Push Mower Timing

[Freddie21]

I am a novice back yard mechanic working on small engines for a while. I have never been able to figure this one out. I understand how timing is related to the magnet and coil. and how if the flywheel moves it could throw it out. What I don't understand is how the blade holder shifting positions affects the timing? Unless it has something to do with the momentum of the blade, but the engine can run without a blade attached. How abouts one of you experts, or not, giving a little 123's on this? Thanks,

 

[slomo]

 

[Freddie21]

My engine is running fine, this is a general operational question

 

[ILENGINE]

Here is my crack at it. the blade gives additional momentum to the lightweight flywheel but doesn't directly effect engine timing. What normally happens is the blade hits a solid object that causes the blade holder to shift, but that sudden stop that shifted the blade holder also causes the flywheel to shift, shearing the flywheel key and effecting timing of fire.

 

[sgkent]

The blade holder and the flywheel are each kept in place with a key, usually a woodruff key or a rectangular key. They will shear if enough pressure is put on them, like hitting a rock or root while running. If the blade has moved compared to the flywheel, one of four things has happened - (1) the flywheel key has sheered (2) the blade holder key has sheered (3) both have sheered (4) the crankshaft has been twisted or is broken. You can certainly figure it out from there.

 

[Freddie21]

Ok, I know what happens to the keys, but I don't understand what the blade holder, or blade, position has to do with engine timing. I hear people can't get their engines to start after hitting something and the responses are to check the blade holder key. Why?

 

[ILENGINE]

Ok, I know what happens to the keys, but I don't understand what the blade holder, or blade, position has to do with engine timing. I hear people can't get their engines to start after hitting something and the responses are to check the blade holder key. Why?

The response should be to check the flywheel key.

 

[Freddie21]

Ok, that makes 100% sense.

 

[bertsmobile1]

There is a lot of rubbish out there published by people who never developed mentally after the age of 5
And for some strange reason these very people seem to be the most convincing and have the strongest desire to spread their ignorance far & wide.

Mower engines are the cheapest engine it is possible to make
In many cases the flywheels and crank shaft webs are so small ( to save weight & metal ) that the weight of the spinning blade is needed to dampen out vibrations & add momentum energy to allow the engine to continue rotating on the non power stroke.
So many mowers will not start if the blades are badly bent, missing or loose on the crankshaft
Those who do not understand conflate the lack of momentum preventing the engine starting because the blade is in the wrong place so is "out of time" with the ignition being "out of time".

The actual ignition timing is done via a Hall Effect trigger that replaces the old time points embedded in the magneto coil.
The flywheel is locked into position by the taper which is commonly used in engineering to transmit limited power and slip in emergencies by breaking free & rotating.
The timing key is there purely to hole the flywheel in the correct position before the taper is tightened to lock it into position.
The key does not do anything to stop the flywheel breaking free it is just a locating device.

Because the actual timing is done electronically it can be adjusted by moving the coil closer ( advances spark ) or further away ( retards spark ) the same way as timing changes when you increased or decreased the points gap

 

[Freddie21]

Very inciteful and supports my thought about blade momentum. I do know that these little guys will start on 1-2 pulls and run without a blade though. Thank you.