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Car Forum / Driving, Maintenance, Tuning / Maintenance and Repair / October 2006(old) ignition question
I have a small (7 hp, 1 cylinder) 4-cycle engine (Kohler) that won't start. It has a magneto ignition. A plug resting on the head sparks; but, for a number of reasons, I still suspect the ignition is 'weak', and doesn't fire strongly enough under compression to produce combustion. But, I'm not at all certain of this, and I don't want to spring for a new coil just to try it. What I'm thinking to do is take an old automotive ignition coil and a battery, and just run it off the points on this engine. If I can find it, the coil would be from a Chevy (straight) 6. My question is whether I need to put a resistance in the coil's primary circuit. I ask because I have vague memories of a resistor like that in the cars (bypassed when the key was in the crank position). Plus, running a single-cylinder (instead of 6), the points would be closed more of the time, and so the average coil current would be higher. TIA, George > I have a small (7 hp, 1 cylinder) 4-cycle engine (Kohler) that won't > start. It has a magneto ignition. A plug resting on the head sparks; [quoted text clipped - 15 lines] > TIA, > George 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 >> I have a small (7 hp, 1 cylinder) 4-cycle engine (Kohler) that won't >> start. .... [quoted text clipped - 4 lines] >George, before you go and re-engineer the ign. system lets start with >the basics. ... 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 >>I have a small (7 hp, 1 cylinder) 4-cycle engine (Kohler) that won't >>start. It has a magneto ignition. A plug resting on the head sparks; [quoted text clipped - 23 lines] > use Nippondenso in my chainsaw and lawnmower, Generator and never had a > problem starting ether. 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. > I have a small (7 hp, 1 cylinder) 4-cycle engine (Kohler) that won't > start. It has a magneto ignition. A plug resting on the head sparks; [quoted text clipped - 15 lines] > TIA, > George 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). >> I have a small (7 hp, 1 cylinder) 4-cycle engine (Kohler) that won't >> start. ... [quoted text clipped - 9 lines] >Yes, you'll need a ballast resistor, you'll also need a 12 volt >power source (battery and generator). What did you use for a ballast? G > >> I have a small (7 hp, 1 cylinder) 4-cycle engine (Kohler) that won't > >> start. ... [quoted text clipped - 13 lines] > > G Off the shelf 1.2 ohm Chrysler ballast resistor. Try draining all old petrol out of it, refill with fresh petrol and start as usual. I've had that problem with chain saw. It was perfectly dead untill refilled with fresh petrol without oil. Andriy >I have a small (7 hp, 1 cylinder) 4-cycle engine (Kohler) that won't >start. It has a magneto ignition. A plug resting on the head sparks; > I have a small (7 hp, 1 cylinder) 4-cycle engine (Kohler) that won't > start. It has a magneto ignition. A plug resting on the head sparks; [quoted text clipped - 15 lines] > TIA, > George 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. > Compression will have no effect on the ability of the sparkplug to > create a spark. As others have stated, eliminate the simple things > first. 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. > > Compression will have no effect on the ability of the sparkplug to > > create a spark. As others have stated, eliminate the simple things > > first. > 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? 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. > 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. > > > Compression will have no effect on the ability of the sparkplug to > > > create a spark. As others have stated, eliminate the simple things [quoted text clipped - 13 lines] > 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 > > > > Compression will have no effect on the ability of the sparkplug to > > > > create a spark. As others have stated, eliminate the simple things [quoted text clipped - 24 lines] > > Toyota MDT in MO OK, I'll play. What is your citation for this. > > > > > Compression will have no effect on the ability of the sparkplug to > > > > > create a spark. As others have stated, eliminate the simple things [quoted text clipped - 26 lines] > > 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... > > > > > > Compression will have no effect on the ability of the sparkplug to > > > > > > create a spark. As others have stated, eliminate the simple things [quoted text clipped - 37 lines] > > 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. > > > > > > > Compression will have no effect on the ability of the sparkplug to > > > > > > > create a spark. As others have stated, eliminate the simple things [quoted text clipped - 41 lines] > 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 > > > > > > > > Compression will have no effect on the ability of the sparkplug to > > > > > > > > create a spark. As others have stated, eliminate the simple things [quoted text clipped - 57 lines] > when the correct replacement coil is probably cheap or even free off of > an old Kohler, but that's just me. 