Ignition question

[enginear]

I have just looked and repaired a McCullouch MH542P virgian hedgecutter not a runner i found that the flywheel key had sheared got the old bit out of crank ok but was puzzled the flywheel part would not come out after checking on net found the key is part of flywheel so new one required at £44 + what a stupid idea the later one’s had a separate key i was then faced with trying to time it, this is my question, where doe’s the spark occur on a flywheel with outside magnet’s in relation to the coil pole’s,when both magnet’s are under both pole’s or when both magnet’s are at the tip of both pole’s i know it would be a guess at what position the piston is btdc, i did loctite flywheel to crank using the slight mark left in flywheel where it had sheared and it ran maybe not spot on, but has anyone ever bothered to check timing on a small engine with a flywheel key i must admit i never had until now, i told owner not to buy off flea bay unless it’s a good make and bin this if it stop’s….

[wristpin]

Would it not be possible to get a keyway cut into the flywheel, using the witness mark of the sheared key to position it, and then use a loose key to align the two keyways? Assuming that the flywheel is a taper fit to the crank , it is the taper that does the work of keeping the flywheel in position, the key just provides the initial alignment.
There is, of course, the question of what caused the integral key to shear; the options being lack of torque on the flywheel retaining nut or an ignition fault causing a kickback, both options overcoming the grip of taper.
I’m guessing that your question about what actually triggers the spark on that electronic ignition system is one for the designers of the particular system to answer. Taking the Briggs and Stratton Magnetron system as an example , it has a visible additional trigger “pole” just before the coil pole pieces.

[enginear]

Ref flywheel unfortunately the mark left by the key shearing was difficult to place exactly so it was just as near as i could get,as for getting a key slot cut it’s down to cost as well as position,cheaper to buy new flywheel but what is a ten year old machine worth £28+ on bay !,when i have come across this problem of key shearing before yes it’s mostly due to shock which on a hedgecutter is easily done,as for ignition what about the old B/S one’s where you can replace the coil and fit an electronic one and do away with point’s and condenser and still use the old flywheel and it work’s………….

[stuart]

You need to file a slot on the inside of the flywheel taper where the cast (integral) key was originally.You will need some needle files to do this.You will then have to make a key to fit the crankshaft keyway and now the flywheel keyway.

[enginear]

Yes filing a slot is a solution i thought of so would putting it on a lathe, even better, but i repeat only if one knows exactly where there original mark was, the crank had obviously turned and possibly moved every time the engine was pulled over thereby partially wiping out where it should be, but if i knew when ign occurred in relation to the coil and where piston should be i could solve the problem the other thing i could do is to make a key offset to alter ignition by 5deg but again we are talking time if it was my machine time is no relation to cost but when repairing for someone else it’s not worth the cost…………

[Author]

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