octane

Joined
Dec 24, 2001
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#1
I am hoping for some help here on this question
on octane in regards to what a motor really needs.
And if it actually changes with the temp of the motor.
This has come to my attention because of a bike that
I have been working on all summer and couldnt
figure out what was wrong with a run on problem
even after replacing crank seals.low and behold my
son won some race gas and we tried it the other day
and the bike didnt run on anymore.Is this pre ignition?
and the bike needed more octane all along or is there
still a air leak and because of the richer fuel it went
away.Like to figure it out.Any Ideas appreciated.
overbore
 
Joined
Jan 11, 2001
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#2
From the Fuel Faq Part 8.2

Run-on is the tendency of an engine to continue running after the ignition
has been switched off. It is usually caused by the spontaneous ignition of
the fuel-air mixture, rather than by surface ignition from hotspots or
deposits, as commonly believed. The narrow range of conditions for
spontaneous ignition of the fuel-air mixture ( engine speed, charge
temperature, cylinder pressure ) may be created when the engine is switched
off. The engine may refire, thus taking the conditions out of the critical
range for a couple of cycles, and then refire again, until overall cooling
of the engine drops it out of the critical region. The octane rating of the
fuel is the appropriate parameter, and it is not rare for an engine to
require a higher Octane fuel to prevent run-on than to avoid knock [27,28].
Obviously, engines with fuel injection systems do not have the problem, and
idle speed is an important factor. Later model carburettors have an idle
stop solenoid which partially closes the throttle blades when the ignition
key was off, and thus ( if set correctly ) prevents run-on.