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Dolmar 7900 Vacuum Leak Issue

Discussion in 'Chainsaw' started by Mr. Piste, Apr 15, 2019.

  1. Mr. Piste

    Mr. Piste ArboristSite Lurker

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    Hey, everyone!

    So I recently noticed that my Dolmar 7900 was leaking bar oil out of the screw holes in the crankcase. The issue was that the screws around the bar and chain oil reservoir were backing out, so I tightened them down and checked the ones around the lower-end, which were all tight and fine. I then preceded to conduct a pressure/vacuum test on the crankcase. It held pressure just fine (any amount at all, it didn't matter), but it leaked a vacuum. It only leaked a vacuum to 6 bar, though, and stopped completely and wouldn't leak off or create more vacuum. No matter what I pumped it to, it always came to rest at 6 bar.

    I'm baffled with this one, because my experience with bad crank seals is that they leak either a vacuum or pressure all the way off. The saw has brand new seals, brand new bearings, a brand new crankcase gasket, and a new intake boot. Only the oil reservoir screws were a little lose, and I had been using the saw at work for almost a year with no leaks or other issues. I even built new rubber pressure/vac testing seals for it, but nothing works.

    Any and all thoughts on this are greatly welcome!
     
  2. huskihl

    huskihl Addicted to ArboristSite

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    Leaking vacuum through the decomp?
     
  3. hotshot

    hotshot ArboristSite Operative

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    WTF?, 6 bar is over 88 psi, do you mean 0.6 bar? Fat chance you would ever get a vacuum to hold lower than 1 bar, and 0.6 bar is roughly equivalent to 8.82 psig.

    The standard nitrile radial shaft seals are rated for +/- 7 psig service.

    Try a vacuum test at 15” Hg, and it should that hold it all day long.
     
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  4. Mr. Piste

    Mr. Piste ArboristSite Lurker

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    huskihl: The saw has a plug instead of a decomp. I do away with the decomps on all of my saws, save for the 084s, because I've seen too many saws come in blown-up due to a bad decomp.

    hotshot: Yes, I meant 0.6 bar (must have been confused in the head when I wrote that). It won't hold anything beyond that. I have had an issue with the rubber testing gasket that I made for the muffler holding seal while testing pressure, but I had resolved that by using a second gasket with the heat shield sandwiched in-between the two. The strange thing is it will hold 0.7 bar for a second or two, and then bleed off to 0.6, where it stays.
     
  5. BuckthornBonnie

    BuckthornBonnie ArboristSite Guru

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    Any wiggle to the crank? I had a 6400 with a spun crank, but there were obvious running issues with that.
     
  6. Mr. Piste

    Mr. Piste ArboristSite Lurker

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    I didn't feel any when I checked recently, but I will check again tomorrow after work just to be sure. I hate to think that the brand new bearings may have gone bad, but it's possible.
     
  7. hotshot

    hotshot ArboristSite Operative

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    A 0.6 bar vacuum test is a good pass, you can’t expect a seal to hold any more!
     
  8. Mr. Piste

    Mr. Piste ArboristSite Lurker

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    There's a bit of horizontal movement on the crankshaft (about 1/16th" at most), but no vertical movement that I can feel. How much horizontal play should there be with this shaft?
     
  9. huskihl

    huskihl Addicted to ArboristSite

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    Horizontal...like forward and back? Or in and out?
     
  10. BuckthornBonnie

    BuckthornBonnie ArboristSite Guru

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    There should be no play in a crankshaft properly installed with good bearings and case.

    Video? Pics?
     
  11. Mr. Piste

    Mr. Piste ArboristSite Lurker

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    It's in and out play. When these bearings were installed almost a year ago, they were tight and remained tight until just recently, and that's with pretty much daily use felling and bucking at work, not to mention milling work for myself later on. I'll see if I can get a video of it.
     
  12. Mr. Piste

    Mr. Piste ArboristSite Lurker

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    Alright, here's a video of the issue:
     
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  13. BuckthornBonnie

    BuckthornBonnie ArboristSite Guru

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    Yah... not good.

    The pto side have a wiggle, too?
     
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  14. Mr. Piste

    Mr. Piste ArboristSite Lurker

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    It's hard to tell with so much in and out action happening, but if I pull the shaft tight to the flywheel side (so as to eliminate any in and out movement) I don't feel any wiggle to either end of the shaft. After I replaced the bearings this time last year, they were all tight. What could have caused this to happen all of a sudden?
     
  15. BuckthornBonnie

    BuckthornBonnie ArboristSite Guru

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    Hard to say. The 6400 that I had on my bench last year wore the crankshaft to where a new bearing was a slip fit with no resistance. Sleeve retainer would’ve been an option if the rest of the saw was in good shape (it wasn’t).

    There isn’t a chance that the flywheel slipped off the taper, right? Unlikely, as it looks like the entire crank is moving.
     
  16. BuckthornBonnie

    BuckthornBonnie ArboristSite Guru

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  17. BuckthornBonnie

    BuckthornBonnie ArboristSite Guru

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    Ahh, that’s right. Other forums are edited here. Google my handle and 6400 to find that thread.
     
  18. BuckthornBonnie

    BuckthornBonnie ArboristSite Guru

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  19. Mr. Piste

    Mr. Piste ArboristSite Lurker

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    Googled your thread. That sounds exactly like what's going on with my saw. I've noticed that the piston has some unusual wear (some might be from carbon build-up, but not all), although the carb tuned fine. Now that I think of it, when I installed the new bearings last year they slid on without any resistance. Back then being new to that kind of rebuild, and seeing how easy the old ones came off, I didn't think anything of it. And for some reason, they worked fine for a while.

    I guess this is more of a common problem than I had thought it to be...
     
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  20. huskihl

    huskihl Addicted to ArboristSite

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    Needs a crank or cases. The bearings have spun on either the crank or in the case
     
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