High compression

[mickdundee]

Tried to start my saw today. With the choke on i gave it a few pulls and it tried to fire. Knocked the choke off expecting it to go it's the normal routine but as I pulled it I ripped the handle out of my hand and it was very stiff when I had another try. I've had it on the bench and with the spark plug out it turns easily. I've had the exhaust off which had no effect. The piston does look a bit burned but I don't know how clean you'd expect it to be. I can't see any scoring on the bore. Any body got an idea what my next step should be. Thank you

[mickdundee]

The bore

[nepia]

I get similar with my 064 sometimes but I reckon it's to do with the pull cord assembly, not the engine: occasionally the starter cord will virtually lock solid for two or three attempts, then free up. Could it be that in your case or definitely not?

[mickdundee]

Wouldn't like to say it's definately not that, but slowly moving the cylinder with the exhaust off the sticking point is consistently as the cylinder approaches TDC, then frees again. I just don't know what can cause this. 2 strokes aren't really my thing

[Macpherson]

just a thought, might be worth checking that the flywheel isn`t making contact with the pickup, that the gap`s correct and not closed up, cheers

[Dinosaw]

That degree of carbon buildup in the exhaust seems a lot. I'd be tempted to pull off the pot and see how much crap is inside.

[mickdundee]

At the minute I don't have a torqs bit that I can get access to the head bolts with to check that

[spudulike]

I am thinking the saw is a MS660 from that exhaust port?

 

If the saw turns freely with the plug out but is hard to turn with it in then it is probably just compression. It is worth pulling the spark plug out and turning the machine upside down as the needle valve may have been leaking and filling the crankcase with fuel!

 

What is it like using the decomp valve?

 

A common one on the 660 is using a too long bolt on the front top cover mounting hole on the recoil side and it comes trough and locks on the flywheel but this would happen with or without the plug in!

 

The kick on older saws such as the 064 is down to the single point of ignition advance of around 25 deg, if the saw fires and the handle hasnt been pulled over with gusto, the piston will fly down backwards and cause this painful phenomenon.

 

Later saws have almost zero ignition advance on start up rapidly increasing to 30 degrees at idle and changes with revs - see my porting thread for further info on this.

 

It is worth checking the flywheel to coil gap - should be a typical business card in distance.

[mickdundee]

Ok few bits to look at again in the morning. Sorry I should have said but this is an old 038 av super. If it has a decompression valve I don't know about it

[mickdundee]

If it had gone backwards at start up spud ( I think that's what you meant if I didn't pull it hard enough ) how would I then correct that