Saw running on and wont idle!

[DCS6800i]

Hey all. Have a saw iv been working on which i want to run past you guys:

Makita 520.

 

Long story short iv had it away and the cylinder honed, and a new piston and rings put into her. She runs perfect at revs, cuts well and doesnt bog down (its a really gutsy saw) but the problem is i cant get her to idle atall: she sits at a very fast idle, wanting to run on. And as soon as i adjust the L screw to rein her in, till the chain stops turning, she just dies.

 

I thought it was a carb issue, so swapped it for a proven one my Dolmar - but same problem. I also changed the spark plug and fuel filter, only thing left to change is the fuel pipe - could it cause this sort of problem?

 

Any thoughts or suggestions are greatly appreciated

[Ray1]

Sounds like an air leak to me, when the new piston was fitted was the saw pressure and vacuum tested? Why did it need a new piston in the first place and was it doing this before the piston was changed?

[wisecobandit]

Are you adjusting the carb correctly?

The L screw should be set a fraction rich from the highest rpm setting and then the idle screw adjusted to bring the idle to where it should be.

 

You shouldn't be touching your L screw to set your tickover.

[DCS6800i]

No, its not tested - maybe that would be a good place to have started.

 

Its a mates saw. He gave me it as spares for my saw - but its in alot better condition as mine (minus this problem!)...... He said it was working fine then just stopped (thats what everyone says though, right?). Looks like one of the rings broke, or melted (exhaust side damage) and did in the piston, cylinder got away lightly and didnt take much to bring it back.

[spudulike]

No, its not tested - maybe that would be a good place to have started.

 

Its a mates saw. He gave me it as spares for my saw - but its in alot better condition as mine (minus this problem!)...... He said it was working fine then just stopped (thats what everyone says though, right?). Looks like one of the rings broke, or melted (exhaust side damage) and did in the piston, cylinder got away lightly and didnt take much to bring it back.

 

 

Ray has made some valid points,sounds like a bad air leak has seized the saw. You have fixed the piston and cylinder but not the cause of failure.

 

Get the saw checked for leak down under pressure and vacuum or it will most likely fail again.

[DCS6800i]

OK cheers (was hoping you see/reply to this Spud). Will get it pressure tested and see what the verdict is and post back. The more i think about it (after being pointed in that direction) the crank gasket is about all thats left could be wrong - i hope - iv been over almost everything else.

 

Cheers again and i will let you know how i get on.

[DCS6800i]

Quick update on this saw. A friend tested it for me mid week, turns out as suggested wasnt holding pressure. Got some time yesterday to strip it down and as i was taking the cylinder off it turned out one of the front screws threading was damaged and the other front had came slack. Replaced damaged, tightened other, rebuilt and tested - running sweet as!! Will get it tested again to double check but hopefully this is all thats been. Stupid little things!

[TimberCutterDartmoor]

air cleaner / compensator ? Well known for that issue but like you said you'd changed carb. hmm

[spudulike]

Thanks for letting us know, you got away very lightly - had an 084 in once that had actually banged the front half of the cylinder out of the casting when this had happened.

 

It is lucky it didn't seize - just shows you to be vigilant if the saws top revs seem high or the idle changes for no reason!

 

A good ending!