Restored Pioneer P51, Rope keeps snapping

[xj04]

Hello,
Sorry Im a newbie here and have to post a question. Before I do, I'll tell you a little about myself. I'm a former mechanic by trade but am back in college. I worked at Sears as a small engine mechanic for 2 years as well so I've worked on plenty of chainsaws (please don't buy a Craftsman.)
Anyway, I restored a Pioneer P51 chainsaw that my father-in-law used to use when he was a tree trimmer and arborist. I rebuilt the carburetor and tuned it, bought a new chain and bar and put on a new pull cord. It runs great. The problem I keep having is that the pull cord keeps breaking. I tie a knot in it and pass it through (as I have done on hundreds of other chainsaws). It keeps breaking the rope right above the knot. I have made sure to even use the decomp valve but it didnt help.
I've also purchased 5.5mm rope which just barely fit, and it snapped that after three starts.
Does anyone have any ideas?? Anyone ever used Cobra Cords? http://www.amazon.com/Chainsaw-Pull...1387162261&sr=8-1&keywords=chainsaw+pull+cord
Thanks in advance!

 

[machinisttx]

A few years ago, my uncle bought a spool of starter rope and found that it was just junk. It didn't matter what he used it on, it would break shortly after installing it. Try a different brand of cord.

 

[cobey]

did you check the handle for a sharp spot?

 

[xj04]

I have used 2 different kinds of rope, but its possible they were both garbage. The handle doesnt have any burrs or sharp spots that I can see and when I pass it through the starter reel, I dont see any sharp edges either.

 

[LegDeLimber]

get some good light and look at the pulley and for good measure, all other places the rope touches
wear can create sharp edges from once radiused ones.
New parts can often have flash/burrs Or simply corners on them that will cut a rope
or a fuel line (see budget string trimmers for sharp edged holes in fuel tanks)

I used to keep a set of deburing tools made from old chainfiles
for detailing every new recoil pully.
Takes a just little bit of work under a chain grinder wheel or rotary tool
to shape up the ends into a concave shape for this.
sort of like a drill bit but in the reverse-ish shape.
Look at your rope an decied if you think it broke (fuzzy stretched fibers)
Or was sliced (the area of damge will be a lot shorter)
if you need or want to, cut the old rope and look at how the ends look
to help tune you eyes to it.

possible Bad rope batch has been mentioned.

edit: I'm a slow typist . looks like you posted while I was busy misspelling words.
I still say take a hard look at the parts though.
also are you heating the knot or putting glue in it?
I've done both before,
(still sorta like the mild heating to set a knot in small spots)
you just need to be carefull
of the unique stress raisers from such methods.

 

[Little Al]

Is your pullcord snapping at the handle knot or pulley knot? If it`s at the handle does it have a metal washer/ plate under the knot in the top T? if the answer is yes to both these try de burring as sugested or replace washer with a hard plastic one. If at the pulley end make sure the hole that the cord comes through into the groove is radiused & smooth not a tight abrupt bend. If possible tie a figure 8 knot rather than an ordinary over hand knot[ spreads the strain a bit] & if you heat seal the end just the single tail & not the knot it`s self. you seem to be aware to use the biggest size of cord that will fit the groove iin the pulley. If it`s the handle end & it still breaks try a different pull handle

 

[LegDeLimber]

Little Al raises a very good point with the knot styles.

I suspect the folks up in the tree work section could just dazzle me with
ways to affect the pinch or twist or general stress spots on a knot or rope.

I flunked out of Cub Scouts and never got good with knot tying.
just Barely got through Granny-knots 101.

So the use judicious use of heat or a couple drops of crazy glue
became my compensations for my lack of knottery skilz

 

[bafarm]

Not sure if will help, but I have learned to short pull the rope, stopping before the last turn on the spool and that stopped it for me. ( most of mine were frayed not cut).

Don

 

[discounthunter]

where did you buy the cord from? i learned the hard way cord from the box stores is not the same as dealer cords.