Pistons - fried like an egg!

[spudulike]

Just fitted a Garden Hire Spares one on another members Stihl 066 Magnum - 170 psi without running in and looks fine. Time will tell - he is running one on a 044 with similar readings with all being good.

 

Other options - Mister Solutions Stoke on Trent - Andy is the man!

[gensetsteve]

Right -No 1 - Thanks Matt for the good rep - your 372 is coming along nicely - found some interesting stuff wrong with it.......

 

Pressure checking, firstly absolutely NOTHING to do with compression or the performance of the top end of the engine.

 

A two stroke engine pulls fuel oil mix from the carb, into the crankcase and then pushes it up through transfer ports in to the compustion chamber, this action also expells the exhaust and then once the piston clears the exhaust port, we are all ready for a good bang and lots of smoke.

 

The air is controlled by the carb and the integrity or the seal of the engine is critical to its correct running, if the crankcase seals, impulse line or carb boot are leaking, you will pull air in to the engine in varying and unwanted amounts and this then can cause a very weak mixture. If you are lucky, the dodgy idle will warn you about the problem, but if you just adjust it out and carry on then the next thing that happens is the exhaust side of the piston melts on to the cylinder wall - a full or partial engine seizure.

 

So - if an engine has seized, new parts are fitted and a pressure check ensures that it wont happen again as long as this is followed by the correct adjustment of the "H" carb screw - tach tune or tune by ear!

 

The early signs of air leaks are poor idle, an idle that has increased unexpectedly - like when your saw runs out of fuel - that increase in speed just before it dies - you may also get the saw hitting higher revs and sounding like a demented wasp you could also get that flat spot between 3,000 - 6,000 revs - like the 345, 350, 346, 357 Huskys when they are fully cold but it will not clear after 30 seconds running - thet wohh wohh sound in the midrange!

 

So - a pressure check = an important bit of the saws health and not engineering sphericals - this single check has saved the 066 Magnum and 048 on my bench further damage after repair.

 

What do you use to pressure check the crank case ?

[spudulike]

All you need is a gauge that can read up to 10psi of pressure, a length of pipe, a suitable valve that lets pressure in and seals back pressure, a fitting that links to the engine that allows the air in and a suitable pump.

 

You can buy a purpose made kit or modify parts purchased separately.

 

The hardest part is sealing the exhaust and inlet ports as even the slightest leak will mess up the results.

[agg221]

As the owner of the 044 and 066 Magnum mentioned above, I can say that I've been very happy so far with the GHS parts on the 044.

 

I bought the saw as a non-runner on ebay, found it needed the anticipated pot and piston, in this case due to one of the rings having let go.

 

Since I was able to make a positive diagnosis, I just fitted the new parts and away it went. I'm using it on a Ripsaw mill, so it's being worked hard, and it's done a reasonable amount now, probably about 30l of fuel through it. It's running nicely and no sign of premature wear. It also has a 1yr warranty, and they know I'm using it for milling which didn't concern them over likelihood of survival for a year.

 

Alec

[oldnwrinkly]

I want to pressure test a Husky 136 so did a search and came across this post.

 

All you need is a gauge that can read up to 10psi of pressure, a length of pipe, a suitable valve that lets pressure in and seals back pressure, a fitting that links to the engine that allows the air in and a suitable pump.

 

You can buy a purpose made kit or modify parts purchased separately.

 

The hardest part is sealing the exhaust and inlet ports as even the slightest leak will mess up the results.

 

 

As I have never done this test I wonder if someone could clarify a couple of points please.

 

RE: ".... a fitting that links to the engine that allows the air in ....". How do I get the air in .......... the impulse port maybe?

 

What is the usual method of sealing the exhaust and inlet ports?

 

Also, is it necessary to monitor the induced pressure over a period of time and, if so, what would be a reasonable length of time for the crankcase to maintain a steady pressure?

 

Dave.

[spudulike]

I want to pressure test a Husky 136 so did a search and came across this post.

 

 

 

 

As I have never done this test I wonder if someone could clarify a couple of points please.

 

RE: ".... a fitting that links to the engine that allows the air in ....". How do I get the air in .......... the impulse port maybe?

 

What is the usual method of sealing the exhaust and inlet ports?

 

Also, is it necessary to monitor the induced pressure over a period of time and, if so, what would be a reasonable length of time for the crankcase to maintain a steady pressure?

 

Dave.

 

You put the air in through either a modified sparkplug base or through the impulse line - you will need to use a valve from a bicycle/car inner tube to seal the inlet pipe.

 

Sealing the exhaust/carb holes is typically done using blanking pieces, bungs or custom made soft rubber sheet sandwiched between the cylinder and muffler/carb.

 

A pressure of no more than 10 psi should be held in the crankcase - more and you may blow the seals out.

 

If pressured to 10 psi, it should hold to no lower than 7psi for a few minutes - I tend to expect zero drop over a minute or two (I also rotate the crank after the initial test) with no visible drop in the needle. I have had engines hold full pressure for 30 minutes plus and don't expect to see the needle moving by naked eye. Stihl say something like the enging should hold some pressure for a minute but I like to see a total seal for minutes.

 

You will need a suitable gauge that will read positive pressure.

 

The other side of this is vacuum check. Pressure checks are great for impulse and carb boot leaks and not bad at crank seals, vac tests are better for leaking seals as it pulls the seal lip inwards.

 

Both tests have saved me hours in finding split hoses etc but is a test more suitable for techs doing repair work all the time than someone doing a one off check.

[oldnwrinkly]

Thanks for the very comprehensive reply, Spud.

 

I have some sheet rubber so will make a 'gasket' for the exhaust and work out something for the inlet port. I also have a gauge and other bits lying around somewhere.

 

I suspect the crankcase seals are a bit iffy as the tick-over is erratic and the saw sometimes surges. The carb boot looks OK - no cracks or splits - and is still a tight fit, Everything else I have checked seems to be in order.

 

Dave.

[spudulike]

No worries, hope it helps. Stihl charge about £250 for their kit but my tools cost £30 for the vac test and £15 for the pressure tester - all does the same job, just got to apply yourself and think about what you are trying to achieve.

 

Use a bicycle pump or foot pump to pressure the crankcase - anything else is likely to give it too much pressure.

 

A few members are glad I did this check on their saws after repairing them:thumbup:

[Al Smith]

FWIW an old blood pressure cuff makes a nice pressure tester . Usually relatively inexpensive from flea bay .