B&S Ignition


Staff member
This thing is going to drive me nuts! She used it for several more hours, then took it to my son's house and it promptly crapped-out. Again, seems like no spark and definitely no joy with starting-fluid. I have been testing for spark with a new plug, but it is an automotive resistor plug. I'll get a new mower plug, just to get past the possibility that the resistance in the new plug is confusing the issue. To recap:

I have two inop mowers, this one with "no spark" at times, and the other with a leaky carb. Both can run fine, so I doubt there is anything seriously wrong with the innards (piston, rings, valves) of either.

The "bad carb" one will start, using starting-fluid, with either of the cruddy mower plugs. The "bad spark" one won't fire at all (again, starting-fluid) with either plug, when it is "not in the mood".

Neither engine seems to have a good spark. In fact, most often I can't see any; yet the "bad carb" one always fires. When I can see a spark, it is very apparent, I don't think this is just a matter of the sun-light being too bright...

This is the world's simplest ignition system, just that coil and flying magnets on the flywheel. I have disconnected the "kill wire" right at the coil. it seems absurd tostart probing with scope to see what is going on with the coil when I crank it, but I guess that is where I am headed after the new (mower) plug doesn't do any good.

My wife and son are going to quickly force buying of a new mower if I can't make headway here, which seems equally absurd, since these beasts can run.
Bill: I can not hand start a mower anymore. That rope pulling action throws my back out.

But prior to giving up on lawn and eliminating the mowers, I installed a large nut on the top of the crank (or maybe it was already there, can't recall) and then cranked it with my 3/8" drill motor with a socket that fit the nut. I probably had to remove the rope hand starter first.

Something like that may help with your diagnostics, since you can have the drill motor running alot longer than one little hand pull crank.
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Staff member
Well, good news... but only for my mental health. I got a new (small-engine) spark plug and saw no hint of spark with the "no spark" engine. I then installed it and there was no firing at all, using either gasoline or starting-fluid. That much, at least, went as expected. I checked compression on both engines and it is the same, in the 60-75 psi range. Yeah, that isn't much, but these are low-perf flat-head engines; and the "no carb" one does start easily.

Jack, for some of my testing I spin the crank with a (hand) speed wrench; pretty much the same as you described. I can get a small engine like this up to 600+ rpm with the plug out.
Bill: I have an old 12 volt inductive timing light that I don't have to use for timing anymore, but it is very useful to just see if there is any spark in a spark plug wire.

I carry it and a fuel pressure gauge with me all the time, so that if I have a breakdown I can instantly diagnose what is wrong.

You can attach it to a 12 volt source and use it to determine spark with the plug still in the engine. Might help in diagnostics.
An intermittent stuck valve could almost fit your situation.

The reason it does not is that you say compression is OK.

BUT it could still be a stuck valve if it was not stuck open when you checked compression, but was stuck open when you tried to start it.


Staff member
Well, I am about ready to (reluctantly) give up on the "no spark" one. Is anybody interested in still playing with me here? I don't want to bother posting my latest "findings" if there is no interest. I don't like typing that much...


Wrench. I help when I can
Yeah, I am stumped, if the coil is installed this side out, cyl side down, and ground is good from coil to engine casing, new coil, new plug, and magnet on fly wheel can hold a small nut or screw, they are fine.

I can only see a bad "new coil". Even if you hit a rock or something and sheared the key, it would still give spark. What are ohms from plug boot to engine casing? You also said you disconnected the kill switch, so that is ruled out also.

Bizarre. I read the article here, https://www.briggsandstratton.com/n...rowse/ignition-system-theory-and-testing.html

And yeah, I don't see anything you have done wrong or any advice others here may have given that was bad.

This site may have better info or ides, maybe give it a read.



Staff member
OK, I'll peek at those links and post the whole new story tonight. I take your reply to mean you are willing to keep discussing this with me, even though I haven't found an McIntosh amp for you at a reasonable price... what would that be, by the way, they seem to be very expensive around here!

