Vintage Mcculloch MAC 70 string trimmer

[Michael Skoien]

I have an old McCulloch MAC 70 string trimmer, after setting the points @016 - I'm trying to check the timing (should be 26 degrees BTDC) there are markings on the flywheel - F and T. Can someone tell me what the F and T mean and what is the other alinement mark. On other engines it's one of the coil legs - using a meter to tell when the points open

Mike

 

[jacob j.]

@heimannm

 

[heimannm]

I am not familiar with those marks as I don't see many McCulloch branded trimmers here.

In general I just set the point gap per recommendation and let it go at that, I have not found it necessary to coordinate the points setting with the specific timing recommendation.

On the small saws (Mini Mac, Power Mac) we did experiment a bit and found that the timing would vary around 1° for every 0.001" that we changed the gap. With a recommended setting of 0.020" to achieve 26° BTDC, 0.016" yielded 20° BTDC and 0.024" yielded 28° BTDC. We were using a timing light with an inductive pick up and a drill to spin the engine over.

This was the set up on a 300 Series engine with electronic ignition. We did see the timing on the electronic ignition models varied around 5-6° as the engine sped up indicating the electronic coils have some advance built into the circuitry. The timing was retarded at low speeds and came up to the factory recommendations as full speed.



Mark

 

[hacskaroly]

there are markings on the flywheel - F and T. Can someone tell me what the F and T mean and what is the other alinement mark.

Doing a quick Google search turned up "T" = Top Dead Center and "F" = Firing position (where the spark plug should fire).

This is Ai's explanation (take with grain of salt):

For correct engine timing:

* T (Top Dead Center): This mark aligns with a reference point (usually a mark or notch on the engine casing) when the piston is at the very top of its compression stroke.
* F (Firing): This mark aligns with the reference point at the specific moment the ignition points should open and the spark is triggered, which occures a few degrees before TDC (BTDC) to allow the fuel charge to ignite and build pressure as the piston reaches the top.

 

[hacskaroly]

I didn't see any other marks on the flywheel, just the letters and their associate hash marks, if I missed one please point it out.