(old) ignition question

George, before you go and re-engineer the ign. system lets start with the basics. First if your getting spark thats a good sign! I would get a new spark plug because sometimes they will not fire under compression when they go bad and don't replace with "Champion, AC, Autolite" they just don't stand up on a aircooled engine. Try Nippondenso or Bosch. I use Nippondenso in my chainsaw and lawnmower, Generator and never had a problem starting ether. Now the most offen cause of a small engine "no start" condition is more often bad gas that was left in the tank and gums up the Carb. To check this out, this is what I do, I remove the Aircleaner and spray "WD-40" down the carb, it dosn't hurt anything and it will fire on it. If the engine fires then you know you have a carb problem. If the gas in the tank smells like varnish ,it's bad! Have you checked compression? Now most larger horsepower small engines had some kind of compression release built in so you could pull start if needed but most had a starter so the compression reading may be lower than what you expect. Thats all I got for now, HTH

Rick

This is exactly what I did with a Briggs 8 horse engine in a garden tractor. Yes, you'll need a ballast resistor, you'll also need a 12 volt power source (battery and generator).

Been there, done that - on my own & in a couple of NGs. (google "weak ignition? (small engine)") Nada. It still doesn't start, I'm stuck.

So, I think it's time to move on. And, to be precise, I'm _not_ re-engineering the ignition at this point: this is just a test. If it starts (or doesn't) with this, I'll know more than I know now.

Thanks, G

What did you use for a ballast?

G

Off the shelf 1.2 ohm Chrysler ballast resistor.

I've got no problem with NGK, but I wouldn't use Bosch plugs in

*anything*, especially not their crappy platinum plug design. And I've had absolutely zero problems with Champion plugs- in fact they're my #1 pick for all applications. My 1994 lawnmower still has its original Champion plug.

Another thing to check on a magneto-fired small engine is the air gap between the magneto coils and the flywheel. If its too large, the spark will be weak.

Compression will have no effect on the ability of the sparkplug to create a spark. As others have stated, eliminate the simple things first.

Most mower and small stationary engines have a magneto ignition, so go through that thoroughly. Use a new plug designated for that motor. Clean and set the points. Check the magneto gap (a long shot).

Check the shear pin on the flywheel to see if it has been sheared if ever so slightly.

Next head for the fuel system. Replace or clean the air cleaner. Drain all the old gas in the tank and bowl. If the mower has been sitting with old gas inside for a long time consider rebuilding the carb. It's a simple job.

Change the oil.

Assuming you mean to say compression has no effect on the ability of a spark to jump the plug's gap (plugs don't "create spark") -- care to cite that with science or fact?

Toyota MDT in MO

-- Plugs are very likely culprits in cases of small engine no-starts. It may not be the plug that failed, it may be fouled from excessively rich operating conditions, the gap may have been unknowingly smashed during inspection, etc.

Kinda hard for me (or anyone else) to prove a negative. How about if you prove a positive, i.e. that compression of a lawn mower engine while being pulled through can cause an electrical current to not jump between the electrodes (create a spark).

If he pulled the engine through enough cycles with no start the combustion chamber could be filling with gasoline and there would be no spark. But that (I hope) is not what you are talking about because it is not compression causing the problem.

OK. You said, paraphrased for relative technical accuracy, "compression has no effect on the ability of a spark to jump the plug's gap". The fact is that a cylinder under pressure requires a higher voltage to bridge the plug gap, therefore more time spent creating the spark and more current drawn to create the spark -- than the same cylinder at atmosphere. Believe it or not, I don't care. I just thought you might come back with some amusing non-truths hence I asked the question.

Toyota MDT in MO

OK, I'll play. What is your citation for this.

Well, the topic is covered in any automotive theory book that covers ignition systems. I'm sure many people have posted excerpts from such a book. Do you want me to spend time googling it for you or can you do that? Do you not want to learn or do you not want to be wrong? You can also *see* when scoping secondary ignition - the effects of low compression on the height and shape of the spark line.

Toyota MDT in MO

Yes, this is straying off topic...

Whether compression shapes the spark is secondary. The real point is that the O.P. should eliminate all of the obvious potential problem areas before trying to patch in an automotive coil.

So you chose option B) you do not want to be wrong...

The OP's question was answered by aarcuda in that a coil can successfully be installed in place of the magneto along with the correct resistor. If he wants to know how to further diagnose the no-start he will probably ask. Saying compression doesn't affect spark is wrong and misleading. Another way compression can affect spark on a pull start engine is that it slows down the RPM and reduces magneto output -- relative to an engine with a compression release feature. A degraded magneto system would suffer in this scenario - perhaps why the OP wants to bypass it.

I think it's pound foolish to put a battery and coil on a small engine when the correct replacement coil is probably cheap or even free off of an old Kohler, but that's just me.

Toyota MDT in MO

Before going off to replace parts like coils that typically don't give problems in small motors like the Kohler it would be wiser to determine that the following are in proper operating condition: Sparkplug Points and gap Magneto gap Old gasoline Air filter Plugged or dirty carb Fuel tank and cap

Having owned lawn mowers for many years I've found that hard-to-start problems are usually solved by checking out the simple stuff I listed above.

Where might one find the old Kohler graveyard? The story I got locally was that a coil would cost me ~$70+. That's more than I want to spend just to find out that I'm mistaken.

George (OP)

Just to be clear, I do think I checked all those things, except the magneto gap. And, FWIW: before it didn't start, it slowly died while it was running.

George (OP)

Sigh...............the facts just dribble out........