Topic: Difference in TDC and firing mark location

[Bob Wessner]

I have done a search and I've looked at the three manuals I have, but would still like someone to confirm my assumption. One manual you can't even see the image at all, the other just references the letters "T" and the "F" and there seems to be some inconsistancy in the location of reference marks, another source has the mark directly over the F/T.

Anyway, attached is a diagram of my reference points behind the points plate. In this case for the 2.3 plugs. I am assuming the marks to the right of each letter are to be used for; valve adjustment "T" and timing "F" respectively, not the letters themselves. If this is correct, it seems I didn't set the timing correctly 20 years ago. Just put new points and condensors in and want to make sure I get it right. Thanks.

[bryanj]

You use the marked lines not the letters and the "static" mark on the casing. The "T" is for top dead centre where you adjust the valve clearances and the "F" is where the points shoild open as you turn the crank by hand (easiest way to determine is a bulb accross the contacts) The other two marks are where the timing shoul be over 3,000 rpm when checked with a strobe.

[Bob Wessner]

Thanks. One other question, er, really confirmation. The combustion process, starting with the spark, begins just a hair before TDC to give it a little time to reach max, correct? I'm asking to better understand the processes and also because I checked the timing statically just before changing the points and condensors and it was occuring a hair to the left of the "T" which would mean it was occuring late, right?

[Raul CB750K1]

The spark, caused by the opening of the points, should occur (static timing) at the "F" mark (not the letter). "F" stands for "firing". As the mixture takes a given time to fully explode, the marks are written in such a way that the actual explosion will occur when the piston is in the "T" mark (Top Dead Center). The time for the mixture to explode is fixed irrespective of the crank rotational speed. The advance mechanism forces the spark to happen even before the "F" mark while rotating, depending on the crankase rotational speed.

That is a mechanical device and prone to errors. Electronic ignition measure the rotational speed and calculate very accurately the ignition moment.

Raul

[Bob Wessner]

I get it I think, combustion takes "N" amount of time no matter what. At idle speed, for example, that time is the degree difference between the F and T mark. As the engine spins faster at higher RPM's the same "N" time encompasses more degrees of engine rotation.

[Raul CB750K1]

That's correct. To compensate that increase in degrees, the advance mechanism advances the spark even before the "F" mark at speeds higher than idle.