Nothing to do with spark advance, hot or cold. Took the 1915 T with the 26/27 engine and starter in it and she runs fine on bat or mag. But when we got home, I wanted to play with the crank start. On bat, the coil buzzes with the timing retarded and she fires right up on one short pull. On mag, however, it seems like the compression changes and it's hard to even pull the crank the 1/2 turn, let alone fast enough to get a spark. Back to bat, cranks like crazy. On mag, hard to crank.
What would moving the key from one circuit to another have to do with the compression? I did not move the timing lever one way or the other on either of the cranks. Maybe I'm wrong, but am under the impression to push the timing rod to full retard on bat and mag starts. On mag to prevent the crank from snapping back and on bat to prevent the starter bendix from getting jammed. Is this the correct procedure?
Try advancing the spark a few notches for starting on mag - The text below is quoted from an earlier post from Ron Patterson:
"If the initial timing (position of the timer when the spark lever is all the way retarded) is set at 15 degrees ATDC as Ford intended, hand crank starting on magneto will be much easier if you advance the spark lever 3-4 notches. (Obviously never do this when starting on battery)."
"The reason for this is as follows: If you don't advance the spark lever you will be trying to start the engine with a spark occurring at 26 degrees ATDC. By the time the piston has moved that far after TDC much of the compression has been lost and it will be very hard to start."
"By advancing the spark lever 3-4 notches the first spark will occur at 4 degrees ATDC and the engine will be much easier to start."
"In an effort to simply explain the Model T ignition system a friend and I wrote an article on this subject. One of the key points was an explanation of why Ford recommended advancing the spark lever during hand cranked magneto starting early Model T's. This issue is depicted best by this diagram."
Mine won't start on mag unless you advance the spark a little. It wouldn't start on mag at all seems like before I got the coils rebuilt. One with recharged magnets I barely have to spin, the other one I spin a little faster.
I would suggest you contact Ron Patterson in Kentucky for some excellent instructions on mag starting. It is different than on battery. Go to this site, More on Model T Ford Timing by Ron Patterson
The reason for this problem is this. When the rotor first contacts the segment in the timer on battery, the coil begins to buzz, however on magneto the coil begins to buzz when the AC wave in the magneto reaches the strongest voltage. This will be a few degrees after the contact is made. To be sure you don't advance too far and end up breaking your arm, try starting on battery and then switching to magneto. Then move the timer lever down a bit and and see where it starts to advance the spark. You will notice a few notches and it doesn't seem to make any difference and then move it one notch farther and the engine speeds up. You should not move it that far when you attempt to start with the crank. Just a few notches. That will give the magneto time to build the voltage. I hope what I just posted can be understood. I know exactly what I was trying to say, but hard to put into words.
"On mag, however, it seems like the compression changes"
The only thing I could think of is on battery the engine is firing and helping it turn over. The compression can not change between the two.
OK, I've read many threads and posts about adjusting the timing between mag and bat starts and I'll become an expert on both in a short while.
But my original concern is that on the mag start, I can hardly rotate the crank. On a bat start, it is almost to easy, I'm expect more resistance than I get.
So if the answer to every mag start issue is adjust the timing rod, does that adjustment change this increase and decrease in compression that I'm feeling through the crank?
Maybe, but I'm just not connecting the dots how it could. If it was just a matter of cranking it the same and adjusting the timing lever, I wouldn't be here.
Thanks for the comments so far.
There is no increase or decrease in compression between the two forms of ignition. None, it stays the same. Compression has nothing to do with type of ignition you are using.
Dang it almost sounds too advanced on mag, like a weak spark occurring a bit too early and making it try to kick back a bit. Dirty commutator grounding a bit?
After reading some other threads, we are going to tighten up all our linkage and rods to eliminate any play in the two adjustments. Then see what we can do.
Maybe mine is worn, but it feels to me that there is a smooth plate at the top end of both rods whre they rub against the backing plate on the steering column before they get to notches at about the mid point on the backing plate. So when people speak of a notch or two advanced or retarded, I'm not even to the notches yet. And I do notice some dead play in the adjusting rods where the gas does not increase or decrease or the timing changes, so I think I have some slop in the adjustments that I need to remove and then start evaluating again.
Thanks to all.
Easy way to explore the cranking difference.
Just crank over the T with the ignition OFF. You should have firm resistance with good compression.
The magneto cranking is more difficult as you have to spin the crank fast to generate the spark.
Battery ignition is constant, as soon as the rotor hits the timer contacts, a plug will fire with that coil buzzing.
It almost sounds like you have bat power going to the Mag. The mag will act like a motor brake if DC is applied. I would put a voltmeter on the mag post to make sure NO DC is getting to it.