Need help on JD 720 Diesel

Howdy,

Got an issue with the 720 and wanted opinions on the trouble before I dissect this thing too deeply. A while back I did some work on the transmission, which required a removal of the flywheel. The tractor ran fine for a while after this with one exception. When going down a hill the tractor would free roll until it hit the bottom of the hill. It would do this in any gear. When the bottom was reached the tractor would take off again in the normal speed with no action by me. That is problem number one.

A while later a problem appeared with the flywheel. It would move out on its own despite my efforts to tighten it as much as possible. It is a replacement flywheel and not like the original with a nut and two tightening bolts on either side of the crankshaft. It has a splined collar inside and a big square nut to tighten. Along with the flywheel issue the tractor has developed a clunking sound in the motor as well as some oil coming around the pony exhaust pipe where it enters the big engine's breather pipe.

Whew!

I realize it is a bunch of stuff but I wonder how much is connected. The transmission work I did was to replace a bearing on one of the top gear shafts. Followed the service book to the letter. However, I'm sure I must have done something wrong or just have bad luck, ha ha. I appreciate any help, ideas or suggestions you can offer. God bless.

--old fashioned farmer

Better ask on the JD forum here. When you tightened the flywheel did you have the clutch thrown out. I wouldn't run it till you get the flywheel adjusted and tight. I don't have a clue why it free-wheels down hills. Had a JD B that jumped out of high range just as you crested a hill, but it wouldn't go back in! Vic

I don't know about the rest, but sounds to me like you didn't get the "taper-lock" flywheel anywhere near tight enough. You need to block the crankshaft to the left side from the pulley side, then tighten the square nut with a 36" pipe wrench, then hit the flywheel with a sledge hammer to drive it further on the taper. Tighten nut, hit flywheel, tighten nut, hit flywheel, etc. Get the picture?

I have a 1956 John Deere 70 D. When I had bought it I found out that there was a tapper lock fly wheel on it and that the person did not install the fly wheel on to it right. The tapper lock was all wrecked up. I had to replace the fly wheel with a stock one off of the same type of engine. Make sure your tapper locks are torque down right if not they will get wrecked. Snap on sells a hug torque wrench but it is alot of money. I should know I had bought one just for this type of work sad to say it got stolen last year out of my tool shed.

The only way that tractor can freewheel is if it's not in gear. Maybe your shift rails aren't adjusted right and a couple of gears aren't fully meshed. It's the first thing to pop into my mind. I'm thinking maybe the free-wheeling comes from the backward pressure on the gears while going downhill throwing it into neutral and then at the bottom of the hill the detent re-engages the gears when the power flow changes to the other side of the gear teeth. The one I'm thinking about is the Hi-Low. I'm more familiar with the A and 630 transmissions but the 730 might have the same characteristics. Let us know what you find so we can be educated too.Jim

Howdy,

Thanks for all the ideas guys. I'll definitely do more on the flywheel tightening. I knew there had to be a trick to it other than what looked like the obvious to me. I learn something new about these old monsters every time I work on one. It's a good education though.

The detent idea sounds like it might be the ticket on the tranny since that is one of the things I did have apart and could have gotten put in wrong or just off kelter. I hate to have to dig back into that transmission again but if that's what it takes then that's what it takes. Gotta have this thing back in running order before harvest. Thanks again everyone. I'll let ya know what comes of it. God bless.

--old fashioned farmer