Dolmar PS 460 Keep Running Issue

[HighRidgePines]

I have been working on my son's PS 460, which historically will start after having "sat" for some time and will run well for awhile, but if you shut it down, it will not restart. I think it did this pretty much from the beginning but he never took it back to the dealer to be diagnosed.

History: This saw is about 12 years old and may have 10-15 hours of run time on it. It has been so frustrating for him that it was put in the corner. I have been working on it not only in an attempt to help him, but as small engine repair practice.

There is good spark on the plug and it is gapped appropriately. To rule out the fuel filter, I removed it for awhile and no change to the condition. The carb did need a rebuild, and I have already performed that. It was relatively clean but one of the diaphragms was very "distorted". It started and ran well for awhile after this work was done, but would not start again after warmed up. (Same condition as before) There is quite a bit of carbon on the top of the cylinder. I pulled the muffler and there is no scoring on the exhaust side of the piston.

The interesting point is it didn't appear as if there was a ring on the piston. I took a small screwdriver and did notice it would go into the groove. I have attached a couple of pics so that those here can take a look. I can't conceive that there is no piston.


My son has a compression tester that we will use on the saw on Friday. Is it possible that compression might be sufficient to start and run the saw for awhile, at least until it warms up and then the compression would not be sufficient to start it again.

Yes, he has the oiler set to feed more oil than the Valdez.

 

[HighRidgePines]

Can’t conceive that there is no “ring” on the piston.

 

[holeycow]

Looks like a ring to me. The pics are slightly blurry tho.

 

[Chevboy0167]

Use a small magnet. Like a 1/4" round mechanics magnet that telescopes out. Place over ring groove. If it sticks, it's gotta be a ring.......
Ring may be there, but wore down or even broken a chunk off.

 

[Fatherwheels]

The ring might be seized, looks like a small mark/scratch on the top of the
ring next the middle as if you scraped it when you went poking, seen in top picture.

 

[HighRidgePines]

Looks like it is a better situation than I thought. Better tuning seems to be doing the trick. I sanded off the limiter tabs when I rebuilt the carb. More tuning has meant better starting.

 

[3000 FPS]

The saw would not ever start without a ring. Also if better tuning is making it better it sounds to me like you had it too lean and it was getting too hot and would not restart until it cools back down.
Do you know how to tune a saw by ear. Listen for the 4 stroke sound and when in wood it goes to a 2 stroke sound.
That means it is tuned correctly and if you do not hear that then the tuning is off.