Technical 2-stroke question (chainsaw)

 

Spodes -

I was doing a little research into hot-rod exhaust mods for chainsaws,
and came across a term I never heard before.  Something about a
carburetor that is called "pop-off pressure".  What the hell is that?
Saws do use weird little pumper carbs that can run upsidedown - maybe
it has something to do with pump pressure?

Excerpt from another board:
'From what I have found, if the carb makers are paying any attention
at all to the pop-off pressure on their carbs for stock saws, they
must be on a secret mission to conceal their knowledge...ie, the pop-
off pressures are all over the board. A 242xp that I tested today, had
a pop-off pressure of 34psi! More often than not, I'm finding stock
pressures above 20psi. Don't know if you saw the email I posted from
Walbro on the subject.... Basicly, a
guy there said it doesn't matter on stock equipment, but did go on to
say that in racing engines, like in karting, the pop-off is more
critical. I guess that they think it is out of their jurisdiction to
have to respond informatively to any question that goes beyond stock
engine requirements. Unfortunately, they act totally vague on the
subject. I still haven't exactly settled on what I find is best for my
modded saws, but I doubt it is 34psi.'

JayC

 


I guess we all know what a pop-off valve is and what it's used for. I
am very surprised to find that someone has determined that you need
one on a carb for a stock chainsaw?

 

danged if i know but this might help
http://www.groupk.com/tec-carbs97.htm

Pop off pressure - This term refers to the amount of fuel pressure needed to
push the float needle valve away from it's sealing seat. Pop off pressure is
checked with a hand pump that is fitted with an in line gage. The pump is
connected to the fuel input fitting of the carb. The return line fitting is
then sealed off with one finger while the pump pressurizes the float
chamber. The pressure reached on the gauge when the needle gives way is
called the pop off pressure. "Adjusting" the pop off pressure is discussed
below. If you don't have a pop off pressure gauge, you should get one that
has a gauge and pump capable of 30 psi. (Most Mikuni distributors carry
them) At the beginning, it's only important to check that the needle holds
the pressure back with no leaking up to the point where it pops cleanly away
from the seat. Perform the pop off test several times to confirm the actual
pop off pressure. Initial pop off pressures on round pump carbs should be
between 8 - 12 psi. Initial pop off pressures on the square pump Mikuni and
Kiehin carbs (unless otherwise specified) should be no less than 25 psi and
no greater than 35 psi.


all i remember from tweaking jetski days was
the pop off pressure was related to mid range throttle response.
john

 

pop goes the wheeze hole.