stihl BG 55 Blower

[bertsmobile1]

As in post # 9 two strokes require a lot higher compression than 4 strokes .
Having said that 2 ring pistons will work with lower compression than single ring ones will.
Most factory manuals do not publish a cylinder compression for good reasons.
1) they are very hard to measure accurately for engines under 100 cc, my gear is definately not up to it
2) they are not as important as leak down and crankcase pressure
Just about every manual has figures for cankcase vacuum & pressures and these are what IS important.
You have 200 PSI under the spark plug but it means diddly squat if all of that is air sucked in from outside & not through the carb.
With my gear 90 psi is a minimum compression

 

[StarTech]

All I can atest to is what my Actron compression tester reads and the minimum compression levels works here. Not compression tester are design to work at the low volumes that many handheld equipment have such as the 21cc engines that I test. A part of the problem is the placement of the Schrader valve and its spring load.

As Bert mentions 2 cycles must have a sealed crankcase to move fuel mix through the engine. Also upper cylinder damage can test at great compression but have all lost the last fraction of the stroke. I got an old 028 Stihl chainsaw that reads 140 psi of my compression gauge that will not even hit a lick. I finally tore it down and found damage right at top of stroke in the cylinder wall.

On top that many 4 cycle small engines have automatic compression releases which testing for compression a problem as even an engine with bad rings or cylinder wear can still test at a specified minimum and still be bad. Even some 2 engines have decompression systems usually a manually set release for most handhelds that have them. I personally have worked on larger mower 2 cycle engine so I don't how those decompression systems work but have work a few ATVs where you manually set them.

 

[bertsmobile1]

If you can find a figure for the compression ratio then you multiply that by atmospheric pressure ( ~ 14,7 psi) to get the MAXIMUM CYLINDER PRESSURE
Usually 2 strokes run between 9:1 and 12:1 so that would equate to 132 psi to 161psi
These are the max theoretcal numbers in practice they would be 10% to 20% lower so they come out at 118 to 105 psi because cylinders never achieve perfect filling which is why turbo chargers were invented.
Stihl do not publish the CR for the BG 55 but they do publish the bore & stroke so you do volume calculation to work out the apparent CR
combustion volume + swept volume : combustion volume and note for a piston ported 2 stroke the stroke is considered the distance from the top of the exhaust port to the end of the stroke not the full stroke length.
This of course only works if you have a flat headed piston.
TO get an accurate reading you need to have a special compression tester with a solid connection between the spark plug hole & the gauge because the volumes of gas are so low.
You also need dozens of heads so the volume occupied by the testers head inside they cylinder is the same as that of the spark plug, a lot more tricky than most would think.
This sort of gear in well over $ 1000 and frankly not worth the money unless you are tuning race engines.
So most of us just note the lowest cylinder pressure readings that from experience we get from engines that actually do run
So on MY ( Lislie) gear a reading under 90 PSI & the owner gets told it is new cylinder & piston time
On Stars gear it is 100 psi
I doubt that either of them would be accurate .

 

[StarTech]

You right neither will be 100% accurate. It all depends on length hoses and the placement of the check valve along with other inaccuracies but with experience with a particular gauge setup we do learn where the eqipment should be reading for things to work. It depends how you testing the compression as with throttle closed reads different than when the throttle is held fully open.

I have been kinda lucky to get the test gauge I have as it reads well within most equipment specs when they have new parts install. I have using the current gauge for well over 6 years now as the last one kinda got crushed. Plus it was important for me to find one with the 10mm spark plug adapter. The most important thing is to have a tester that is designed for small engines and not just automobile engines, again it all about the volume of the compress air/fuel being measured. I did try one from Harbor Freight. It was the worst tester I ever brought but I still have the hoses for my homemade lead down tester.

 

[Fish]

I would say that the clanging noise on shutdown is a bad sign.
Go get a new BG50 for $140, and give this one to a kid to tinker with.