This has me stumped.
 

You may or may not recall but a little over a year ago I did an engine swap in my 782. I swapped in an M20 from a Huskee. Before the swap the M20 started having a spark issue, so while I had it out I replaced the coil and starter.
It's been hard to start ever since and has little and intermittent spark. (yes I have a spark tester)

Several times in the last few months I have hooked the spark tester to one spark plug and have no spark or intermittent spark. I found if I take the air cleaner off and put my gloved hand over the intake it gets what looks like normal spark. So I put the plug wire back on and do the same thing and it starts.

Question is, how does that affect the spark? Once it starts it runs great.:bash2:

You might have a plug wire that intermittently shorts to ground. I would suggest, that you look at the plug wire(s) real good and see if there is any frayed spots. Let us know what you find.

Isn't the wiring different for the M20 on the 782?

Cub Cadet 123

Yes. One has to make modifications.

Recently, another member Diesel Doctor had a Mag twin that was running badly. He rebuilt the entire engine and it still ran poorly. Finally, I believe, he replaced the coil...again...and it runs very well now.

The new coil came with the spark plug wires attached so they should be good.
Yes I had to do the relay mod for the M20. I just don't understand how cutting off the air to the engine has any affect on the spark.

I know it's hard to believe but I tried it several times to make sure I was seeing things right.

Once it starts it runs good, That's whats got me baffled. Seems if the coil was bad it wouldn't run so well.
You have to pull the engine to get to the coil, so I'm not looking forward to doing that.

Have you changed the spark plugs? Are they the correct ones for that engine?Have you checked the choke cable linkage to see if the choke plate is closing completely?:bigthink: Please don't misunderstand...I am not being condescending...but you have not mentioned anything about these simple items. :beerchug:

Yes I put new correct spark plugs in when I did the swap. The choke is closing all the way but the choke plate has like a quarter inch hole in the middle of it so it doesn't shut the air completely off. When I put my hand over it and shut all air off is when it gets spark.
I know, sounds strange but that's what it's doing. Since I found this method, at least I can get it started when needed.

Did you set the air gap @ approx .010 on the new coil?
Now as far as starting with your hand over the carb.
You are doing 2 things:
#1 increasing the richness of the incoming charge.
#2, decreasing the compression pressure by restricting the amount of air entering the cylinder.
That makes it easier to ignite the mixture with a weaker spark.

It is not unheard of to get a bad new coil, I understand your not wanting to spend 30 minutes removing the engine to get at the coil to replace it.
As a test:
Decrease the spark plug gap to .020 and see if it helps starting.
If it does, it reaffirms the idea that it is ignition related.
Also, remove the white magneto kill wire plug @ the connection closest to the coil and see if that helps.
I've seen them shorted, or shorted @ the Ign. switch.
lets us know what you find.
Edit,
I've also seen aftermarket coils that do not produce spark at less than optimal cranking speed.

Maybe his hand is creating a better ground for the engine, is your engine properly grounded?
Stand on a wooden piece of wood and see if it does the same.

:HeadScratch:

Ok, Unc......now I'm confused, which doesn't take much....

Cub Cadet 123

IIWII and no China wood either.

Some good responses here. I thought of the possibility of a wire shorting near the air cleaner cover, maybe?

:ExtremeFunny::ExtremeFunny::LMAO1::ROTF2::High5:


Maybe I'll just sit this thread out...... :biggrin2:

Or one can stick their finger in their ear and say "tinga, linga, loo" :beer2:

Are you using new, solid core ignition wire that is property terminated? While I suspect that resistor wire should work fine too, I'd double check the ignition wire and plug as they're typically the culprit in frying coils.

Ignition cables are much cheaper than coils anyway. Even if the ones on the engine are new, I'd be tempted to replace them. Toss a new plug on there too.

I wouldn't worry too much about the value of the resistance measurements. You're in the right ball park and relatively low value resistance measurements are not particularly accurate with a 2 wire DMM anyway. Your resistance reading sounds fine as does to voltages measured.

This IS a good puzzle!

If you miss and hold a leaky plug wire, that's not ALL you'll be saying !:bigeyes:

Why are you sitting this one out? I was hoping you would have the answer to the original question. "How does cutting off the air affect the spark?" Just cranking the engine there is little to no spark, when I block all air going into engine I get normal spark, using a spark tester.