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. >... > [quoted text clipped - 12 lines] >problems are usually solved by checking out the simple stuff I listed >above. 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) > >... > > [quoted text clipped - 16 lines] > magneto gap. And, FWIW: before it didn't start, it slowly died while it > was running. Sigh...............the facts just dribble out........ > .... >The OP's question was answered by aarcuda in that a coil can >successfully be installed in place of the magneto along with the [quoted text clipped - 9 lines] >when the correct replacement coil is probably cheap or even free off of >an old Kohler, but that's just me. 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) > > .... > [quoted text clipped - 17 lines] > > George (OP) You could call all of the small engine repair shops and power equipment stores and ask about used parts or where to find them, or you could search ebay etc. for a place to start. If you knew anyone with the same engine family on their equipment you could try a parts swap; be sure to check the gap on the donor engine and reinstall the coil to the same gap when replacing it. Toyota MDT in MO > OK, I'll play. What is your citation for this. It's a fairly well known fact. One of the reasons those long fluorescent bulbs bulbs can support an arc that is so long is because of the low pressure in side. The fact that he has spark under normal atmosphere pressure doesn't mean it will fire under increased pressure of compression. The correct method to check if it will fire under compression is to widen the gap. The amount of voltage that the magneto can produce is proportional to the gap. On a lawn mower with relatively low compression the spark should be able to jump a gap of at least .1" to be considered good. although just having spark doesn't mean it is firing at the right moment. -jim >>>Compression will have no effect on the ability of the sparkplug to >>>create a spark. As others have stated, eliminate the simple things [quoted text clipped - 8 lines] > while being pulled through can cause an electrical current to not jump > between the electrodes (create a spark). That's simple. just look up the dielectric strength of any common gas (air, for example) as a function of pressure. > Compression will have no effect on the ability of the sparkplug to > create a spark. Absolutely incorrect! The voltage required for dielectric breakdown of any gas between the a pair of electrodes INCREASES as a function of pressure. A weak coil that will spark fine with the plug lying on the cylinder head may not be able to produce a spark AT ALL at normal engine compression. Back when coils were a lot more fragile than they have been for the past 30-odd years, that was a fairly common cause for a sudden "no-start" or a mystery misfire that would only show up at high manifold pressure (and consequently higher cylinder pressure.) Its also 'exactly why we have pressure reglators on high-current gas dischcarge closing switches for pulsed power systems. Controlling the pressure in the switch controls how much voltage it will hold off before it self-triggers. I was a pulsed power engineer in a previous life (and still have all my fingers and toes, thank God...) > > Compression will have no effect on the ability of the sparkplug to > > create a spark. [quoted text clipped - 15 lines] > it self-triggers. I was a pulsed power engineer in a previous life (and > still have all my fingers and toes, thank God...) The point of my post was the original poster had several more obvious and likely causes of the non-start problem to rule out before trying to fit an automotive coil to a lawn mower engine. >The point of my post was the original poster had several more obvious >and likely causes of the non-start problem to rule out before trying to >fit an automotive coil to a lawn mower engine. <snip> Why would anyone seriously consider doing that? Just asking...and marveling.... > >The point of my post was the original poster had several more obvious > >and likely causes of the non-start problem to rule out before trying to > >fit an automotive coil to a lawn mower engine. <snip> > > Why would anyone seriously consider doing that? Just asking...and > marveling.... ???????????????? >>The point of my post was the original poster had several more obvious >>and likely causes of the non-start problem to rule out before trying to >>fit an automotive coil to a lawn mower engine. <snip> > >Why would anyone seriously consider doing that? Just asking...and >marveling.... As noted above: it was an _experiment_. I had some spark, but it looked weak. I wanted to see if it would run with a good spark. This was an idea to generate same. George (OP) |
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