bmp01

Yes that should happen. Now open each screw, one at a time and tell us which one is allowing the air through…. I think that’s ok too. You are blowing into the metering chamber only, so long as you have the little valve and it’s gubbins plus the diaphragm in place. That’s the chamber where fuel is supplied to the jets….. So here you are creating a little bit of pressure on the fuel side of the jets, you are blowing through the main jet and low speed jets in the direction they are supposed to work. The one way valve in the main jet should be open in this direction. With the low speed circuit there is sometimes a little one way valve at the entry to the low speed circuit, which would also be open in this direction. What is not ok is the purge bulb (or you) being able to suck through the main jet one way valve. And it’s not ok, being able to suck through the low speed circuit one way valve if there is one, less of a big deal, but still not good. Worth mentioning, both one way valves are often a little leaky with air, less so with fuel in them. Secondly, if the purge button pops back really slow and fills with air, its likely the fuel filter inside the carb is probably blocked …. bulb can’t suck fuel so it does what it can and sucks air, through the jets. And finally, leaky fuel line (between carb and bulb) or the bulb itself will allow air into bulb, I think this is pretty obvious. But less obvious maybe is that a leak in this area can easily allow air to be sucked into the carb metering chamber when engine is running…. Food for thought.

Had a bit of time this afternoon to clear away some sawdust, sluice out the bottom end with some fuel and inspect the main bearings and cages with a mirror on a stick. Tried one of those endoscope connect to you phone thingees - waste of time only focuses down to 4 or 5 cm. Seems like everything is sound, rotated it through a couple of revolutions to make sure I didn’t miss any sections of the cage hidden by crank weights. There must be some regions of the inner race I couldn’t get to see but I’m going to say it’s good.

Yes, I understand that, amazed the bit(s) didn’t leave a single mark left on its way, but if that’s what experience says then I’ll check the brgs. Maybe I’ll find some damage in the bottom end if/when I end up splitting the cases. My blow-by question was relating to the black on the crank weights. Same shade of black as the carbon deposits on the piston skirt above and below the ring. Which is combustion gas getting past the ring. (Piston hasn’t been hot enough to cook the oil). Thanks for your reply’s.

Thanks for comment. Two or more suggestions for bearing - ok, better get over my laziness then …. Blow by past the piston maybe?

Thanks for that, can’t fault the logic. Have to admit to being negligent as far as checking further, I know I should but it looks pristine in the bottom end. Plus I can’t think of anything in there (brgs, cages) that would provide flat debris like that. I like the screw driver theory ….. right opposite the spark plug hole. Ties in with a DIYer trying a Chinese replacement plug sourced from an online store, cheap as chips. Why am I leaning towards boggery rather than mechanical unreliability ? 🤔🙄

Ring got forced back into the groove about 1/2 mm, I was surprised there was enough depth in the groove for that to happen.

New plug is Chinese and healthy, I think it’s the correct spec al-be-it Chinese. But the fact it’s new might be a clue…. Those bits could be the curved electrode of a plug. If any of the dents in the piston or cylinder were curved in shape that would have been conclusive. But the long dents are straight. Also worth mentioning some of the dents are carboned over.

Good thinking and apologies for not elaborating - who ever investigated it stopped at the filter. Carb wasn’t removed nor was the cylinder. Plenty of undisturbed sawdust and oil there. That’s the right course of action I’m sure. It’s good enough to warrant getting back to full health.

It’s there, present and correct. (And finally he uses the quote function to reply to the suggestion 🙄 )

Circlips present and correctly fitted, probably as fitted 30 plus years ago. Carb looks good. Exhaust is cosmetically good condition on outside, all looks carbon grey through the port and exit hole. Something back through the exhaust gets above the piston though.

Air filter present under the cover but not fitted, just floating about. I wouldn’t be surprised if someone gave up trying to fix it, just threw it back together, top cover screws were loose too. Something dropped down the inlet or fallen off the carb was my first thought - seen this before too. BUT I’m discounting this because there is absolutely no damage below the piston…. not a mark. Am I wrong ? Tillotson carb on it if that helps.

Let’s see what the Arbtalk brains and experience can come up with. Then we can talk about repair, upgrade piston and cylinders 🤔😊

I don’t know the answer BTW, it’s a saw I just bought on market place for next to no money and is otherwise good condition. It does have a new Chinese spark plug fitted with signs of combustion …. Talking of which the other thing that looks odd to me is the leading edge of the crankshaft weights that are really black.

A few bits of extra info… there is no damage below piston, none. The bits I found in the piston and cylinder are ferrous. It’s not the piston ring as that’s in one piece…

Right then time for a little puzzle…. What broke? Have a look at the pictures for the damage.