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F-20 backfire

E

EM Iowa

Member
It's backfired on me twice while cranking it ever since I timed the mag. I've probably cranked it only 30 times because it starts up fairly quick. My question was should I set the magneto back one hole or should I live with it? The motor doesn't sound perfectly smooth at idle, but it sure sounds great under a load. Is this one of those cases where the spark ignites before TDC? But then it wouldn't run at idle so I should be fine...
I thank my grandpa for teaching me how to correctly crank a F-20 (6 o'clock to 11 without wrapping the thumb around the crank). That lesson may have paid off today, but it wasn't as bad as I was made to believe, but then again my elbow wasn't locked nor did I have my thumb on the crank!
It rolled backwards 3 times in a second if that means anything. The first time it only turned backward half a turn.
These 2 events happened on separate days.
 
Sounds like you've got it advanced.Retime it to where it is retarded at starting.Once it is running,then you can advance the timing by moveing the lever to the up position.
 
yup, I agree, sounds like your timing is a wee bit advanced, which is not cool for cranking for sure. Yup, under load, the advanced timing makes her purrrrrrrrrr. My suggestion is to set it as the manual says, and it should be TDC when the impulse trips. set it that way.
 
On my F-20, I move the "Lever" up about 30 degrees to retard the spark when cranking and down after it starts.
 
You and F-Dean vary on which way to turn the mag lever to advance the spark. Down is fully advanced right?
 
Yup,I was wromg.Down is advance,up is retard/off.Sold my 20 several years ago,guess I forgot....
 
yes and no Evan. The advance lever does nothing for starting. (what it does is spark later once the engine fires and runs making it start and come up to speed better) The advance moves the cam disc inside causing the points to open sooner or later when the tractor is RUNNING. During starting the magneto is STOPPED until the impulse trips at which point it quickly rotates a quarter turn making the spark. So no mater where the advance lever is pointing on impulse it is going to spark RIGHT NOW. That is why the impulse MUST TRIP AT OR JUST AFTER TDC, [u:654c4848f0]NEVER[/u:654c4848f0] BEFORE. Set the mag accordingly. Am I clear now?
 
(quoted from post at 12:11:54 08/03/13) Yes you are. The mag trips after TDC, but it backfired those two times which is odd to me.
Click to expand...

Nothing odd about it at all, you are right on the edge. Meaning you are firing just slightly before top dead center. There is a lot of iron your spinning there and usually its momentum overrides the early fire, but on the kickback occasions the momentum was not enough to overcome the early spark. Listen to Cowman and Reset your timing before you experience a full fledged kickback that breaks something or launches the crank. Its a he!! of an eye opener when they bounce around the shop or your arm hurts for a couple weeks
 
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