Chainsaw won't start with bar installed

[Mauibuck]

Stihl MS 460, 27-inch bar, couple of decades old. Chainsaw starts fine without bar and chain. It won't start with the bar installed. If I support the weight of the bar, the chainsaw will start and run fine. If I support the weight of the bar and chain, it will start and run fine. There is something about the weight of the 27 inch bar that is causing the problem.


Here is the rig I made to support the bar and chain while starting with the chain break engaged. It is a half inch copper pipe, inside a half inch PVC pipe, inside a 1 inch PVC pipe, supported by 4 bricks. Ugly, cheap, works, but obviously just for troubleshooting.

What is causing the weight of the bar to affect starting?

 

[Bob Hedgecutter]

Um, no.
Something is wrong.
Does your bar have two adjuster holes per top and bottom of the bar slot?
Is it missing one of the bar studs?
Does it start with the bar and chain on if the chain brake is off?

Need to take the cover off, take photos of the bar tail mount and the mount position on the saw, the clutch area and drive sprockets etc- then get back to us.

 

[Mauibuck]

Yes, it will start easily with the chain break on or off as long as the weight of the bar is supported. I tried starting it with the bar on but with the nuts only finger tight to make sure the tightness of the nuts was not a contributing factor. It wasn't. Just the weight of the bar seems to be the issue. I could try putting heavy lead weights on the studs with the bar removed and try starting it. I think the issue will come down to the torque on the two studs caused by the weight of the bar. This is a really weird problem.

 

[Bob Hedgecutter]

If the bar nuts are tight- the bar does not move- so the bar really has no bearing on the function of the engine.
It is the correct type of bar,

My best guess is something is binding within the PTO side of the saw- clutch, needle bearing area.
You should NOT be able to "relieve" weight off the bar if it is fitted correctly.

 

[Bob Hedgecutter]

Outside possibility- if the AV is that flogged out, the saw can be moving excessively between tank and crankcases- if there is a bare wire in the mix somewhere, it could be earthing out with the additional weight of the bar and chain.

More likely perhaps is a failed bearing on the PTO (clutch) side of the crank and chain tension is pulling open the oil seal- air leaking enough to hinder starting, Take the weight off and the seal seals again.
Although I would expect if this was the case- saw starts and then lifted from the cradle revs would rise dramatically.

 

[Mauibuck]

I don't think a bare wire grounding is likely because once the saw is running, it continues to run regardless of position. A grounding wire would cause it to run intermittently or stop.

I agree with your philosophy that the presence or weight from the bar should NOT have any impact on starting. But it does. The presence or absence of the chain has no impact. Only the weight of the bar, or more likely the torque caused by the weight of the bar seems to create the issue. Since the issue is the same with or without the chain, I think the clutch is not a candidate.

The rope is MUCH harder to pull with the bar weight/torque present. My feeling is that something is loose and the weight/torque of the bar Is causing something to shift and open/close. The difference in effort to pull the rope with and without the bar load is a very significant clue. But WHAT?

 

[Bob Hedgecutter]

Has the saw always been like this since you have owned it?
Has it been dropped at any stage from a generous height?
Has it been bar pinched in the cut and had a tumble?
Has it been run over or a log rolled on it?

Because the only way the "weight" of a clamped bar is causing the symptoms you describe is is the crankcases are cracked and the crack is opening with the weight of the bar.

 

[Mauibuck]

This is a new acquisition to me. I can't answer any of those questions. As I write this, it is night in Hawaii and I will check the chainsaw tomorrow and report back. Many thanks for your dedication to solving this mystery!!!

 

[Bob Hedgecutter]

Ah, then my guess is you may have ben duped.
I would be pulling the clutch apart first and checking that plus any play in the crankshaft- then I would be stripping everything I could off that bolts on to inspect the cases- including removing the gas tank.