Ryobi post mortem!

[John Rumm]

A follow up to my non starting Ryobi trimmer thread a few weeks back.

Recap: line trimmer driving a hedge cutter attachment - started and ran fine for perhaps 20 mins or so before cutting out, and then would not restart - even when cold. The usual diagnostics seemed to show that fuel was getting into the cylinder, there was a spark, and air but no joy.

Anyway, I dropped it off at a local garden centre service place for investigation. Just got a call to say its beyond economic repair since the cylinder is scored and hence its not drawing the fuel mixture in properly on each stroke. They said it looks like it had been run without two stroke oil in it.

Just wondering what could have happened here. Its only about a year and a bit old, and has had a total of a can and a three quarters of fuel through it in its life. Fuel was mixed with the right ratio of oil[1], according to the instructions with the oil - the complete container of oil to 5L of petrol. The petrol was super unleaded, and the trimmer spec says it required 95 RON or higher. So what could have gone wrong?

I presume the damage was done on its final run (the first of this season) - since it started easily and ran fine for a while. I was using up the remaining fuel in its tank which was about 3/4 full - that had probably sat in its tank since last autumn.

So does the two stroke oil degrade in some way? Was the higher than required octane a problem? or does this sound like problem related to product quality?

The oil in question was Stihl oil like this:

[Chris Bartram]

>>

Was the 2 stroke oil mixed in with the fuel properly (i.e: did you give the container a good shake)?

[The Natural Philosopher]

Id take it back and get a second opinion.

Otherwise they may actually be lying to you.

As I said, I don't actually believe their story.

>

[Dave Liquorice]

What does that work out as a ratio?

My Ryobi strimmer wants 20:1 fuel:oil, this is much higher than other two stroke engines I've looked at (chainsaws and the like) they ask for 50:1. Personally I think it's probably to much oil as it smokes quite a bit but that's what the instructions and sticker on the tank say...

I don't give much to petrol and/or oils "going off", at least not in fairly well sealed fuel tank or proper fuel container. I guess the oil might have settled out in some way, the stuff I use does require a decent shake to get it dispersed into the fuel when mixing it.

[Jules Richardson]

Hmm, crap got in there somehow? Difficult to imagine how it got past the carb, though, unless it ended up in there via the exhaust (the small petrol 2-stroke stuff I've seen don't have fuel filters, but that should just result in tank junk getting stuck in the carb, not making it all the way to the engine).

Piston ring failed and broke up? Possibly due to poor-quality part, possibly severe overheating.

Make sure you get it back from them, and tear it down yourself or give it to someone else to look at; I agree with TNP about a second opinion.

cheers

Jules

[John Rumm]

As far as I can tell - it was a uniform pink colour, and would have had a reasonable shake getting the tool ready before starting.

[John Rumm]

Yup, the thought occurred to me... I will have a look myself and possibly take it elsewhere.

Yup, I am certainly suspicious of the diagnosed cause - since I know what I put in it, and have never lent it to anyone. I thought I had better check there was not a chorus of "oh did you not know two stroke oil turns into pumpkin juice after 3 months non use!!".

[John Rumm]

Good question - I will go find the manual and check...

It was in a decent plastic 1 gallon fuel can with an air tight cap...

[John Rumm]

According to:

21)

50:1 - so what I was using should have been ok.

[Tim Downie]

They might still be right about it being beyond economic repair though.

I used to have a friend who worked at a mower servicing place and even dealing with contaminated fuel was frequently considered BER as the workshop time involved in cleaning lines, carbs etc. was more expensive than a new engine in many cases.

Tim

[The Natural Philosopher]

Most synthetic 2-stroke oils are FINE at 50:1

I have never had separation problems.

If the bore is scored, I doubt its from siezing up either. More likely dirt or a bit of broken metal in it.

And I bet that you can source a new piston/cylinder from somewhere.

[John Rumm]

I shall perform a dissection in a bit and see what it looks like.

You can get them - I have seen suggested prices of about £50 for the whole cylinder, crank, and piston assembly. Whether that it worth it however is doubtful given a new one can be had for just over 90 and that would include a new line trimmer head as well.

[Mike]

I think this is down to product quality, certainly the small motorcycles coming out of China seem to have a life expectancy measured in weeks, lawn mowers seem to be just as bad.

