MGB: Compression on a Leaf Blower??

Hi Guys and Gals,

My employers Leaf Blower, a STIHL BR 320 backpack type blower is not running. We gave it an overhaul here at work, but it still will not start. My co-worker figured we should check the compression (tomorrow, when I bring my gauge in).

Problem is, I have no clue what it should be..

The Engine specs are as follows.

STIHL single cylinder 2-stroke engine.

Displacement: 2.74 cu.in (44.9 cm)
Bore: 1.61 in (41 mm)
Stroke: 1.34 in ( 34 mm)
Engine Power: 2.7 bhp (2.0 kW)

I did a search online and have the manual here, but I couldn't find out what the compression should be. Have any ideas?

Thanks,
John

My searches keep bringing up a figure of 120psi.

OK, info gleaned from a bunch of sites, pressure when new should be 160psi, 120-160psi is very good, 90-120psi is worn but it should run, starting would probably be tough though, below 90psi it probably won't start.

Thanks Mike,

For some reason my searches wouldn't bring anything up.

John

Pull it through slowly. It should feel like it has compression on both sides of the piston, sort of a high then low compression feel. The low side is compression in the crank case which forces the fuel- air mixture into the combustion chamber when the piston comes down. If you don't get that double kick it's the reed valve right under the carburetor is broken of has something under is holding it open. This is pretty rare but will keep a 2-cycle from running when It's getting spark and you presume fuel too.

Update...


We pulled the carburetor and cleaned it up, it was a bit dirty, but nothing that screamed problem. We changed out the spark plug, new air cleaner and generally did a tune up on it.

We then checked the compression and got 160 psi. This seamed odd because the machine is a few years old. We scratched our heads a bit over this... a machine this old with a lot of usage shouldn't be giving "like new" compression.... Then it hit us. We never checked the spark arresting screen on the muffler... sure enough it was clogged solid. In to the solvent bath it went and with some diligent scrubbing it came clean. We rechecked the compression and were getting 135 psi.. that made more sense. Next, we went outside and it fired right up... problem solved.

Why is it always the last thing you look at ?

John