor, it will advance the vacuum about 18-20 degrees.  Vacuum advance is mostly used to advance timing at cruise to improve fuel economy.  Your engine will run if vacuum advance does not work, but fuel mileage will drop off significantly.

Your carburetor has two vacuum ports. One is below the throttle plates and is connected to engine vacuum; the other port (ported vacuum) is above the plates, so that it only draws vacuum as the throttle plates open and are above idle.  If vacuum does not advance beyond 14 degrees when you reconnect the canister, then the line is connected to ported vacuum, or the canister is bad.  You need to manually apply vacuum with a vacuum pump, or connect to a full vacuum port before you measure.

Your total timing should be base+mechanical+vacuum, or no more than about 14+20+20 = 54.  However, you can't easily measure them all at once, since if RPM is high enough for mechanical to kick in, then vacuum will often be too low to bring in vacuum advance.   The only time you get them all working together is at a steady highway cruise.
Measure base timing- 14 degrees BTDC
At idle, pull vacuum on canister and measure base + vacuum - 32-34 degrees
Plug vacuum line and measure base + mechanical at 3000 RPM - 32-34 degrees

Bruce

Thank you Bruce. Very very informational. As soon as I google translate what you just said I am sure I will get it down. Thank you again my friend.

Trey

[Irish_Alley]

http://forum.73-87chevytrucks.com/smforum/index.php?topic=32085.0

[VileZambonie]

Specs are BTDC with the engine warmed up, choke open, vacuum advance disconnected and plugged at idle.