Scored piston on MS 260.

RoyaleBlue

An MS 260 (relatively new with ~ 100 hrs on it) at my work has stopped running. It pull starts fine (feels like the same compression as before) when I try to start it, but it never fires up after several attempts by several people over a few days. My co worker thinks he ruined it by starting and revving it without a bar and chain attached. However, we took it to a Stihl guy and he said that the ignition module will keep the saw from overreving without the load of a bar and chain. He popped off the muffler and said the piston was scored and the saw wasn't worth fixing. I looked in there and saw about a centimeter wide area of scoring on the piston, facing almost straight out of the exhaust port. The dealer thinks its because we use Opti-2 in our mix. Another guy at my work swears by Opti-2 and is hyper defensive about the scoring being caused by Opti-2. Rather he thinks it is caused by bad 87 octane gasoline. Another older saw, an 026, also seized up recently (the starter cord pulls really hard) using the same gas and Opti-2. Is this bad gas, bad mix, or a weird coincidental. Also, how hard would it be to put in a new piston and cylinder assemble in the saw for a guy who has never cracked a saw open before?

superfire

What part of the PNW are you in? Sounds like bad two stroke mix. The saws are fairly easy to fix. Knowing where you are can help get some one close by to be available to guide you throu the rebuild.

RoyaleBlue

I am from Olympia, but I'm currently working in west texas for a few months.

superfire

The saws are fairly easy to take apart. Just remove the top cover,then remove the impulse hose. Then remove the muffler. Then remove carb to cylinder boot clamp off the cyclinder side. Then remove the 4 T27 torx that hold the cyclinder. Then remove the cyclinder off the saw and then remove the piston. It is basically reverse with the new parts. You will need some special tools to do the job.

KevinE

Putting a new P&C onto a 260 is as easy as! I'm no mechanic, but I recently did one, took me about 3 beers to get it done! I did the whole thing with the tool that you get when you buy it + a flat screw driver + cir-clip pliers. The hardest thing was getting the cir clips back into holding the gudgeon pin (wrist pin) in.

You can get the kit off ebay very economically, although I preferred to buy mine through a former member on here. It was $115 delivered to my door & the quality of the engineering was first class!

RoyaleBlue

Thanks for the help. A few questions:

1) What's the impulse hose and where to get cir clip pliers?

2) Has anyone ever started a saw with a bar and chain?

KevinE

Dunno what an impulse hose is (scary, coz I've done a rebuild on a 260) but you can get the cir clip pliers at most any hardware store for a few bucks.

rogue60

(impulse hose) i think he means spark plug lead. As for the bad fuel thats not it, would only mean saw is down on power. lower octane = low power,id say it comes down to the 2 stroke mix. What is the mix he is running? ie 25 to 1 or say 50 to 1 ??..And running a saw without a chain and bar wont hurt the engine as long as its not held flat out for minutes at a time! id be more worried about the clutch flying to bits! When you back off, the clutch disengages and the outer clutch basket free wheels at 10000rpm. not good!

superfire

The impulse hose fits on a nipple on the right side of the saw on the crankcase. The rubber boot to the carb only needs a small skew driver to remove the clamp. The easiest way to understand what I am talkin about is to remove the top cover and look at all the stuff I have mentioned.

KevinE

Piston & cylinder removed; where's the impulse hose?



I didn't use a ring clamp, I just squeezed the rings with my fingers. It was quite easy to do.

Attached Thumbnails

 

superfire

Kevin the photo shows no details. The impulse should be on the right side towards the rear front handle bar mount.

RoyaleBlue

So low octane gas = lower redline= less power. How does the saw detect pre-ignition? Does it also pull spark timing? The mix is Opti-2 mixed at the recommended 70:1. They package says that this ratio is good in any two-stroke/any ratio, and some people swear by it. They saws have been running Opti-2 for a year or two, so it may have been very gradual scoring if the Opti-2 is the cause. Can pre-ignition from bad gas cause scoring?

rogue60

my god 70:1 i think you will find thats it no oil! if i run 70:1 here in summer in my saw,the piston would look like yours in less than a tank of gas!, its not the oil as such its the mix..my self i mix no matter oil brand 32:1 but thats me and big cc saws... some dudes run more or less up to 50:1 ive never heard anyone run 70:1...there is a thread on here all about 2 stroke mix and what people run there mix at...anyways good job pulling the saw down save heaps of $$ doing it your self and its not that hard

APSE

Opti=2 is not bad oil for homeowner style saws. BUT....70:1 is nuts! You really should be running that saw at 50:1 or 40:1. Sustained RPM, cylinder temp, ambient temp, will all cause a lean oil mix to deteriorate very rapidly. If that is the mix ratio you use in all the other saws, it might be worthwhile to remove the mufflers and give them all a good inspection. An afterthought for you, it is ALWAYS best to clean the saw prior to disassembly. Less risk of damage from crap falling into places where it will mess things up and it gives you an opportunity for proper inspection and a good diagnosis. Also these little guys are prone to air leaks, there is no way to inspect for an air leak on a dirty saw

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KevinE

I think you're getting 2 posters on this thread mixed up?

RoyaleBlue was running his saw at 70:1, I don't. All my gear gets run at 37.5:1

Yeah, maybe the saw could have been cleaned up before I pulled the P&C off, but it was no biggie. I knew what my problem was before I started pulling the saw apart; the rings had gone soft. The old P&C are fine & no holes (saw was idling fine with L screw out 1 turn). I was offered a mini BB kit for more or less the same price as OEM rings. No brainer, the BB kit went on. The paper towel did it's job keeping crap out of the crankcase. The big end bearing was fine, so I didn't have to pull it down further.

If the BB kit fails, I'll just buy OEM rings & put the original top end back on.

APSE

Sorry Kevin E,
You're right I did get you guys mixed up, it's a little easier to see that this morning with out my beer goggles

__________________
Vintage Chainsaws, Parts, Restorations, Custom Work, Special Saws on Request and Much More.
Please visit our webalbum:
http://picasaweb.google.com/APSELLC
or our website https://apse.mybigcommerce.com/
APSE-LLC
Email:apse-llc@live.com
Phone:720-260-0157
Business Hours: Tuesday Through Friday 9-6
Saturday 9-5 Rocky Mountain Time, USA
If ya got sumthin' ya can't find we'd be glad to try and help ya!
Brian and Tina
:

KevinE

No worries mate, I've been guilty of that myself more than once lol!

rogue60

yeah sorry mate im guilty as well didnt even see your name on that pic of the saw, just thought RoyaleBlue had pulled hes saw down...

sharkey

For the folks that arent sure what 'impulse' is or does;

Impulse is the name for the pressure and vacuum pressure differentials that occur below the piston crown in the crankcase of an engine. On a saw engine the pulse cycle is piped off to run the fuel pump which is located within a diaphragm carb. The pulse can also be used to run bar oil pumps on some of the older designs like the Homelite and McCulloch.

Some units use a hose to connect to their carbs and some have channels cast into their intake adapters and blocks. If the pulse is lost or impaired the engine can run lean and fail.