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JD 420 Engine Vibration

2575 Views 14 Replies 8 Participants Last post by  jd_430
I’ve got a JD 420 with the Onan 20 HP “B” engine that has a nasty engine vibration. It gets progressively worse as the engine rpm is raised. I’ve taken the engine apart and inspected it but can’t find anything that would lead to this type of vibration. I haven’t had the flywheel balance checked, but there isn’t any sign of mechanical damage to it. The keyways are all in good shape, there’s no debris in the oil pan, and the crank, pistons and valve train all look fine. I meant to uncouple the engine from the driveshaft and running it to see if the vibration went away before removing it, but forgot. However the driveshaft ujoints are tight and not binding. Minimal play is present on the driveshaft end going to the pump. The engine was securely bolted down before I removed it.
The engine had been using a significant amount of oil the last couple of years and is due for an overhaul, but I’m hesitant to spend the money on overhauling it unless I find the source of the vibration. If I can’t nail down the cause of the vibration, I’m leaning towards replacing the Onan with a Honda engine. I love the tractor (bought another one this spring) and would have no problem spending the money on the new Honda engine, but I’d sure love to figure out this problem first. Anyone have any ideas? I keep thinking I’m overlooking the obvious.

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I would think its probably your drive shaft.I would take it to a driveline shop and have them spin it.you could also check the bearing in the front pto.if it was your crank causing this vibration I would think you would have broke a rod by now.also it would be ticking like crazy.if theres no ticking noise I'll bet its in the driveshaft.

Now would be a good time to invest in the heavy duty rear yoke! It has the 2 bolts that clamp it to the hydro input shaft.....but I run mine with just enough clearance to let it slide on the shaft.
I have a 420 setting in my garage , like Bret it has a vibration problem . It started with the father-in-law telling me it had a hydro leak, then i took the seat pan off the guy , and looked the seal at the shaft of the tranny was leaking. Easy fix, dropped the drive shaft off replaced the seal , good to go ! NOT!!
I began to mow with the lil guy and at first it worked great , then when i used reverse there was a bad vibration like Bret is talking about . I looked under the 420 , and i have a nice hydroleak again. Now I'm like Brian wondering about the origin. I know the front of the shaft has a little movement , but not very much . So i'm wondering if what Bret has is a frequent thing ?
Jim can you expand on the heavy duty yoke?. As im doing research on it , while im setting in this chair recovering from knee surgery LOLOLOLOL
and thanks
What happens with the stock slip yoke, it wears over time and gets sloppy on the hydro shaft causing a vibration.
The heavy duty yoke was used on 430's and is an option in the parts list for a 420. It has a split collar with 2 bolts acting as clamps and can be tightened to the shaft to reduce clearance in that area.
Ok Jim , I can get this still at JD?
and will it cure the vibration ? and I take it I need another seal at the front on the tranny?

As far as I know you can still get them from deere. I took care of the vibration I had on my 420 years ago.
The seal may need a new bearing or bushing by now too.
I guess it would depend on how bad things are loosened up from running it with the vibration?
Got ya Jim, And thanks I appreciate it.

Bret , I'm sorry i feel like i hijacked your thread , I apologize .

No problem Keith.

Actually I should have added in my original post that the engine vibration was so bad that it broke the muffler bracket and shook the intake manifold bolts loose. This would lead me to believe the vibration is located in the engine, especially since there isn't any vibration in the rear of the tractor (seat area) like I would expect if the driveshaft was binding. I pulled the driveshaft out and inspected it. The splines were a little dry (no grease) but have little play, and the rear ujoint doesn't bind, but certainly isn't as free as the front one since I neglected to grease it for the past couple of years. Shame on me. I guess I just forgot the grease zerks were there. Anyway since I already tore the engine apart and couldn't find any cause for the vibration, I plan on replacing the yoke with the heavy duty piece per Jim's suggestion and put everything back together again.
Thanks to everyone for their input.

My dealer just fixed my drive shaft vibration with a kit from Deere that clamps on the transmission shaft with two bolts, thus eliminating play from worn splines. The kit was $70.
You may also want to check the pump shaft and bearing. I had a vibration and leaking seal on a 318 that I thought was being caused by the drive shaft, turned out that the pump shaft was worn.
Keith, if it is leaking that soon you probably need to replace shaft, Very pricey.
Ok guys , I ended up replacing the needle bearing in thew pump , a new seal. and i got the new heavy duty splined end for the drive shaft, all new bearings on the universals. As of today I have mowed the yard 4 times (acre yard) , and i have no oil leaks, and no vibrations . I hope I got it this time. Thanks to all
Hi All, Gary Parsons was right . Not only did i replace that shaft but the whole pump and tranny. The shaft was bent something close to 40 thousands out of true in a full rotation. Have started it several times. No vibration, no leaks.........................so far.....
I'm working on a LA105 lawn tractor for a friend.
After about two laps around his front yard the right rear wheel begins to squeak. Is there a bearing or bushing at the end of the trans case where the shaft exits? We're thinking if so, it is worn or the shaft is running dry in the bushing causing the squeak.
Anyone else ever have this issue?
Thanks in advance for any replies.
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