Qualcast Suffolk Punch 35s - ignition spark problem

[paulbeddybedford]

Thanks again for the help guys. Now to get that blooming fly wheel off!! Ta

[Tim Lamb]

In message , [e-mail] writes

I'm a bit out of date but iron horse engines used to be both. The make and break operated by a cam lobe to co-incide with the magnet passing the coil. I thought Briggs and Stratton were similar?

regards

[Tim Lamb]

In message , [e-mail] writes

Try heat first.

Check the thread direction before getting heavy with it.

I used hub pullers because I have some:-)

regards

[paulbeddybedford]

Thanks again guys Ok i think the plan might be to persuade the missus to let me bring it in the house for a few days. Wish me luck on that!!

[Dave Liquorice]

Might not be to good for the magnets...

Aye the nut might be lefthand thread.

Took mine down to the local garage and their pullers struggled but it did eventually come off.

[Arfa Daily]

Well, I'd have to dispute that. My first petrol mower was a brand new Qualcast, probably 20 years ago, and that most certainly had a set of points ...

Points or fully or partial electronic interuption, it never-the-less is still there, and has to be get a) a voltage of sufficient amplitude to produce a spark at the plug and b) to ensure that spark occurs at the correct time. If the system tried to rely on just the magnet moving past the coil, the transition from magnetic field present to magnetic field gone, would not be sharp enough to induce a sufficiently sized voltage in the HT winding to produce a spark.

Arfa

[Dave Liquorice]

Bollocks. My 5 to 10 year old 4 stroke B&S based rotary mower has no points, neither does my 2 stroke Ryobi strimmer. Both have sealed ignition units.

You're making the assumption that there is only one magnet. IIRC there are

4 quite powerful ones around the edge of the flywheel of my fourstroke. Not sure how the timing is done but it sure isn't via points and a cam/push rod.

[Arfa Daily]

- that you can't see inside ...

It is not bollocks, as you so eloquently put it. If you took the time to actually READ what I've said in any of the posts, or follow up on any of the links to what others who know more about it than either of us, you would have actually understood that the interuption scheme does not *have* to be points, merely that it *can* be. Just because you can't *see* a set of points, you cannot assume that it all works in the fashion you believe.

*Some* interuption schemes work by having an additional coil to the LT side of the magneto coil, with the purpose of picking up a narrow pulse at the appropriate time, to drive the 'switch', which may be a transistor. The transistor is the equivalent of the points. The pulse is the equivalent of the mechanical cam to drive them.

I am making no such assumption. Two or four magnets. It makes little difference to the fact that the rise time of the flux in the core of the magneto coil, will be too slow to produce an HT pulse of sufficient amplitude to cause a spark at the plug with any accuracy of timing, if at all. The fact that you say that you don't understand how the timing is done but that it's not done by points, a cam and a push rod (?) shows that you do not understand enough about ignition systems, to comment.

Try actually *reading*

understand about timing in magneto fired systems.

Arfa

[The Natural Philosopher]

I think the issue is whethr or not you have a bettery.

If you hav a battery, then a CD or othr discharge system works well. Normally hall effect sensors and a magnet do th timing.

CD systms use a 200v HT in a capcatir, that is chargd by an inverter, and than 'applid' to teh coil primary.

Non CD electronic systems simply short the coil across the battery, and 'open circuit' it at the correct point to give a primary rung voltage around 200v, and th secondary up around the 15KV nedeed to fire the spark.

On battery less engines, the only way to generate the voltage is via a magneto. The traditional mechanism was a magnet moving past a coil, which was held shorted. This formed IIRC the primary of a transformer, and th contacts then opened, and the collapsing field generated a spark in the secondary windings. Contactless versions merely have a bit of electronics probably fed off the primary circuit that replaces the contacts with something like a switching FET that is coupled to a hall effect sensor.

On single cylinder 4 stroke its normal to have a wasted spark on the exhaust cycle, rather than taking the hall effect of the the camshaft.

[Mark]

Lol On a piddling lawn mower engine it doesn't have to be that complicated at all. Just a simple thirister will suffice

in fact B&S used to do an add-on of the above to replace the points on old engines.

Often wondered how people could be over qualified for a job, now I know.

-

[Arfa Daily]

Yes. All absolutely correct, and exactly what I have been trying to get Dave to understand. Also, correct on the way CDI systems work, and the fact that an inverter, powered from an external source (a vehicle battery / alternator) is used to produce the high voltage to charge the coil coupling cap. I believe that I said just that in an earlier post.

Arfa

[Arfa Daily]

Well that actually tells us nothing except that Rooster make a black box with three connections which replaces the points. What exactly leads you to believe that there is a thyristor in there ? There might be, but nothing on the cited document actually says so. Even if the switching element is a thyristor, it's still going to need to be triggered, which means that it is unlikely that there is *just* a thyristor in there.

Arfa

[Mark]

Because I use one (a thyristor) on my own lawn mower and motor bike

You need to do a Lot more reading on how a thyristor works.

-

[The Natural Philosopher]

And what maks youy think that that is just a 'simple thyristor'

Often wondered how people can determine the internals of a three wire block just by reading an advert.

[The Natural Philosopher]

Spent many happy hours doing just that.

[Arfa Daily]

I have been working with thyristors for some 35 years, so I have a pretty good idea how they work ...

So, are you telling me that you have a 'thyristor ignition module' on each of your engines, or that you have an actual single thyristor on them, that you have bought from Farnell or wherever, and grafted on yourself in place of previously existing points ?

Arfa

[The Natural Philosopher]

I think we already know the answer arfa..;-)

[Arfa Daily]

That was my thought ...

Arfa

[paulbeddybedford]

Update Right folks. Here is what i did. New spark plug. New HT lead cap. Recoil starter off. Flywheel off. Used a wooden spoon of all things to prize it off. Removed magnito. Half an hour on it with wife's hair dryer. Dried out inside of flywheel. Reassembled and hey presto!! Started first time!! Thanks for the help guys

[The Natural Philosopher]

And did it have 'just a thyristor' in it? :-)