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JD 720 carb problems

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Jonathan

04-10-2000 19:01:04




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HELP! I 'rebuilt' the carb on my JD720 AllFuel. We removed and disasembled it. Put it in a fancy SnapOn carb cleaner and let it aggitate for 1 hour. Blew out all the passages. Bead blasted it. Blew it out again. Ran a wire through all the passages. Blew out each passage with carb cleaner. Reassembled with all new gaskets. Put it back on the tractor. It did not run any better than before. The tractor will only run smooth with the choke about half way out. It seems to be too lean. The adjustments on the carb seem to have no effect whatsoever. We are going to try a carb of a 60 that runs perfect just to make sure it is the carb. If the intake is leaking, it must be internal. We sprayed eather on the intake gaskets while the tractor idled and saw no change. Therefore, we 'assume' the intake does not leak. Any ideas or pointers for cleaning and tuning these carbs?

Thanks
Jonathan

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clooney

04-11-2000 16:57:49




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 Re: JD 720 carb problems in reply to Jonathan, 04-10-2000 19:01:04  
Jonathan, it sounds like you still have a plugged idle passage. You will probably have to run the proper sized drill bits through those passages to get them clean. There are 3-4 very little holes in the side of the throttle bore on each side of the throttle plate (they are very hard to see), there should be a brass screw on the outside of the throttle bore right across from them to allow access to them. You will probably have to remove the throttle plate to clean them... "Roberts Carb" has the proper sized drill bits for sale.

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Jonathan

04-11-2000 20:31:20




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 Re: Re: JD 720 carb problems in reply to clooney, 04-11-2000 16:57:49  
I agree that it must be in the idle circuit. The tractor runs decent wide open especially under load. We put the carb off the 60 on it and it ran perfect. At least we now know it is the carb. I can spray carb cleaner in the idle circuit screw hole and it comes out of both of the little holes on each side of the throttle blades. I can spray cleaner in the hole with the brass jet, from the choke side, and it sprays out of both of those holes. I still agree that the problem must be in that area somewhere. Closed throttle or no load is the most problem. With the choke closed half way it idles close to what it should. When you load the tractor or open the throttle, it is way to rich then and the choke must be closed. Above the idle adjustments, there are two plugs, one in each barrel, on the top of the carb. They appear to be pressed in not screw in. I assume then, you are refering to removeing the idle adjustment screws and running a drill through the passages?

Thanks for the help. This isnt the first carb I have rebuilt, but it has been the most frustrating.

Jonathan

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clooney

04-12-2000 14:10:48




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 Re: Re: Re: JD 720 carb problems in reply to Jonathan, 04-11-2000 20:31:20  
Jonathan, one thing you should check is to make sure the throttle shaft bushings are pressed in far enough to be flush with the inside of the throttle bore. If they are not flush with the inside, air will "bypass" the throttle plate in that area and won't pick the fuel up at the little holes... Note: not all carbs are drilled all the way through, some have a step bored in them to press the bushings against, on those carbs disregard this post. Also make sure the throttle plate is not worn and is centered in the bore.

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