Scag / Kohler engine has me stumped

[Boit]

After reading the initial problem description( which was very good), I'd like to throw in my opinion. It appears that the backfire was from unburnt fuel in the exhaust that just so happened to ignit when the valves were open and sent excessive pressure back thru the intake hence the tube being blown off the carb. I would guess that something caused the ignition to be cut off(interlock switch maybe) and the raw fuel was ignited in the exhaust from normal heat accumulation. Now, what caused the ignition to be killed is up for discussion. There are a few safety switches in the system, such as the obvious; seat, brake release.etc. Since the number of switches are limited, I'd look at those first. Good luck and please report back when you have solved this problem.

 

[MiserableOldFart]

I'd still give a crack at that key start. If it's the old standby "Indak" type switch, they can corrode and stuff gets worn and they can cause problems. They are, as mentioned above, inexpensive and easy to replace, and if it's not the problem, you will have a spare..

From there, I'd check the safety switches, especially the seat one and if there's one on the blade clutch. They are generally very easy to get at. Then wiring, then ignition and timing.

Sometimes the contacts in the back of the key switch corrode and can be cleaned with a sharp knife or file. Had that one happen this year on a much older switch.

I've even had one or two occasions when a safety switch was got to work again with a shot of WD40.

 

[Kernopelli]

Not saying there aren't some great guys here on TBN but Lawnsite.com repair forum has some Kohler gurus that are real gems and Kohler certified mechanics that have helped me immensely in the past. Might join the site and see what they have to say.

 

[aczlan]

Page 25 of the Kohler PDF :

Engine Starts But Does Not Keep Running
1. Restricted fuel tank cap vent.
2. Poor fuel, dirt or water in the fuel system.
3. Faulty or misadjusted choke or throttle controls.
4. Loose wires or connections that short the kill
terminal of ignition module to ground.
5. Faulty cylinder head gasket.
6. Faulty carburetor.
7. Vacuum fuel pump malfunction, or oil in vacuum
hose.
8. Leaking/cracked vacuum hose to fuel pump.
9. Intake system leak.
10. Diode in wiring harness failed in open circuit
mode.

I would guess that 4 or 10 is the issue here.

Aaron Z

 

[aloha]

Thanks for the additional suggestions. I have posted on lawnmowerforum.com and will try lawnsite.com if I don't get it fixed.

aczlan's guess may be on target. I had concluded that this must be an electrical problem because the only difference between the spark plugs firing and not firing is the position of the ignition switch -- they fire when the switch is in the start position, stop firing immediately when the switch goes to the run position.

Today or tomorrow I should receive a new ignition switch and a relay/diode. Logically, the problem seems like it has to be in the ignition switch (something that could be likened to a connection shorting the kill circuit), but testing the diode indicated it had a short. Because all parts need to be shipped in I figured that I needed to order both rather than risk letting the grass continue to grow if the first part did not fix it. I will report on the results.

 

[retiredmgn]

I had a kohler 6hp on a machine. Went from running fine to dead. Finally found the coil had died. Never seen such a thing before on any small engine. Put in a new coil, no problem since.

 

[aloha]

As previously posted, I thought I had the problem narrowed down to either the ignition switch, since the spark plug stopped firing when the switch went from start to run position, or a diode, since my multimeter reported the diode was bad. Replacements for both parts arrived today. There is no change in the symptoms.

A quick recap:
The engine fires and runs with the ignition in the start position, but the spark stops instantly on changing the ignition from start to run.
Disconnecting the wire for the kill circuit makes no difference.
All safety switches check out ok.
All wiring connections appear good.

I am well and truly stumped by this. There is an electronic module that apparently does some magic involving the ignition switch and safety switches, but the manuals give no clue on how to test it, and it is too expensive to get a new one on the off chance it is bad. Troubleshooting is complicated by the Scag and Kohler wiring diagrams not exactly matching what is on the machine (even though wiring is all original).

All suggestions will be most welcome. Thanks.

 

[sld]

Could it have jumped time from the backfire? Does it use a metal or plastic timing gear?

 

[aloha]

sld - thanks for your thoughts. If I understand your question, the answer would be that it is unlikely the timing changed. Timing is controlled by magnets mounted on the flywheel which pass near ignition modules to trigger the spark.

The sparks seem good and strong and correctly timed while the ignition switch is in the start position, but no spark as soon as it goes to the run position. I will not be able to get a timing light on it to confirm timing until I can get it to run without the starter running at the same time.

 

[aczlan]

The sparks seem good and strong and correctly timed while the ignition switch is in the start position, but no spark as soon as it goes to the run position. I will not be able to get a timing light on it to confirm timing until I can get it to run without the starter running at the same time.

What happens if you pull the trigger wire going to the starter solenoid while leaving the key in start?

Aaron Z