diagnosis

[openspaceman]

Your repairing seized saws with no obvious air leaks ect and you have not noticed much issue with old fuel ? on a big saw you might hear it start to detonate before its to late .

 

Maybeso but these saws were in regular use and had fresh fuel. The bit about old fuel was a general observation about starting difficulties and that problem is over within 30 minutes use.

[openspaceman]

Piston Failure Analysis Piston Failure Analysis

 

Thanks for the link, I can read the thread but cannot see the attachments he refers to.

[Stubby]

I've never noticed much issue with old fuel, I recently repaired the ignition on a husky 136 which stared fine with its 5 year old fuel.

 

One thing about leaving old fuel in 2t engines is if there is any leak into the crankcase the petrol evaporates leaving oil, when the engine turns over this can foul the plug but if it fires then the problem clears before the engine is hot enough to cause a problem.

 

Don't discount old fuel . It may start and fun for a bit but danger lurks there my friend .

[spudulike]

I tend to find that saws running lean will bleach the plug white or even light grey through overheating.

 

The plug tends to stay normal colour when straight fueled as it kills the saw very quickly, not fast enough to colour the plug.

 

The pistons tend to look very much the same, too much heat destroys the exhaust side.

[spudulike]

Been asked to list this PDF by another member:001_tongue:

English_Piston_Failure_Analysis_Guide.pdf

[njm]

Maybeso but these saws were in regular use and had fresh fuel. The bit about old fuel was a general observation about starting difficulties and that problem is over within 30 minutes use.

 

Fair enough but personally i would not run a saw for 30 seconds on old fuel .

[Cut4fun]

Thanks for the link, I can read the thread but cannot see the attachments he refers to.

 

Must not be a member. Forgot about the links not showing.

 

There is 2 of them.

 

Will try to upload for you all here.

 

I give up. Cant find them on my HD. Only on the site.

[GardenKit]

I tend to find that saws running lean will bleach the plug white or even light grey through overheating.

 

The plug tends to stay normal colour when straight fueled as it kills the saw very quickly, not fast enough to colour the plug.

 

The pistons tend to look very much the same, too much heat destroys the exhaust side.

 

A few years ago I had a customer who seized his new Tanaka brush cutter. I diagnosed lack of oil in his fuel, but he said he had used the same fuel for the last three or four times he had used it, in fact it was from new some 4 months before. But we checked his can and found neat petrol. He admitted he had put no oil in as he thought it did not need it.

So, in short he had straight fuelled it from new and it survived 3 or 4 cuts of around 20 minutes each.

 

I found this hard to believe (although I trusted his word) so I straight fuelled a secondhand strummer I had around, taped the throttle wide open and ran it. It just kept going, using up a full tank of fuel before stopping, but restarted after refuelling with more straight petrol, after which it ran for another 20 minutes before seizing. A run time of over an hour on straight fuel.

 

So in fact, although I had always believed a straight fuelled engine would die within minutes, I was way off the truth.

 

A strip down revealed a piston scored all round and a bone dry and rattly crankcase.

[Richard 1234]

A few years ago I had a customer who seized his new Tanaka brush cutter. I diagnosed lack of oil in his fuel, but he said he had used the same fuel for the last three or four times he had used it, in fact it was from new some 4 months before. But we checked his can and found neat petrol. He admitted he had put no oil in as he thought it did not need it.

 

So, in short he had straight fuelled it from new and it survived 3 or 4 cuts of around 20 minutes each.

 

 

 

I found this hard to believe (although I trusted his word) so I straight fuelled a secondhand strummer I had around, taped the throttle wide open and ran it. It just kept going, using up a full tank of fuel before stopping, but restarted after refuelling with more straight petrol, after which it ran for another 20 minutes before seizing. A run time of over an hour on straight fuel.

 

 

 

So in fact, although I had always believed a straight fuelled engine would die within minutes, I was way off the truth.

 

 

 

A strip down revealed a piston scored all round and a bone dry and rattly crankcase.

 

 

I thought you could kill a 2 stroke in seconds/minutes without oil in the fuel. Seems hard to believe!!

[spudulike]

A few years ago I had a customer who seized his new Tanaka brush cutter. I diagnosed lack of oil in his fuel, but he said he had used the same fuel for the last three or four times he had used it, in fact it was from new some 4 months before. But we checked his can and found neat petrol. He admitted he had put no oil in as he thought it did not need it.

So, in short he had straight fuelled it from new and it survived 3 or 4 cuts of around 20 minutes each.

 

I found this hard to believe (although I trusted his word) so I straight fuelled a secondhand strummer I had around, taped the throttle wide open and ran it. It just kept going, using up a full tank of fuel before stopping, but restarted after refuelling with more straight petrol, after which it ran for another 20 minutes before seizing. A run time of over an hour on straight fuel.

 

So in fact, although I had always believed a straight fuelled engine would die within minutes, I was way off the truth.

 

A strip down revealed a piston scored all round and a bone dry and rattly crankcase.

I remember your test, very interesting although on a saw, you are pulling another circa 3000rpm+ and is under a lot more load in a big cut so may be different - you got any saws you want to test with sraight fuel?

 

All interesting stuff though!