Just a quick question, sometimes, and only sometimes when I switch over from battery to mag, the engine will cough, sputter and backfire. If I leave it running for a minute or so it will smooth out and run fine with a nice low idle. Any ideas why this happens ?
The switching should be smooth and quick. If too slow unburnt gas fills the tail pipe and muffler and kaboom! Your switch contacts may need cleaning and takes a bit for them to make full contact.
Yep - don't pause between battery and mag
You should do the switch at idle. First start on batt then when it is running smoothly, turn the switch quickly to mag. If you are driving the car and realize that you forgot to switch to Mag either pull over and stop while you make the switch or put it in neutral and push up the throttle and quickly switch then let out the clutch and advance the throttle. If you try to make the switch while you are driving along it will make the big bang and damage the exhaust system, either or both blow the manifold gasket or the muffler.
If you are idling in neutral and it sputters when you switch to mag, your mag is weak. It should actually smooth out and run faster and better on mag than on battery. If the mag is weak, it might sputter at idle and run better with speed. That would usually improve if you recharge the magnets. In some cases the problem would be from the mag coil grounded near the ground end of the coil thus eliminating a few coils from working or the endplay in the crankshaft is causing the coil to be too far from the magnets.
Switching over at as low an RPM as possible could help too.
Norm beat me to it.
Yeah, mine does that same thing.
It's possible you're mixture is set a tad lean. To get a fair, ballpark setting; let the engine warm up and set it for a fast idle, then slowly enrich the mixture (counter-clockwise) a quarter turn or so. If the engine speeds up, you were way too lean. If not, slowly lean it out (clockwise) until you can hear the rpm's start to drop. Now, enrich just past the point where you get maximum rpm's. As Model T carburetors don't have accelerator pumps (which temporarily enrich the mixture for smooth acceleration), it's a good idea to enrich maybe another 1/8th of a turn. Don't over-do that.
Two other things:
It may be that you've been idling too low when making the switch-over from battery to mag. The faster a magneto spins, the more juice it makes, so unless you've got freshly charged magnets set at the perfect distance from the mag-coils, your magneto's output probably won't equal that of your battery when the engine is ticking over at a low idle. What you want to do is find the lowest idle that will allow you to make the switch from battery to mag without orchestration.
Mark and Fred are right; the switch-over from battery to mag must be instantaneous. Pausing in between can "load up" the engine (also known as "over-priming," in airplane parlance) and the resulting "backfire" can burst your muffler, so snap that switch all the way over.
And just because I enjoy being verbose and pedantic (and gimme a break, cause it ain't often I know what I'm talking about):
Over-priming doesn't cause an actual backfire. As the engine whirls without benefit of ignitionódepending on choke, throttle and mixture settingóraw gasoline can get dumped into the exhaust system and if the car is pointed uphill (as in your driveway), some of the drip (and vapor) might make its way into your muffler. And you don't want anything to go bang in there.
Heck, everybody beat me to it!
The car runs better on MAG, so why would you wait until it warms up? You should switch to MAG immediately after starting. Like this:
Thank you everyone for your thoughts and advise.
It's still a mystery to me cause it only happens infrequently and only when starting a warm engine
Sure the switch is making good contact when switching to mag? Mine has to be given a slight nudge to run when its switched to mag.