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First Post! Tried the Search function, but didn't quickly come up with the answer.

Background: I'm an experienced mechanic (cars, ...), and have had this Yardman 999 since 1998 (MTD, but built by Kwik-Way). Runs/mows great, with maintenance. Has the original Kohler Command CH22 engine in it.

Battery great, fairly recent new coils, doubt the flywheel key is broken. No fuel/air problems. New OEM ignition switch installed (no change).

First start of the day, cranks great but won't fire. Release the key and it (correctly) springs to the run position. Then often "BOOM!" -- backfire through the exhaust. Not a stuck valve.

Quite convinced that the coils are being shorted somehow while cranking (switch in the START position), but then are active when in the RUN. If I crank briefly and let go of the key, a few times, it will barely start then run fine. I'm guessing a short somewhere?

We are going to need the full engine number.
If you have one of the SAM type ignitions with a powered ignition timing control then it is most likely not getting power.
I have a fleet of hire cars and they have the same problem, when cranking the voltage to the ignition unit drops to 10V and they won't fire.
The exact same engine with points fires right up while cranking
If this problem has only started happening since you changed the ignition switch, then the problem will be in the switch.
The power to the ignition might be coming from the alternator output which is 0 till the engine fires on some configurations

Otherwise the ignition is self contained and is controlled by grounding the module in which case it has a separate circuit inside the switch,
M + G = off
M + nothing = on
Letters are on the back of the switch

However before we start guessing we need more information.
1) full model number from engine
2) what type of ignition ( the SAM type will have more than 1 small wire going to the coils )
3) a full service manual for yo engine is available from Kohler and from https://mymowerparts.com/about_us.php

And because I always like to know this stuff
When was Yardman acquired by MTD ?
As MTD started off making press dies then did pressings & finally complete assembly I was lead to believe that acquired all of the other brand names over the years
Thus I would have imagined that Yardman would have had this mower made by Kwick Way direct rather than Yardman ordering them from MTD who subcontracted the assembly to Kwick-Way.
Not that the latter was impossible but from here it looks like most mower makers went belly up or merged between 1980 & 2000 and the smaller brand names got acquired by the bigger companies.

bertsmobile1 said:

And because I always like to know this stuff
When was Yardman acquired by MTD ?
As MTD started off making press dies then did pressings & finally complete assembly I was lead to believe that acquired all of the other brand names over the years
Thus I would have imagined that Yardman would have had this mower made by Kwick Way direct rather than Yardman ordering them from MTD who subcontracted the assembly to Kwick-Way.
Not that the latter was impossible but from here it looks like most mower makers went belly up or merged between 1980 & 2000 and the smaller brand names got acquired by the bigger companies.

Click to expand...

Yardman /MTD was in the 1990s., maybe earlier...

bertsmobile1 said:

And because I always like to know this stuff
When was Yardman acquired by MTD ?
As MTD started off making press dies then did pressings & finally complete assembly I was lead to believe that acquired all of the other brand names over the years
Thus I would have imagined that Yardman would have had this mower made by Kwick Way direct rather than Yardman ordering them from MTD who subcontracted the assembly to Kwick-Way.
Not that the latter was impossible but from here it looks like most mower makers went belly up or merged between 1980 & 2000 and the smaller brand names got acquired by the bigger companies.

Click to expand...

MTD acquired Yardman in the 1970's. White and Cub Cadet where purchased in 1981, Aircap and Arnold in 1982, Then Troy-bilt, Bolens, and Remington in the 2000's

ILENGINE said:

MTD acquired Yardman in the 1970's. White and Cub Cadet where purchased in 1981, Aircap and Arnold in 1982, Then Troy-bilt, Bolens, and Remington in the 2000's

Click to expand...

Thank you gentlemen

Yep, it was made by Kwik-Way/KW Manufacturing, in South Dakota, for MTD.

I replaced the ignition switch to try to fix this problem, but there was no change.

The new ignition modules ("coils") were OEM Kohler 24 584 36-S. The instruction sheet mentions a "SAM"; I'll have to research what that is and if the engine has it.

Family SKH624U1G2RA
Model No. CH22S
Spec No. 66527
Displacement 624 cc
Serial No. 2509301107

Thanks for the help!

Yep, you have the Smart Advance Module ( SAM) fitted you your engine.
It is a failed method of getting a proper and responsive advance curve for the timing.
I will guess that the power supply for the module either comes off the wrong terminal on the switch or it has a bad ground so is suffering a big voltage drop when cranking or you battery is just plain past it.
Try jumping the mower from a good known battery or your car

bertsmobile1 said:

Yep, you have the Smart Advance Module ( SAM) fitted you your engine.
It is a failed method of getting a proper and responsive advance curve for the timing.

Click to expand...

Interesting, thanks! Has worked well for me ... until now.

I will guess that the power supply for the module either comes off the wrong terminal on the switch or it has a bad ground so is suffering a big voltage drop when cranking or you battery is just plain past it.
Try jumping the mower from a good known battery or your car

Click to expand...

Switch/wiring is OEM, so unless a terminal in the plug, etc. has gone bad ... a failed ground will be my guess (vibration or corrosion). Know where the ground-point(s) are?

The battery isn't new, but seems quite strong -- I'll try the jumpering first (probably this weekend).

Had (only) a few minutes just now, so I did a couple of tests:

Battery at rest: 12.6 V

Cranking: 10.3 V

And, of course, it started up fine. :confused2:

There is a rectangular black-plastic module-box on the engine, on the left-bank valve cover/housing in the spot opposite the fuel pump that's on the right. I assume that's the SAM box.

Next try will likely be Sunday ~midday. I'll likely put my little, plain 1.5 A charger on it for a few hours before, and during cranking.

{Each winter I take the battery out, clean it, fill it with distilled water, then charge it for a month or two with a different pulse/desulfuring trickle-charger. I never use those pulsing-chargers directly on equipment with electronics, including this tractor.}

Re: CH22 Ignition Short? [Solved; thanks!]

Yardman999 said:

Next try will likely be Sunday ~midday. I'll likely put my little, plain 1.5 A charger on it for a few hours before, and during cranking.

Click to expand...

Worked like a charm; started right up.

Thanks for the clue-in to the low-voltage problem for the ignition module! {Despite it cranking strongly.} Sites' forums, like these, are sure great for us grease-monkeys.

So now on the to-do list, in the weeks ahead, is to check/clean the battery cables/ends, and likely to replace the battery. Then hopefully smooth sailing for another 20+ years. :thumbsup:

P.S. The old battery did die a few months later. New (same type) battery was cheap, from Walmart, so should have replaced it back then.