I think you did originally say that the were silver deposits in the crankcase.

[Harry Bloomfield]

The oil needs to be throughly mixed with the fuel, otherwise it will just sit there. Did you give the container a good shake before using it?

[Grimly Curmudgeon]

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember John Rumm saying something like:

Hah. Think of all the thousands of 2-T motorcycles that had scuffed and seized pistons even when run on the proper oil and fuel. There's only the one problem - two-strokes are shit, is all.

[Peter Parry]

Ryobi usually specify 50:1 with synthetic or 25:1 with normal 2 stroke oil. Is it possible a non-synthetic was used?

Doing so would kill it quite quickly. A contractor I know had a dozen Ryobi chainsaws die in one day when someone inadvertently made up their bulk fuel with conventional 2 stroke oil at 50:1. Life of the saws was about 30 - 60 mins.

[John Rumm]

The instructions on mine make no reference to the type of oil, beyond:

"Use high a quality 2-stroke self mixing oil for air cooled engines. Do not use automotive oil or 2-stroke outboard oil."

Ryobi's own two stroke oil is a semi synthetic, I think. The Stihl oil I was using may have been a normal mineral oil, but claims to be suitable for all machines.

Hmmm, well I got about 20 mins life on its last run which sounds similar, however, that was after using probably the best part of 6L of fuel through it last year using the same oil, petrol, can etc. Not sure how many hours running in total that was - but a fair number.

[andrew]

Much as I expected, wrong or lack of oil in the mix is the usual suspect but because of the way the simple diaphragm carbs work on these little two strokes there are other ways to cause overheating and partial seizure. As I said in my original post you can normally see this through the exhaust port. The ring becomes stuck in the grove as the aluminium deforms over it and hence won't seal the compression. Adding a small amount of oil to seal the ring for a few firing cycles indicates this and then confirm by removing the silencer. The worst damage is normally on the part of the ring which passes the exhaust port. If this is not the case I would supsect something else, like foreign objects.

As the mixture strength is set for a given rpm the throttle becomes an on off switch in use, running from lo idle to full power. Set it to a mid way speed position and the mixture may weaken but the major effect is there is not enough cooling air. This became a big problem when nylon cord started to replace steel saw blades, people would extend the cord such that the engine could not reach full revs and the engine would overheat and seize quite quickly.One make, from a cold country, was also notorious for forming vapour locks in the fuel system even in our summers which effectively weakened the mixture and caused erratic running and overheating in hot weather.

Back then we mostly used leaded fuel and this delayed the onset of problems, the change to unleaded lead to a spate of seizures. A solution ( which was also prompted by the need to reduce NOx levels) was to provide a bypass jet in the carb to prevent unintentional weakening of the mixture. A 2 stroke actually produces more power when weakened beyond its safe mixture as though torque is reduced the extra revs more than compensate.

AJH

[John Rumm]

If it is that, then obviously I want to work out how I got it wrong, and also for that matter, how it took six litres of fuel through it to reach the point of no return.

From what I can see of the rings through the ports, they still look to be in decent condition.

See the photos here:

the ring for a few firing cycles indicates this and then confirm by

Just realised I forgot to photograph that, but just went to have a look. The rings through the exhaust port look fine. No (or very very slight) marking under them on the base of the piston either.

Now this is a bit of operator error that I will admit to. At the time of the failure I was using the hedge trimmer, and with that there is a tendency to not use full throttle all the time- having said that the usage would have been a mixture of idle, full, and perhaps 20% partial throttle (was hacking a pampas grass - so you spend a fair bit of time working out what cut to make next, pulling dead and cut stuff out etc).

Temp at the time was probably 14 deg C ish - not cold but not excessive.

Any thoughts taking that into account and looking at the pics?

[John Rumm]

There is a simple fuel filter on this on the end of the klunk weight in the tank. The fuel ought to have been pretty clean anyway though.

From what I can see at the moment (anyone know how to get the clutch assembly off the end of the shaft so I can remove the plastic case - that would let me take the cylinder right off?)

Sounds like one possibility - but I am not sure yet what melted.

There is enough metal crap floating about though:

> So what could have gone wrong?

Yup - disassembly photos were after I got it back.