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I have a 2006 HPX gator with an FD620 gas engine with the twin carb. I had trouble with it starting so I pulled the carb and went through it all. Pulled all the jets and everything. I had one damaged jet so I replaced that jet with the correct jet from JD. Put all new gaskets everything. Put it back together and started it up and the engine takes off on me going full throttle without throttle linkage moving. I can move the linkage and the engine will stall out. Talked to a few friends and they said it might be getting too much air. I run the engine again and close the choke partially down and the engine throttles back down to a nice idle speed. After about a minute of running at an idle speed with the choke half away it begins to take off again. I figured it was sucking air in else where. I did have a minor oil leak so I broke the top half of the engine down replacing gaskets. When looking at the pistons and cylinder there was hardly any build up assuming that rings on the pistons were ok (have not confirmed compression yet). Putting it all back together and putting the engine back in the gator I start it up and same situation. When it's running I sprayed carb cleaner around the joints as a friend of mine said if air is leaking the carb cleaner will block it momentarily and can stall the engine down sometimes. Could not find any leaks. At this point I am second guessing everything. I have confirmed the jets with the JD parts and confirmed placement of the jets from JD pictures on the parts ordering website. It appears my main jets (75) goes into the bowl of the carb and my idle jets (56.3) goes into the carb. At this point I am stumped. Hoping for a gain a lot of knowledge and guidance from everyone here. Thanks for the help. If any other information is needed I will be happy to provide it and try anything.
 

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Forget to put a linkage on or perhaps put it in the wrong orientation or forgot to put the throttle return spring back on.

You have something wrong during reassembly and it will be nearly impossible to give better guidance without seeing in person.

Perhaps compare the carb linkages and such with those in the repair manual.
 

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Thanks for the response. As far as I can see the linkage is all correct as I took pictures before taking it apart. The throttle return spring is back on and on loosely as designed. I believe it has to be something internally with the carb. I would love if someone had a book and tell me the correct orientation of the jets. I don't think the online catalog is correct. I appreciate the help and your comments.
 

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Have you consulted the workshop manual for your model. They are a bit expensive, but I've found them valuable in certain situations. It is a gamble if it will have the information you seek though.

For the RPM's to increase the throttle has to be opening. Typically the only way for this to happen is to have a problem with the governor, linkages or springs.

Not certain what online resources you're referring too.

Can you quantify how many RPM's it will reach?
 

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I have a hard time believing this is a jet problem as air has to be introduced go full throttle. This can happen in two ways: throttle movement or a vacuum leak allowing unmetered air into the combustion chamber. Fuel (gas) by itself will not cause this. It may cause poor running condition, but not send it to full throttle. In fact too much fuel will flood it.

Put a spring or other mechanism on the throttle to make certain it remains closed. What happens?
 
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