755 Yanmar Engine Vibration

[BBO]

This is a 1987 model. 755 Tractor with a 60" mowing deck and a 3 cyl diesel engine.
The hour meter was not working for several years but I suspect ~1500 hours.
The engine was serviced regular for the 1st 10 years then not so much after it was passed down to a relative.
Last year it had been sitting for 2 years and would not crank. They had said it vibrated really bad at low RPM's that they suspected the engine was bad.
They had a shop look at it late last year and they found a bad glow plug; replaced all three.
Then they decided the injector pump needed to be rebuilt. $ 750, done...

It cranks fine now but at low throttle the engine really shakes.
Also it seems low on power as any amount of grass will slow the engine down and as it bellows out black smoke as it's catching back up to speed.

After I got it I found the deck gearbox was bad. This is now all fixed after $500. Also a new universal on the deck, plus a new universal joint up from the engine to the transmission.

This all helped the vibration and power issue some, but it's still excessive.

I had one mechanic (large tractor guy, not a mower man) listen to the vibration and he said to shut it off because the "harmonic balancer" on the front of the engine was about to fly apart.
So back to the parts page I go to NOT find anything called a "harmonic balancer" only the main pulley in the front.

He says "if you take the front end all off and that pulley you will find your problem, as it will be obvious."

So as of now I am still running it some and I adjusted the low throttle speed up somewhat. At wide open throttle, it seems smooth.

I kinda suspect the timing may be off with the rebuilt injector pump, but I do not really know how to adjust this at the moment.

I was worried that the rings might be worn or damaged but the thing is not used enough yet to see if it's using oil.
Some on this forum say these little engines will last for 2 or 3 times the hours I have on it now...

So far I have not added any engine oil, but I have changed it twice because it was so black and thick when I got the tractor.
The fuel system was dirty and I am on the 3rd fuel filter as I am getting it cleaned out with Seafoam in the tank.

The question is does anyone have experience with the pulley on the front of these engines?
This fall I could take the radiator and fuel tank off so I could access that pulley. I hope I would only need to replace the pulley and a seal.

Any other suggestions?

 

[JDTank]

The purpose of the Harmonic balancer is, like it sounds, to compensate for the pistons chugging up and down at different times.

Harmonic balancers are filled with a silicone like material, almost like liquid but thicker than that. After about 6 - 8 years, the material inside of the harmonic balancer will and does harden. Once it hardens, it can no longer do it's job, since it cannot move around.

This definitely could be an issue, and if the balancer is not too expensive, it would be worth it to try and replace it.

You could have a bad fuel injector with a fouled spray pattern. At some point though you have to stop dropping tons of money on this thing, unless it holds a special meaning to you.

 

[BBO]

This definitely could be an issue, and if the balancer is not too expensive, it would be worth it to try and replace it

Well the little tractor does have sentimental value to me.

But I do NOT see and "balancer" on the parts list. Does this engine have one?
I see an M89138 item 7 pulley that is $55.
Am I looking in the WRONG PLACE? :confused2:

View attachment 755_20-16.pdf

 

[JD755]

Just a comment FWIW. My 755 runs very smooth and strong above 1500 rpm. Below that it vibrates. Sounds like maybe not as much as yours, but enough that when I idle the engine I set it to just above 1500. It has been that way for so long I can't remember if there were ever a time it did not do that. Hope you get your 755 up and running better.

 

[BBO]

My 755 runs very smooth and strong above 1500 rpm. Below that it vibrates.

Thanks.
The lack of response leads me to wonder if it is NOT at least a "common" issue.
So far no one has suggested that these engine have any type of balancer at all.
I might need to take the 3 injectors out and have them mist/pump tested. Seems like they are about $100 each.
If one was partially stopped up I suppose this could cause the symptoms.

 

[BBO]

Just a comment FWIW. My 755 runs very smooth and strong above 1500 rpm. Below that it vibrates.

I was going to adjust the timing but it looks like the 755 is not really adjustable. It's a 20/1000" shim.
The manual shows to NOT use the old shim (which I suspect they did) but there is no timing marks like on the 855.
Remember the PO had some shade-tree mechanic remove the pump to have it rebuilt.
Strange...

Also on the low idle speed it looks like the manual wants it at 1450 (+/- 50) RPM. (It was at ~1300)
It says 3425 RPM max and I was never able to get mine adjusted over 3100 RPM.

Manual pages attached if anyone else wants to look at them...
View attachment 755_Timing-Idle_398-399.pdf

 

[tiresmokin]

I also have a 755 and it seems to run quite rough around the 1400 and lower RPM range, although my tractor surges at start up no matter where the throttle is positioned but clears up after about 20 to 30 seconds. My 755 only has 345 hours on it so no one has touched the injector pump, I replaced the fuel filter and both air filters at which time I had to bleed the filter and the injector pump to get it started so I'm thinking that air in the line is not the issue. I put seafoam in the tank in hopes that if its an injector getting clogged that this will fix the surging and shaking.
If it works I will let you know, but if someone else has any suggestions I'm open for them.

 

[DrBill]

755s do not have a harmonic balancer. They have a solid steel front pulley. The 3 cylinder engine does not run well below 1450 rpm.. Engine performance should be measured at 3000-3400 rpm.. The engine governor has an adjustment that does make a big difference at running speed. Most people nor dealers know about the adjustment as it's not published or explained by the factory.. Also, The engines are mounted on 4 light rubbered mounts that do shake a lot at stall speed..

Your research was correct about .020 timing clearance. Timing is not too critical on these engines..