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Advance timing

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Jameswest21

New member
So my carb guy who rebuilt my carb said it's good to advance timing in these older boat to help with the lower quality fuel we get. Boat in question is 87 Grumman gsf. 2.5l omc with omc outdrive. My question is the Mark's say btc and atc... which way am I going with this?
 
Your so called “carb guy” is incorrect! Poor fuel quality is more likely to undergo “marine load ignition induced” detonation!!! If anything, the advance should be held back.

As to your question ......... (no offense) ...... since you needed to ask, I would suggest letting a professional take care of your ignition advance timing!


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Jameswest21 said:
.................. My question is the Mark's say btc and atc... which way am I going with this?
Click to expand...

OK....... to further explain this;


All Std LH rotation engines turn CCW when viewing the flywheel........ this is an industry standard means of determining engine rotation!
This means that when looking at your timing marks, the harmonic balancer will rotate CW (when viewing from the front of the engine).
Any harmonic balancer degree markings (intended for ignition timing) will be to the right side of the TDC marking.
(not all balancers are equipped with anything other than a single mark).
Any timing tab degree markings (intended for ignition timing) will be to the left side of the TDC marking.... of which is often a V notch.


When a piston is at the absolute TOP of it's stroke, this is considered to be TDC or Top Dead Center.
This will occur when the harmonic balancer "0" mark is aligned with the "V" notch in the tab.
This also occurs 2 Xs during 4 engine cycles..... once at TDC on the exhaust stroke, and once again at TDC during the compression stroke.

(the term ATC (ATDC) refers to After Top Dead Center)


Combustible Gasoline/Air does not explode within a cylinder, it "burns" within the cylinder.
This "burn" requires a duration in order to reach it's maximum expansion rate of which occurs ATDC or After Top Dead Center as the cylinder has entered it's Power Stroke.
(max expansion is best if it occurs @ 12* to 14* ATDC)

All of this means that the ignition spark must occur during the up-stroke of a given cylinder's piston.... or BTDC.

Since the ignition spark must occur before the piston reaches the TDC area, we refer to this as BTDC or Before Top Dead Center.
This is referred to as ignition spark advance.

All gasoline engines must be given a variable spark event (i.e., ignition spark advance).
The ignition spark advance is less at lower RPM, and progressively increases as the engine approaches higher RPM!




So........ if you want to make this adjustment yourself, you will need to look up your engine's OEM ignition advance curve.
You will adjust the BASE or Initial advance at the prescribe idle RPM .
You will then increase RPM as you verify the Progressive Advance and as you verify the Total Advance.


Many mechanics will set BASE or Initial Advance only, and will NOT look any further.
IMO, it is best to look at all progressive phases of the ignition advance.
If it does not occur as per the OEM specs, it may cause engine damage!




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