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Bent Crank?

IndyIan

IndyIan

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Hi Guys,
I think I've got a bit of saw (372)trouble... Last night after I was cleaning it up I noticed that I had a tight spot on the chain when I spun it around by hand. Did some research online and found that it could be a bent crank or an egg shaped sprocket. Since the crank is not turning it has to be the sprocket! YAY! they're cheap!

Then I put on the compression release, pulled the chain out of the groove at the middle of the bar and turned over the motor... I could feel the chain tighten and loosen significantly with each revolution... not good... this can only be the crank since the sprocket isn't turning...

So am I in for a new crank? I was planning to take the saw in this week anyways as it has starting revving to high for my liking unloaded. Related maybe? Still cuts fine and still "four strokes" a bit but when compared to the sound bite on madsen's saw tuning page it revs alot higher. Could be different saws I guess but I don't want to risk it.

Any advice? I have been quite careful not to over tighten the chain when hot but I have got pinched a few times and have seen the chain pulled pretty far off the bar while I was freeing the saw... Tryed not to pry it out but I guess I did... Rookie mistake :(

Anyone have an idea what the allowable run out is for the crank? Can I measure it with the clutch on? I'll have a look tonight pulling it over with the bar off and see it I can see it moving.

My buddy has a 371 so I'll go over to his place tonight and see how it compares. Maybe I'm making a big deal over nothing.
Thanks for any help,
Ian
 
Dennis

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Remember, every time the crank makes a revolution the magnets go by the coil...they want to stay there....therefore creating what may feel like a tight spot. You say it sounds higher than the sound byte on Madsens...that could just be a different muffler...if its still fourstroking...then you probably aren't too lean....but its very hard to tell without actually having the saw here. If you are concerned about the tight spot and cannot tell what is causing it...pull the plug...take the bar and chain off....and pop the ignition out of there....then rotate the flywheel by hand....that should give you a perfect freewheeling crank.
 
eyolf

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I doubt if you've got a bent crank...you'd be noticing a lot of vibes. If you're using rim sprockets, many of them are out of round at least a little, and I've also seen them start OK and wear in out of round too...harder metal in some places, I guess.

Pull the plug and watch the end of the crank while you pull it through. You should be able to see as little as .010 out with no trouble. If you're not sure, put the saw on a sturdy surface, hold it down tightly, and try to set up something you know cant move as a tell-tale next to the end of the crank while you pull it through. I wouldn't get too excited about removing the ign...there is a pull from the magnets, but it won't be any stronger than crankcase compression.

I bet you're fine!
Good luck.
 
mikek711

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I'm betting its the chain also,
All of my bikes with chains have a tight spot.I've never noticed it on the saw though.I probably dont tighten the chain to the point of noticing it.
I know that all chains do have that tight spot in them so I wouldnt worry.Let us know though.
Mike.
 
glens

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Mike, try to avoid hitting the (bike) chain with a pressure washer.  I ignorantly ruined a chain on my VF750F that way many years ago.  I guess it gets by the o-rings and washes the lube out, then the o-rings seal back up... (assuming you've got o-ring chains)

Ian, it could also be the clutch hub or it's bearing.  Got a dial indicator to use in conjunction with the other advice given?  That'll be the only real way to totally eliminate the bare crank itself.

Glen
 
IndyIan

IndyIan

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Thanks for the help guys,
Last night all I had time for was to check how the mixture was doing. Bought a new plug and did about 20 cuts on an old 8" birch log. I've attached a pic of the plug.
Part of it looks fine but the white part scares me. I've got the high jet as rich as it will go. The low seems fine as the saw idles and accelerates good. No holes in the air filter, I checked closely.
Could be the fuel filter but I've only got about 40 hours on the saw and haven't been dumping saw dust in the tank...

I think I'm going to just take the saw back to the shop and let the mechanic look at it. I've still got 14 months of warranty on it and I hoping if they do all the work on it, if things go bad I'll have a better chance on being helped out.

Doesn't seem like many cranks get bent so hopefully its the clutch bearing or something cheaper than a crank!
Thanks again,
Ian
 
bwalker

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Ian, The condition you describe is actually quit common. Depending on the oil you are using the plug should be completely white/very light mocca colored.The darkness you are seeing is just a indicator the plug isnt getting hot enough on that side to burn off combustion deposits.Besides plug color is a very misunderstood way of determinng a motors state of tune. To properly read a plug you dont even look at the tip rather you inspect the mixture ring on the insulator down inside the plug where it meets the metal shell.
 
tundraotto

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and this is only done while you run the saw under load and then hit the kill switch and inspect the plug. light brown is good. whiite is bad and black is, well normal for a homolite etc....:blob2:
 
Tzed250

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Bent Crank

The only reliable way to tell if the crank is bent is to place it between centers and put dial indicators on both journals. This will show any radial or axial runout.
 
IndyIan

IndyIan

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Thanks for the info again guys.
Last night I took the clutch housing off, the bearing seems fine. I took the plug out and had my girlfriend pull the saw over slowly while I held it down with the end of the crank just touching a block of wood. I'm 99% sure that is bent. The saw was moving around a little on its antivibe mounts but I would bet a lot of money that its bent.
It's tempting just to run it as the saw runs fine but I think this maybe a case of 'pay a little now or pay a lot later'...

I'm curious how long it takes to tear down a saw completely? By a mechanic that is.
Thanks,
Ian
 
bwalker

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and this is only done while you run the saw under load and then hit the kill switch and inspect the plug. light brown is good. whiite is bad and black is, well normal
otto, What you describe is the propper way to do a plug chop/reading. However the color scheme you mentioned has nothing to do with it. What you are looking for is the mixture/soot ring on the insulator. The closer the ring gets to the metal shell and the lighter it gets the leaner the jetting is. The ring it self will bealways be black in color
 
IndyIan

IndyIan

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Hi Dennis,
The side bearing is the one in the crank case? No play on the end of the shaft that I found by trying to move it by hand.

I think what bent it was when I was bucking some large sugar maple tops earlier this winter. I got pinched a couple times just as I finished the top cut and the 12" branch slid down grabbing the bottom of the chain pulling it pretty far off the bar... Lately I've learned to angle some of these cuts and no more pinching.

Thinking back, I probably should've bought a small walmart saw to learn with and make all the expensive mistakes with it and then use the 372. I haven't even done much falling yet so I might take my own advice, it'd be real funny to replace the crank and then squash the saw! :mad:

Next week I'll be able to take the saw in and I'll let you know if the cank is bent for sure.
Just so I know, the crank is three pieces, is it possible just to replace the piece that the clutch rides on? If its the only bent piece?
Thanks,
Ian
 
Newfie

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"Thinking back, I probably should've bought a small walmart saw to learn with and make all the expensive mistakes with it and then use the 372."

A couple of $3 felling wedges would make cheap insurance from getting the saw stuck in the cut. Plus you won't have a bunch of slanty firewood next year.;)
 
IndyIan

IndyIan

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Good point Newfie, I used them this weekend using my Dads "powerMAC". 35 cc's of raw power! Actually it cuts surprisingly good with the narrow chain. The anti-vibe leaves something to be desired though... It's better than no saw though!
 
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