Zen and the art of Ignition Timing.

[Lance Leitzel]

First a little background. (Sorry for the long post)

1) I have never worked on a points type ignition before. Actually I've never done anything except replace cap, rotors and wires before on my car.
2) Last year after I purchased the tractor, I gave it a "tune-up". New plugs, points, cap, rotor, wires, etc.
3) I learn best by messing with things.

So, finally in reading enough posts here on the forum, I deduced that I had to check my timing as my tractor never did like to slow idle. It ran much better faster my engine was reved. In helping my father-in-law cleanup his shop, I stumbled onto a timing light in some dark corner. Also, I'm managed to purchase a used dwell - tach meter (but clueless about it)

Here's what I found so far. My timing was retarded. Basically at fast idle it was firing at TDC. At slow idle I'm guessing it was firing at 16 degrees ATDC. (My guess due to me replacing the points last fall.) Amazingly enough, when I adjusted the timing to 16 degrees BTDC (at 475 RPM or higher), I was finally able to engine to idle at below the magic (375-400 RPM) to test the TDC timing.

Now, if I were smart, I would have just stopped there, but I just HAVE to play with that dwell-tach's dwell switch. Right now I see about 70 degrees of dwell. I can't find the exact number of degrees of dwell the cub should have, but my gut feeling is this is too high? So, I'm guessing that my points are too close together as I don't see any other adjustment on the distributor. Am I on the right path here?

Also, I have another odd question. Am I correct in thinking the rotor in the distributor turns at the same rate as the engine. Meaning that if the rotor is pointing to cyl #1 for example (and the timing pointer is pointing to TDC), that you can't tell if the motor has just completed the compression or exhaust stroke? (Sorry I'm thinking ahead to the day I check the valve clearance, and the reason to pull the plugs and feel for compression)

Am I on the right path here or am I all wet? Thanks - Lance

[George Willer]

Lance,

If the engine runs and the wires are in the right position in the cap the cylinder has just finished the compression stroke when the rotor points to #1. If you then turn the crankshaft one revolution, you'll find that the rotor then points to #4.

If the point gap is correct when the rubbing block is on the highest part of the lobe, the dwell will be automatically correct whatever it reads. Dwell meters were used to set points externally and are of no use on the Cub.

[Eugene]

70 degrees of dwell would be about right - indicating that the point cap is correct - or very close to correct. George is correct. Just set the points and forget about the dwell. The Cub engine is a low RPM engine and the dwell is not critical.

The distributor turns at 1/2 engine RPM. So if the rotor points to the #1 distributor cap tower and the pointer on the front cover points to the TDC mark on the crankshaft pully, the engine has just completed the compression stroke. At this point the ignition points should just start opening - to fire the spark plug.

Valve clearence and compression test. My opinion, If the engine is running ok, I wouldn't bother. Just leave things alone.

Eugene

[Lance Leitzel]

Hi George,

That makes sense on both counts. Good reason not to have dwell specs then in the manual. It makes much more sense not having an "extra spark" during the exhaust stroke. Sorry I didn't notice that extra gear in the distributor assembly. My mind kept saying that since the governor is driven 1:1 to crankshaft that the distributor must be as well. Which it is, but didn't think to look at the distributor itself for gearing.

Since the engine runs pretty well, I'd best just leave things alone.

Thanks again,

Lance

[400lbsonacubseatspring]

Lance,

I had a lot of success playing with the idle mixture screw on the carb in order to get it to slow idle better.

Originally, the tractor would not run below about 1/3 throttle....now I can bring it down to about 1 notch above bottom after its warm. That's close enough for me.

[John *.?-!.* cub owner]

Now that you have all that figured out, i will tell you what my theory is on slow idle of old tractors. i was raised working them for a living, and still give my cubs a pretty good workout. only time they have an easy life is in parades at shows. I tend to idle my engines higher, around 500 or so rpm to provide better cooling after working them hard. just my personal preference.

[Lance Leitzel]

I have one last question. I have my high idle set at 2000 RPM per the service manual. But from what I have gathered here on the forum that's too fast? Is 1800 the right number or 2000? Thanks - Lance