Kohler Magnum 18 Running Badly

[mattd860 161]

I went to use my WH 418-8 with Kohler M18QS yesterday but the motor was running very badly. If the throttle is at 1/2 or higher, the motor rpm surges up and down. It also surges very badly under load. After about 15 minutes I stopped and pulled it back in the garage for some diagnosing.

I bought new spark plugs for it a month ago but never got around to installing them. So I went to install them and noticed that the plug facing the front of the motor was very hot. I know they're supposed to be hot, but this thing was practically red hot, and A LOT hotter than the other plug.

Does that tell you anything? Maybe a bad ignition module? Or an overheating cylinder? My next step is to take off the carb and intake manifold and clean them out thoroughly because I know they're very dirty. I'm also going to adjust the valves because that could be a source for over heating the cylinder and possibly the plug. But what do all of you think?

Note: After replacing the plugs, the engine did not run any better and the front facing plug was still scalding hot.

[Horse Fanatic 49]

I would start with the basics, check both cylinders for spark and test compression on both cylinders and let us know what you find.

[TT-(Moderator) 1,128]

Sounds like the back plug isn't firing - or is only firing periodically.

Pull the wire off of the rear plug, start the engine & see if it acts any differently than when the wire is attached.

Try doing the same with the front cylinder & I bet it won't even run.

[stevebo-(Moderator) 8,059]

Just to add what was said if it is too hot that tells me that it is running too lean (not enough fuel) however not sure how the intake is set up on that motor. It may not be possible for the cylinders to have different fuel delivery :thumbs:

[mattd860 161]

Each cylinder receives and equal amount of fuel. The carb outlets into a single intake manifold.

I think I found the cause of the overheating though. I took some of the engine shrouds off last night and found a HUGE mouse nest in front of the front facing cylinder. It was blocking any and all air flow over that cylinder. All I found was the nest though - no cooked mice

The carburetor and intake manifold are also very dirty. I'm going to disassemble everything and clean it out good. I'm also going to drain the gas, change all fuel lines and add a fuel filter. Currently there is no fuel filter which I think is odd because I thought they came standard on the 87 models. The little rodents practically chewed through the gas line going from the fuel pump to the carb.

I have to imagine all this cleaning and tune up will make her run better.