Kohler Magnum 20 - No Hot Re-Start

I have a Craftsman lawn tractor with a 20HP Kohler Magnum engine. (Inherited it with the new house last fall, so I don't know all the history.) On the first start, whether it's 50 deg or 85 deg., 3/4 to full choke, and 3/4 throttle fires immediately. I can then open the choke and throttle down to warm up for five minutes or so. (It will go right into running full throttle with no choke, too, but I generally don't do that). No hestitation, and engine runs great with full power while mowing.

My problem comes when attempting to re-start a warm engine. It will not re-fire no matter what the combination of choke/throttle, and seems to be flooded; however, remove the plugs and they are dry.

Air filter and fuel filter are new. Plugs are old, and will be replaced, but cleaning and re-gapping did not help. Only after everything cools off will it re-start, and then with no choke (or throttle) needed.

Time for a carb rebuild?

Thanks for any responses.

Andy

Well, $80 and 20 minutes later, a new ignition module fixed the problem. Used the thing all day hauling much around the yard with nary a hiccup. In fact, the "puff/poof" I was hearing in the exhaust is gone, and it runs smoother than ever.

Thanks everyone for your help.

Andy

Here is what I found out about our mutual problem from my friend I call The Master, who works for Cub Cadet:

The condition is most likely a vapor lock, and there are a few things you can do about it.

First, install a turbolizer kit. This is an easy baffle to install that goes between the carb and the intake, and frequently helps with this condition and with loping at idle. (That won't help mine, I have it on there already.)

The carb may be in need of a rebuild, or it may need the float set richer. Most of these Mags used a Walbro carb with no high-speed adjustment needle, to please the EPA. There is a variable jet instead that moves inside the carb body, and it can malfunction as it gums up. (This is likely my problem, as the plugs read lean and sometimes even after a rebuild, which I have done, the carb still won't perform right.)

In my case, he says the best overall solution will be to replace the carb with a KT-17 carb, where I can have an idle and high speed adjustment screw. Then I can richen the mix for best performance. Second best is a new variable jet carb.

Also, I might add that HVAC suppliers and installers often have insulation for small tubing that you can buy and slip over the gas lines to help keep fuel cool and prevent vapor lock. I have noticed they do use that on the newer CCs with Command engines.

Hope it helps ya out. Heck, I hope it helps ME out. ;-)

Thaks for consulting Master Yoda on the matter. Vapor lock is looking more and more like the culprit, but the "puff/poof" I am hearing still has me wondering about the ignition module. The "turbolizer" sounds like a performance spacer one would put under a Holley 850 on a built 350. Hmmm.... 20 HP now, 200 with a carb spacer??? Muahh-ha-ha-ha!

Thanks. I'll keep you (and evryone) posted.

Andy

I pulled the covers off the engine sides last night, and discovered that Mrs. Mouse had built a nest under the left-side cover. (I won't detail my findings, but let's just say she won't do it again.) Cooling on that side was definitely compromised.

I also changed out the plugs (which I found were two heat ranges too low, and actually an automotive plug).

Buttoned everything back up, and ran at full throttle for about 30 minutes, and couldn't duplicate the problem.

One thing I did notice, though, is that while running full-throttle, there is an occasional "poof" sound in the exhaust. I never noticed it with ear defenders on and the blades turning. Maybe I *do* have an ignition module that's starting to go south. Time will tell, since the grass is still growing, and I'll be mowing again soon. If she doesn't start up after that, I'll check the module, and the ignition wiring.

Are cooling fins and shroud clear of debris for optimum cooling?

buy a can of ether (says starting fluid on the can)

when it won't start - take off the air cleaner and spray a teeny bit into the carb - crank it right away

if it fires, you have a fuel starvation problem

if it doesn't fire, you have lack of ignition

ether can destroy things if overused

I ruled out the ignition module at first, since it *runs* fine, but simply won't re-start. Electronics gremlins like to do strange things, tho. Will a direct-connect timing light suffice as a plug tester?

I'll also look at the cooling fins, etc. and make sure there's no obstruction, and play some compressed air on the carb to cool it off to rule out (or find) vapor lock.

Don't know if I have the low oil switch or not, but could be the culprit, as well; however, does this switch cut the motor while running (i.e. grounding the ignition), or only prevent it from starting (by interfacing with the start circuit)?

Just as a note, in the "hot-no-start" condition, I've seen a small amount of fuel in the carb throat. Seems strange it would be there. Could've boiled back due to vapor lock??

I'm kinda stumped, and have just lived with it. Suggestions?

Ignition modules can cause this but more likely it's high resistance at a terminal due to dirt/corrosion. However, "vapor lock" due to the carburetor being heat soaked after stopping is much more likely. You can easily test this... pour warm (not cold) water on the carb for a minute or two and then try to start it. If you use cold water you will crack the head. Vapor lock can be caused by poor quality gas, wrong gas (winter gas being used during the summer, they are different), some additives in the gas, engine running too hot due to wrong timing, cooling fans/shrouds missing/damaged, dirt/grass clogged fins on head/shrouds, exhaust restricted due to wrong/damaged muffler.

If the plugs read lean, overheating is almost certainly the problem.

It stumps me why it won't even try to fire with gas in the carb throat/manifold. That would indicate no spark. But pull a plug, and you'll get spark. This engine comes with the Oil Sentry feature, and I wondered if somehow the switch (which is brand spanking new from Kohler) could be shutting it down like there's low oil pressure. I have had some trouble with Kohler-supplied parts before. I may try getting it hot and then jumpering that to see if it makes a difference on restart. Or it could be a defective wire to the switch, too, I guess.

It's not many engines that stump me, but this one is one of 'em. Appeciate the input.

Yes, a faulty oil pressure switch or wiring can cause the problem as well.