So u are telling me that if he grounds plug to head and gets a strong blue spark jumping across the spark plug electrode , that the coil could still be bad enough to prevent engine from starting or revving up? And im guessing this guy that wants to get his trimmer running doesn't have any fancy spark testing tools, and neither do I, and I've been able to determine a bad coil without it. Why do u people have to make things more difficult than they are..
Because we fix these things on a daily basis
What appears to ba a good strong spark at atmospheric pressure can fail at combustion cylinder compression ( same for the plug )
There are 3 simple tools for testing the strength of a magneto's spark.
one is the 3 point tester , the next is a clip on spark plug with a 1/4" electrode gap and the last one is a HT lead with an adjustable gap.
All of them are around $ 20, not fancy or expensive & takes the guess work out of diagnosis
Back in the old days we used clear biro tube with a fixed wire in one end and a sliding wire at the other
Add to that hand helds are designed to run 7,000 rpm to 12,000 rpm
Magneto generated sparks tend always to appear poor at cranking speeds because the flywheel is turning so slow.
If the coil is wound to make a big fat blue spark at cranking speed then it will burn out at full revs.
If it had a capacitor discharge ignition or a battery coil ignition like your car then yes a fat blue spark at cranking speed is vital because both of those systems produce a better spark at idle than they do at running speeds because the coil has longer to saturate.
Your initial reply was very good .
It is quite rare for the flywheel keys to shear off , but it does happen I have seen 1 that sheared off in use and one that got mutilated durning assembly, and this is in 11years of roughly 300 invoices a year .
Now back to Bill's original problem.
Cases like this are near impossible to make a diagnosis on because he has just kept on throwing parts at the trimmer of unknown quality
I get at least 5 jobs through the gate every week where the owner has fitted cheap parts that were faulty right out the box.
Usually they insist that their new parts are all good and it has to be some sort of secret adjustments that they are unaware of because most will not accept that they could have made it worse.
So now we have a dubious coil, dubious carb , dubious spark plug . plus a trimmer that just quit during use
This is near impossible to sort out on a web forum as there are way too many variables to work through .
From what Bill has posted , the current problem appears to be a carb that is way out of adjustment and to me it sounds like it is starting & running on the high speed jet alone .
but it still could be the wrong or faulty coil so a badly timed or failing spark is limiting the revs
So if it came into my shop right now I would start by putting all of the old parts back on then seeing if it will start by itself or with carb cleaner down the plug hole .
If not then I remove the kill wires at the coil & try again with a flashing spark tester fitted ( my preferred tool also under $ 20 ) and a brand new spark plug , gapped as per the manual.
On mowers the gap is not all that important but on high reving hand helds accurate gaps are vital .
If it was confirmed that the coil was at fault ( fairly rare ) and not the kill wires or switch shorting ( very common ) then the magneto gets replaced with one from a know supplier of good quality parts and that rules out ebay , amazon , face book market , and even craigs list ( in the USA ) .
If the tool will not run properly when the new coil was fitted then the first suspect would be the new coil.
The coil has a timing chip in it and most of them are different however the mounting centres for coils tend to be fairly standard .
So a lot of wrong coils will bolt up and look right so the starting point would be to put a dab of paint on one of the fins to use as timing mark then a dab of paint on the body some where to correspond to TDC aligned with the dab of paint on the fin and hook up a timing light to see just where the spark is
If you are really into it then you can print out a timing disc for free , glue it to a piece of tim or alloy and actually measure the advance
printable timing disc
My first guess would be you have the wrong magneto , particularly as it has the same symptoms with both carbs, but a new plug correctly gapped is cheap and if not needed becomes a spare for latter on .
