John Deere STX38 runs very fast

I changed the blades a few days ago. Started the mower and set the throttle down low to let it run a bit. I shut it off. I fired it up again and the engine raced and sounded like it was trying to blow itself up. The throttle had no effect on it slowing down or speeding up. I moved the lever a bit and can see 2 arms moving. The 2nd arm only moves when the throttle is up to the choke position. I let it sit over night. Went out yesterday and mowed for a long time. Everything was normal. It ran out of gas so it died. I filled it up. Started it again and the engine raced again. I have started and shut it off multiple times. It still races. I have had governor spring, suggested. Problem is, I don't have any idea what that looks like. I've never repaired one of these engines before. I'm planning to clean the carb anyway, could it be something in there causing it? I need to get this working again soon as it's my only mower.

Thanks
Jason

 

It really sounds to me like the governor gear came off inside the engine. It has little plastic tabs that hold it on the shaft and they sometimes wear off, allowing the gear to drop into the oil pan.

 

How hard is that to fix? Am I in for some major $ here?

Jason

 

Not much money if you do it yourself. It takes a little time to do, but it's not hard. Before going that far, check your throttle shaft in the carb. These commonly break off just below the point where the shaft exits the carb and lets the butterfly come wide open, or even get sucked into the engine. Can you slow it down by manually closing the throttle? I would check this first (I didn't think of it before posting my earlier reply).

 

I haven't tried to slow it down manually yet. It's running so fast that I don't let it run long enough to even get to full speed. I can just tell that it's not right. How risky is it to let it run at this speed?

Jason

 

It's not just risky, it's inevitable, your engine will blow. Do not run it at this speed, it won't make it long and the flywheel could explode, sending shards of heavy cast iron directly at you at high speeds.

 

Well it took it apart today. The plate on the inside closest to the engine seems to move freely independent of the arm on the top. The side with what I believe is the choke plate has a spring on top. The throttle side doesn't. Should it? I also see a flange on the side with the starter. It looks the same as for the carb or for a muffler. It has nothing going to it though. Sould it? So, can the dealer sell me a throttle plate and spring if its needed? Or can I repair the one I have? I tightened the screws on the plate and it made no difference.

Thanks
Jason

 

The plate farthest from the engine is the choke and the one closest to the engine is the throttle. The latter should be controlled with a rod in the arm attached to the throttle plate. That rod goes to the governor. Normally the operator control on the mower acts on the governor which opens and closes the throttle plate with different rpms.

If the throttle plate moves without the governor control the rpms would run much too high.

 

Yep, it sounds like the throttle shaft is broken. You can get one from the dealer. Take the screws out and pull the shaft out and install a new one. Don't worry about the spot on the side of the engine. That is where a fuel pump would be mounted on engines that were made for use with a fuel pump.

 

The throttle shaft does not have a spring on it. Like Cheese says, get a throttle shaft from dealer. Also take model number and spec number with you, without it he cannot order parts for you.

 

I was going to take the whole carb with me as well. Thought that might help more than my descriptions

Jason