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Suddenly, my Kohler Courage 20HP (TroyBilt) is running flat out, revving well above its usual highest RPM. The throttle control has no effect and the throttle linkage on top of the carb (circled in red on drawing) seems to be frozen and will not move/pivot either by hand or with the throttle cable. I know absolutely nothing about carburetors (including the basics of what they do and how they work)... any ideas why this might happen and how/whether I can fix it?

The engine governor linkage has come loose, or the carburetor shaft is stuck from dust or debris and will no longer move. Try spraying the upper lever area with WD-40 lightly and let it soak in for a couple of hours, then get it moving and spray it a little more. Keep working it back and forth until it loosens up nicely.

If that does not resolve the issue....

A rare possibility is if the governor has failed and come apart.

My suggestion is to find a reliable and reputable independent local small engine mechanic before considering a regular shop. Regular dealer shops have a high turnover of people with little or no experience.

Edit:

As the reply following this one state, there is a chance one of the throttle blade screws came out and now it is jammed wide open. The intake will have to come off and the carburetor separated for inspection.

There could also be a chance that something got in the intake and made its way to the throttle plate to hang it open.

Screws might have come loose and the throttle plate is wedged in the carb.

Auto Doc's I hit the throttle linkage with penetrating oil a couple of times last night and WD40 this morning after reading your post, and am seeing no improvement. I'll pull the carb today or tomorrow and take a look. Thanks for your advice!

VegetiveSteam Thanks for replying to my post! It may be a day or two before I get the carb off and I'll update the thread with my findings.

It looks like VegetiveSteam called it. I found that the top screw on the butterfly valve was missing, the bottom screw was loose, and the valve was jammed up, off-center, inside the carb.

I did not find the missing top screw. Is it likely to be inside the head? How could I find out, and how big a problem is the screw likely to be if I leave it wherever it is? It looks like the bottom screw hole is pretty well chewed up, so maybe the top screw has been out for a while without causing any other problems?

By now the screw is either in the muffler or imbedded in the head or top of the piston. It shouldn't really be a problem unless it's imbedded in the edge of the piston where it could groove the cylinder. If you have or can get a small inspection cam you could maybe see inside the spark plug hole and look around. They are pretty cheap and work amazingly well.

VegetiveSteam said:

By now the screw is either in the muffler or imbedded in the head or top of the piston. It shouldn't really be a problem unless it's imbedded in the edge of the piston where it could groove the cylinder. If you have or can get a small inspection cam you could maybe see inside the spark plug hole and look around. They are pretty cheap and work amazingly well.

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I have a cheap borescope and finally managed to wrestle it around to view the entire piston top... no screw, so I'm just going ignore it and hope I don't see it down the road. Looks like now my biggest problem is going to be finding a replacement butterfly valve and screw set... Google has not been kind so far. I may have to buy a used Nikki carb off Ebay or pick up an aftermarket knockoff carb and hope the valve and screws are interchangeable.

Thanks very much for your help with this, and Auto Doc's too. I'd still be out there poking the engine with a stick if you guys hadn't stepped up to help me.

cmayo said:

I have a cheap borescope and finally managed to wrestle it around to view the entire piston top... no screw, so I'm just going ignore it and hope I don't see it down the road. Looks like now my biggest problem is going to be finding a replacement butterfly valve and screw set... Google has not been kind so far. I may have to buy a used Nikki carb off Ebay or pick up an aftermarket knockoff carb and hope the valve and screws are interchangeable.

Thanks very much for your help with this, and Auto Doc's too. I'd still be out there poking the engine with a stick if you guys hadn't stepped up to help me.

Click to expand...

I'm retired now but I wasn't allowed to suggest any other fix than a new carb for this failure. Some carbs had a throttle shaft kit available but not the throttle plate.

VegetiveSteam said:

I'm retired now but I wasn't allowed to suggest any other fix than a new carb for this failure. Some carbs had a throttle shaft kit available but not the throttle plate.

Click to expand...

Yeah, I had just started to Google "throttle plate" instead of "butterfly valve" and am seeing more hits for shaft-and-plate kits. Haven't tried to dial one in for my Nikki 2005395 carb but worse comes to worst, I'll snag a cheap aftermarket carburetor and bolt it on or cannibalize it for parts.

Just wanted to close out this thread with my results. I bought a used Nikki carb off eBay for $25+shipping. It turned out to be a different carb but the throttle plate matched and the screws were there. After replacing the throttle plate, waiting for parts that never arrived (so I had to use my old ones) and finally figuring out how to get the speed control bracket adjusted back up, the engine is running like a top and I was finally able to mow yesterday. Yay!

Thanks to all who helped me with this! I wouldn't have had a clue without you guys!