Magneto Timing question

Going to put the magneto on the 4H project super A tomorrow -

I'm not too experienced with magnetos.

I thought I knew what I was doing until I read the manual...

Tells me to put cylinder 1 at tdc - compression stroke. I'm good with that - makes sense. Figured I'd even go a hair past to make hand cranking safer.

Then turn the (not yet installed) magneto so it's ready to fire on plug 1.

Easy enough - mine has the little window - I can see when it's ready to fire on 1. Nice and easy. I tested it with a spark plug to be sure I had it right - sparks just like I expected with one click.

Next step is install the magneto - and retard it fully.

Ok - makes sense...

But then it tells me to crank the engine exactly one revolution back to TDC - which of course would be the exhaust stroke.

Then turn the mag towards advance until it clicks - and lock it in there. which would make a whole lot more sense to me to have done without cranking the engine that last full crank..

... ?

What am I missing here? Why crank the engine? wouldn't that make it spark on the exhuast stroke?

I assume the manual's correct and I'm missing something - but what?

Am I NOT supposed to have the mag ready to fire on one? perhaps it should have just fired on 1 when I install it???

Very confused here.

The manual is correct. Here's the deal:

The magneto "fires" once every 1/2 revolution. So once the mag is synchronized to the #1 cylinder firing, the final step (rotating the mag backward until the impulse trips at TDC) could easily be accomplished by turning the crank only 1/2 revolution - ie. until the next cylinder in the order hits TDC.

However there is no timing mark 180 deg from the #1 TDC. So the crank must be turned a full revolution to bring the timing mark back into view. The fact the mag is now firing #4 cylinder is inconsequential. You are instead synchronizing the impulse trip point to TDC for each of the 4 cylinders.

The procedure in the manual can indeed be confusing. But follow it and you will be good to go!

Thank you for the explanation.

What you say does make a lot of sense now that I think it through. I could have just followed the directions and been fine - but I prefer to actually KNOW why I'm doing it that way.

The key was that it fires every half revolution -That's obvious when you do the 3rd grade math - 4 cylinder engine each firing once every 2 revolutions.

(But only obvious now that you pointed it out!)

So here's how I'm reading it:

Set mag ready to fire on 1 - put it in the block. Don't worry about exactly when it's going to click - just make it greatly retarded so you can be sure to make it click manually on the last step.

Rotating the engine one half revolution makes it fire on 1 (late) - and go to cyl. 3.

Another half revolution would fire late on 3, and get it ready for cyl. 4

1 and 4 are again at tdc, but now 4 is on its compression stroke.

Go slightly past tdc since you want the mag firing a little AFTER tdc to be safe - especially when hand cranking it.

Advance the mag by hand until it clicks, and tighten it down there. That's where it'll actually fire in relation to the piston in 4 - slightly after tdc.

And obviously if it's timed properly for one cylinder it's timed for all.

I have seen the light - all makes sense now.

thanks again.

JR,You will be fine,I beleive your trying to over think the situation,Keep it simple

jimmy

Well as I said - I'm sure I can do it and be fine, but really like to know what's happening in there.

Mostly out of curiousity - but also because I hand start this thing. Just can't swing a new starter right now.

If you've ever felt a kick back you'll understand that I have a vested interest in knowing EXACTLY what this thing is doing and that it's set up correctly!

(quoted from post at 07:58:25 05/16/12)

. . .
So here's how I'm reading it:

[b:cf96fc9775][i:cf96fc9775]Set mag ready to fire on 1[/i:cf96fc9775][/b:cf96fc9775] - put it in the block. Don't worry about exactly when it's going to click . . .

Click to expand...

Turn the mag BACKWARDS to get to that point. That way you are not fighting with the latch-in of the impulse coupling to get it in position. Doing it this way, the engine will have to turn to (at least) the next firing position so the impulse is latched in before you can complete the timing process.

Now to confuse things:
If you could attach the magneto in a WAY, WAY, WAY retarded position, so far that the impulse hasn't latched yet, you could do the whole thing without the extra engine rotation. But you don't have that much leeway in attaching the magneto.

(quoted from post at 09:35:01 05/16/12)If you've ever felt a kick back you'll understand that I have a vested interest in knowing EXACTLY what this thing is doing and that it's set up correctly!

Click to expand...

Hey JR - been there, done that! As recently as yesterday in fact cranking up a friend's hand start JD A. (A sudden kickback on a flywheel is not nearly as dangerous as a hand crank.....but it still gets your attention!)

Anyway I suggest this as an additional final step to verify the timing is correctly set:

Attach a timing light to the #1 plug wire. (If your tractor does not have a battery you'll need to supply the timing light from a separate 12 VDC source). Now slowly turn the crank while observing the timing mark. If the mag is timed correctly the timing light will flash at exactly TDC.

I then like to retard the mag 3 or 4 degrees AFTER TDC to prevent kickback while cranking.

Do the timing light test several times (several more revolutions of the crank) to make sure the mag trips consistently 3 - 4 deg after TDC.

The broken wrist you save may be your own!

yes - that's exactly what was causing so much initial confusion - I thought I could put the magneto on - in the fully retarded position - and just advance it till it clicked. And be done with it.

I did'nt get why i had to crank it at all - but when I figured your point out that it has to sort of set first - I figured a full two cranks back to compression on 1 would be in order.

But I do get it now. It's really the same thing - just using a shortcut of timing it on 4 after 1 crank instead of going all the way back to 1 with two cranks.

Yeah I was going to pull the #1 plug and watch for a spark - but the timing light idea might make it a little easier in the light of day.

I've had this thing kick back on several occassions in the past - you really do need to be holding it the right way to avoid breaking bones.

Even holding it properly - it'll still give you a good enough jolt that forces a new found respect for the amount of power in even these relatively small engines.

Only reason it happened more than once to me is that this was my first engine that needed hand cranking - I thought I was doing it wrong. Didn't realize the timing was off.

Must have been timed close because it only happened if I went slow - and didn't give it a good quick lift.