520H starting to get alot of vibration

[DrabHorse 2]

Hi, my main mowing tractor, a 97 520H running a 60" deck has been running great for the last few years.  It has 995 hours on it.  Just the last 2 times I mowed I am noticing alot of vibration on the foot boards.  It is comming from the tractor itself, not the deck.  I looked it over real well last week and checked all the regular stuff, made sure the drive pulleys were clean, put a new drive belt I had on it, changed oil and filter on engine and trans as they were about due anyway, engine mount bolts are tight ect.  Pulled the plugs and they looked really good, and checked the compression also, I dont know if my comp tester is accurite but atleast it showed that both cylinders had compression and were about the same, they were 110 front and 115 rear.

The engine sounds good and makes good power, it just has developed this vibration that is worse with higher rpm's to where you dont want to keep your feet on the foot boards, it also is making the hood rattle at the hinge on the front some.  Im not going to run it much like this unless I can figure out what is going on.

Edited August 25, 2013 by DrabHorse

[Kelly 1,033]

Eliminate things, you said you changed the drive belt did you run it with out the belt on it?? if no vibration engine is good, then check trans input shaft, fan, loose pulley on trans ect....  if the engine is the problem, pull the PTO bell off check it, again, it could be as simple as a build up of dirt/grass on the flywheel, a broken fin on the flywheel, but first see if it's engine or drive train issue. 

[DrabHorse 2]

thanks Kelly,  yep forgot add that the flywheel looks good and the tranmission fan and pulley are good, and did not notice any play on the tran input shaft.  It does not seem to vibrate much after first starting and running a few minutes, but after getting fully hot after about 20 minutes it is much worse.  I did not try it with the drive belt off with it fully warmed up, so tomorrow I am going to mow for a while and then take the belt off and see what it does then.

[MalMac 1,333]

One other thing on the motor is check to make sure your not getting a misfire on one of your plugs. Onans can fool you by still having power. You can still have good compression with a misfire. Just some times a misfire can cause a vibration especially if it is a misfire that is intermediate.

 

Also check your idler or tension pulley on drive belt. If the pulley is good then check where the pulleys mounting rod goes through the hood stand. They will wear there big time and cause vibration. I just had to replace mine and bush the hood stand. My tractor had a lot less hours than yours and had worn quite bad.

[Shuboxlover 482]

These guys might be on to something...If it starts to smell more gassy when it starts to vibrate, then I would definitely say it's an ignition problem....bad plug, wire, coil etc...

 

My buddy has a JD 318 and shortly after he bought it, he called me and said it's running like crap.  I went over there and found that one of the plug wires were off.  It's amazing these things run as well as they do with only one cylinder firing.

 

Good luck

[DrabHorse 2]

Just got done working on it some more.  I ran it without the drive belt and it is still getting the vibration.  Seems to be less vibration with less load on the motor,  so with the mower deck engaged it is the worst, with it off its a little better, and with the tras drive belt off its alittle better again but still there.  It is kind of a light thumping feeling at lower rpms and gets more buzzy at higher rpms.  I would say that it is not really bad but not normal for this tractor since it was not like that before.  I put my timing light on each plug wire and the stobe looked steady, but that might be hard to tell if it had a very small misfire going on.

I do have two new plugs I can try, just havent done it yet since the plugs looked good, I will try that after it cools down some.  Other than the plugs I dont have anything else to try without buying wires, coil, ect.  I have a few kohlers around but this is my only Onan so I dont have anything to swap parts from to try.

 

note to MalMac - I did have to repair my belt idler bushings on the hood stand last year

Edited August 25, 2013 by DrabHorse

[brandonozz 168]

These guys might be on to something...If it starts to smell more gassy when it starts to vibrate, then I would definitely say it's an ignition problem....bad plug, wire, coil etc...

 

My buddy has a JD 318 and shortly after he bought it, he called me and said it's running like crap.  I went over there and found that one of the plug wires were off.  It's amazing these things run as well as they do with only one cylinder firing.

 

Good luck

Can you just alternate pulling the plug wires off the plugs and see if will run worse or the same?  Ought to let you know if it's just running on one cylinder.  Safety message from Captain Obvious: if it's already running when you pull the wire use an insulated tool to remove the wire!

[DrabHorse 2]

Well here is what I found today.  I remembered that not long ago I noticed the front two bolts of the motor mount plate where it bolts to the frame were just a little loose, so I had tightened them down.  So just to try it out I loosened up those two bolts again and almost all of the vibration along the frame and foot boards went away.  So that didn't really fix anything but atleast its something. 

I was already planning on de-carboning the heads and adjusting the valves this winter.  I guess it will need looked at closer and see if there are any measurments out of spec.

[DrabHorse 2]

One other thing I just though of, when I was looking at the engine I grabbed ahold of the crank to see if there was any play in it and there was not any up and down, but there was a good bit of crank endplay in and out on it.  I see it should be between .006 and .012 and I did not measure it but I know for sure it was alot more than that.  anyway thanks all

[MalMac 1,333]

One thing I just remembered, I know you said you checked the flywheel. Kelly said check for dirt, etc. I had a friend that had an exceptional amount of crud built up on the inside of the flywheel on one of his Onans. He had a little vibration from that. Just a thought.

[DrabHorse 2]

One thing I just remembered, I know you said you checked the flywheel. Kelly said check for dirt, etc. I had a friend that had an exceptional amount of crud built up on the inside of the flywheel on one of his Onans. He had a little vibration from that. Just a thought.

That is a great idea, I will check it out.  Was your friend looking to fix a vibration problem or did it just happen that he found that by luck?

I do remember the last time I had the flywheel screen off to clean it and the flywheel, I was blowing with the air gun around the flywheel and I blew across and around those 4 bolt holes in the middle part of the flywheel where the screen bolts on and those holes must go all the way through because some dust was blowing out of there.  Thanks

[MalMac 1,333]

He knew he had a vibration but was tearing the engine down when he discovered the flywheel just packed full underneath when he took it off. Just lucky that was it. He thought it was internal.