Slightly confused at starter cord locking up.

[openspaceman]

5 hours ago, Will C said:

other thought is have you taken the starter cord side cover off? Unusual but could be something amis in there that is binding

I had a saw that jammed the starter occasionally and that was because the spool the cord wound on had a split and occasionally two lays of the starter cord would slip side by side on a pull and expand the reel against the case.

[bmp01]

I dont see how an internal engine fault can come and go so I'm gonna say there's a fault in the in the pull start mech too ?

 

Don't suppose you've changed starter cord in both saws have you ? Just thinking dodgy cord might be bunching up to be too big diameter to go through the hole in the starter mech ?

 

Quick diagnostic test and get a friend to help... Pull it over until you get a hard to turn over event, and while keeping tension on the cord remove the plug. Does it suddenly free up ???

[lurch_917]

could the piston be too small for the bore so under compresion it sometimes comes up on a cant and part bind's ? have had this on a crosser before  thats why i ask .

[Darkslider]

Last two replies are helpful and just the sort of thinking I lack sometimes. The saw is a 2018 model and came to me damaged and pretty neglected so anything could have happened to it, I think a wrong diameter piston being fitted is unlikely but not impossible

[pleasant]

Will be interesting to know what the outcome is  a "wrong size piston' ie smaller diameter isnt goiing to give you just over 95psi conmpression

[lurch_917]

1 hour ago, pleasant said:

Will be interesting to know what the outcome is  a "wrong size piston' ie smaller diameter isnt goiing to give you just over 95psi conmpression

you may be right but thats what i thought with my crosser but when i measured things the skirt of the piston had worn in a oval under sizing it by only a few thou so every now and then it would come up uneven and the ring would bind .

[Stubby]

3 hours ago, lurch_917 said:

you may be right but thats what i thought with my crosser but when i measured things the skirt of the piston had worn in a oval under sizing it by only a few thou so every now and then it would come up uneven and the ring would bind .

Ah but it wasn't the " wrong size " piston , just the right size but worn ?

[Darkslider]

Did a bit more tinkering with this today.

 

Removed spark plug and replaced with a mityvac bung I can hold in with my thumb while pulling over. Still locks up every 2 pulls but if I keep pressure on the cord and let my thumb off the bung it pops out with a bang and the starter cord comes out the rest of the way with it. Tried this with the coil, carb and exhaust all off to rule them all out as causes.

 

Thinking this definitely points to over compression now I try the Gunson tester on it with the 10mm adapter from the Sealey one. It's got a nice light spring on the Schrader valve and has given me reliable results on all my other saws, 5 or 10 pulls including a few that lock up and it's reading 160 psi. About what I'd expect from a 4 year old saw and I think I can trust this result now, however doesn't help with my problem.

 

Decide to strip it all the way down next, but that doesn't reveal anything untoward. Bit of carbon build up on top of the piston crown but everything else looks in good nick. Conrod looks nice and straight and no signs of scoring or gouging where the piston might have been nipping up or canting or anything. 

 

I think this one might be going to the dealer for them to try. I had a good play around with the recoil mechanism too couldn't get it to stick or jam or anything.

[Stubby]

20 minutes ago, Darkslider said:

Did a bit more tinkering with this today.

 

Removed spark plug and replaced with a mityvac bung I can hold in with my thumb while pulling over. Still locks up every 2 pulls but if I keep pressure on the cord and let my thumb off the bung it pops out with a bang and the starter cord comes out the rest of the way with it. Tried this with the coil, carb and exhaust all off to rule them all out as causes.

 

Thinking this definitely points to over compression now I try the Gunson tester on it with the 10mm adapter from the Sealey one. It's got a nice light spring on the Schrader valve and has given me reliable results on all my other saws, 5 or 10 pulls including a few that lock up and it's reading 160 psi. About what I'd expect from a 4 year old saw and I think I can trust this result now, however doesn't help with my problem.

 

Decide to strip it all the way down next, but that doesn't reveal anything untoward. Bit of carbon build up on top of the piston crown but everything else looks in good nick. Conrod looks nice and straight and no signs of scoring or gouging where the piston might have been nipping up or canting or anything. 

 

I think this one might be going to the dealer for them to try. I had a good play around with the recoil mechanism too couldn't get it to stick or jam or anything.

Hard to see in the pics but is the piston ring pin missing ? ( the pin that stops the ring rotating ) If it is could it be jamming something up ? 

[Darkslider]

54 minutes ago, Stubby said:

Hard to see in the pics but is the piston ring pin missing ? ( the pin that stops the ring rotating ) If it is could it be jamming something up ? 

I see what you mean looking at the photos but nope both rings have their pins, gudgeon pin and circlips there, crank feels nice and smooth when rotated I really am at a loss 🤣