Many years ago when the 084 came out

jomoco · Nov 14, 2006

The best chainsaw shop here in San Diego ( D&D Saws ) gave me an 084 to demo for them in some big wood ( Euc ). I ran the saw and it worked well, but since it was a demo, I got a bit aggresive in the base cut, and the saw reved up real high and died. I sheepishly took it back to the shop with the bad news.

The chainsaw mechanic told me that when I torqued real hard on the saw in the base cut, that I popped the base impulse line loose, and that had fried the saw. The owner of the shop told me not to worry as that was the reason they had me demo the saw, and that he would contact Stihl and suggest they lengthen their base impulse line.

My question has always been, what was the reason a base impulse line coming off would fry the saw? Is it because it sucked air through the line into the crank case causing it to over rev and fry the piston etc.? Could it, can it happen that fast? What was the chain of events? I would have thought it would simply quit pumping gas and die without massive damage internally.

Any experts care to enlighten me on two stroke fundamentals?

jomoco

GASoline71 · Nov 15, 2006

jomoco said:
My question has always been, what was the reason a base impulse line coming off would fry the saw? Is it because it sucked air through the line into the crank case causing it to over rev and fry the piston etc.? Could it, can it happen that fast? What was the chain of events? I would have thought it would simply quit pumping gas and die without massive damage internally.

Any experts care to enlighten me on two stroke fundamentals?

jomoco

My take is yes it can happen that fast. I've seen other 2-stroke stuff (i.e. dirt bikes) go through the meltdown fairly fast. Even when we tore down the engine thinkin' we got it stopped in time, there was a lot of damage. The saw was prolly runnin' on whatever fuel was left in the carb, and then experienced a massive lean-out condition. Just my .02 cents.

...oh yeah... ignore the mule. He's obvioulsly trollin' tonight.:bang:

Gary

Lakeside53 · Nov 15, 2006

It's not just the gas left in the carb... a big tank surface area will generate a LOT of pressure - way more then a carb needs for fuel... and the vibration from a fast running saw will foam the tank plenty... (the tank vent doesn't vent out fast enough). It probably wouldn't have idled though as the mixture would have been too lean, but it will sure run at high throttle where the impulse air contribution is smaller.

EDIT: [Added]: Anytime the throttle dropped, the carb probably ran lean, so "gunning" a saw (to keep it running) that will only run at higher rpm and not idle is a real bad thing... Same for "goosing" out the last few drops in a tank.

lesorubcheek · Nov 15, 2006

jomoco said:
My question has always been, what was the reason a base impulse line coming off would fry the saw? Is it because it sucked air through the line into the crank case causing it to over rev and fry the piston etc.? jomoco

If the impulse line comes loose, no more fuel is being pumped by the carb (that's what the impulse line does!). This is the same as running out of gas in the cut which causes a lean run condition under load. Now this happening once and totally frying the saw seems rare, but others have posted the dangers of running out of fuel while in the cut that may share their experiences. Its certainly not good to run dry in the cut, but how much any given saw can take requires some metrics collection.

Dan