Edit: It absolutely is not the flywheel key, that is "off the table"


Wrench. I help when I can
Sorry Bill, yeah, I too could only find some really extremely expensive McIntosh amps and will go back to my original plan from last year. I will build my own. I have seen a few MK30 mono kits for sale under 400$. So two of those would be in the price range I was willing to invest. But they seem elusive. http://mcc.berners.ch/power-amplifiers/MK30.pdf

So don't waste anymore time on this.

I will just have to do more research and compare with some of the other more common asian tube kits out there. Since the MK30 kits are replica's, I would assume they share alot of the same hardware as their asian counterparts.

This is just a long term dream of mine to either own a proper tube amp or build one myself.

Every time I go to my local used CD/Vinyl shop, I keep staring at his vintage collection he has set up in the back room and I just kind of drool.

I quite like the look of this one.


Will have to compare spec and such, and also see what I can maybe scrounge up when I go to montreal, they have alot of shops there that would help me save on shipping and such.
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Staff member
I know tube stuff fairly well, and I built a "high power" tube amp from scratch when I was a kid. I kind of copied a sample circuit from the RCA Receiving Tube Manual (still have the manual) but substituted 6L6GCs in place of the 7027s. The tube amps look kind of sexy with the output tubes pulsing blue in time with the sound. However, I gotta say... I feel there is no technical reason why a semi-conductor amp can't provide as-good or better sound quality.


Wrench. I help when I can
I know I went off topic, but yeah, I agree with the sound quality, but you said the magic word, it has a sexy feel and look. In the days of I-pods, and I phones and MP4, MP3,s, optical, analog we live in now days, it just calls to me.

Plus I get the added bonus of putting a small part of myself into it.

I have been looking at some interesting designs, and probably would just change some of the hardware to more reliable caps and such. I miss that scratchy feel of an old LP turning. It's a whim, I know. But would be really nice looking with my decor of my living room/listening room.

I can listen to fiber optic any day of the week thru my PC.

I am a bit older now, so I don't really need those 100watts per channel anymore making my ears bleed and windows rattling. Although a good movie on the 5.1 surround still is a thrill.

A nice soft tube amp at 12 or 16 watts should happily drive my 1975 Merak (MK14201's) "canuck" speakers. Those are being reserved especially for this stereo project.

The McIntosh would probably have been overkill anyway. Miles Davis goes a long way on low wattage and good crisp mid range and SEAS tweeters.

Thx anyway Bill for having looked into that. I hope you may have found some new insight from those links for BnS mowers/engines, but I feel you knew most, if not all of it already.

Would suck to throw away two perfectly good running engines/mowers for a shyte carb and a problematic ignition circuit.


Wrench. I help when I can
The kit I was looking into seems to come with the EL34 tubes. So for the single end HIFI look and feel I am looking for, I may just do what you did back then. I found these here.

Then there are these ones, pricier, but state that they are in fact RCA made in usa tubes made specifically for applications such as the MC30 McIntosh. But again they are mono. So still need more knowledge and direction as to where my budget vs REAL needs lye.


I still have alot of research to do.

Found this wiring from here. http://lilienthalengineering.com/wp-content/uploads/McIntosh-MC-30-ed.-1960-vers..png

A wealth of info on this site. I am looking forward to this summer scouring for parts and will probably have all the proper knowledge of those 60's amps and hardware for those cold winter days and nights.

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Staff member
Good luck trying to duplicate that MC-30 circuit! Inductors in both the cathode and anode circuits? A feedback tap in the output transformer secondary? There was some real "black magic" being sold there. You can get good sound, and all the tube-sexy, with a much simpler circuit.

Getting back "on topic", those links actually have been of benefit. If nothing else, to remind me of something I knew, but was trying to ignore. I should, and maybe will, describe more; but I do hate writing, so not now.