Grounding the engine has nothing to do with it, it's all self contained. You could hang the engine from a tree on a rope and it would run on the mag.

I've been working on this problem for about a year, so yes I've tried everything everyone has mentioned. New plugs, old plugs, wider gap, shorter gap, grounding engine, check kill wire, plug wires, relay, high CCA battery, rebuilt carb., fuel pump, fuel lines, cleaned tank, it's parked in shed with wood floor, etc.,etc,etc.

And George, if you can pull the engine in 30 minutes you're a good man, it would take me that long to gather up the tools and get things arranged to do the job.

Jbrewer, I think your comment is on the wrong post, should be on the 1650 frying coil post.

I'm with George, less compression. What kind of spark do you get if you pull the plugs? I'm thinking the new ignition module is the issue.

I'll have to get back to you on that, I'm busy this morning but this afternoon I'll pull the plugs and see. I thought the first new coil was bad so they sent me another one, I guess it could be bad too.

Go back to post #5. Everyone here helped chase that guys problem and he found out it was faulty ignition part that, I think, was new. Research his thread, it was very educational and certainly worth filing away. If they sent you one bad coil, seems to me the next one could be bad as well if they were from the same lot. Not saying that is the problem, but all things are worth considering.

Yes, mine was a bad "new" coil. There glue holding the wires into the coil became an insulator then it had a weak enough spark causing a skip.

Lol. It was too entertaining to not sit back and watch a while.

George nailed it in post #9. To fix this, it would probably be easier with the engine out. He's right on about what plugging off incoming air is doing, and why it seems to help. Sounds to me like you have a bad magneto.

With both plugs out and grounded, I get a weak but consistent spark on both.

The general consensus seems to be a bad coil so I'm going with that. I'm going to order an OEM this time instead of aftermarket.

I'll report back when I get it installed. Thanks for all the input everybody.

I tought it was a puddy cat, I thought that was quite imaginative with the wood thing.

I figured out quite quickly why spark plug wires have thick insulation on them when video taping this buzz box for my hit and miss engine.

https://www.youtube.com/watch?v=dGVoOPqjBOI

https://www.youtube.com/watch?v=4cWu8K5hm-M

Let's just say it got my full attention quickly.

When you get your new OEM coil, make sure to set the air gap between the legs of the coil and the magnets in the flywheel. As others have mentioned above, this can make the difference between a hot spark and a weak one.
When installing a coil, I loosen the mountings screws and slide the coil as far away from the flywheel as possible, then lightly snug the screws to hold the coil there. Now after rotating the flywheel until the magnet is directly under the coil, I place a standard thickness paper business card between the legs of the coil and the flywheel. Now loosen the screws and the magnets will pull the coil into place. Tighten the screws securely, and rotate the flywheel to remove the business card. Your air gap should be right around the .008" spec. The paper of a business card is right around .008 - .010" thick and is the perfect tool to quickly get the coil installed evenly and at the correct distance from the flywheel.

Question is, who's business card do you use? :biggrin2:

Maybe I can get some with a picture of Hillary on them....:bigthink::biggrin2::biggrin2:

Actually Jon it's a business card from one of our sponsors at the top of the page. It was included in a shipment of parts a few years back, and I threw it in the top drawer of the tool box "just in case". Been using it as a gap tool ever since! :biggrin2:

Sounds like it replaced your “out of spec” matchbook :biggrin2:

And there are people here who won't get that, lol. :biggrin2:

I don't get it, don't you use the matchbook to set the gap for the mechanical PTO ?.

Only if you're standing on wood!

:biggrin2:

Will do, that's how I set the one that's in there now, as someone on here had mentioned. The coil has been shipped so I'm going to start tearing it down in the morning.

Update on coil
 

First, I guess you can pull the engine in 30 minutes. It took me a little longer cause I took my time and wasn't in a rush. I had it tore down and ready to go
when the new coil arrived today. I just got it in and back together, she fired right up, happy, happy.
I'll try it first thing in the morning when it's still a little cold out, if it starts I'll know it's finally fixed.
Thank you everyone for the comments and advice. Ray :beerchug:

Good work Ray.:beerchug:
Hope you got it figured out for good now...

Good job Ray! It's a shame that in today's world NEW doesn't mean GOOD !

Glad you got it!

You lucked out that Stan just had the same issue, so it was fresh on his mind.... and everyone elses. :beerchug: