Art Bianconi <British-Biplane@...>
Placement of the probes should be done in such a way that they are
accessible for replacement without having to remove the headers. Remember
that the hose clamp that is commonly attached to the probe has to be
reachable while the engine is in the plane and the orientation of the
tensioning bolt on the clamp can be important.
Ideally, the probe should be as close to the exhaust port as possible
consistent with your ability to orient the thermocouple leads. Usually
this is somewhere between 3 to 5 inches from the flange that secures the
manifold to the cylinder head(s). It's important that the distance be
uniform on all cylinders so as not to introduce unnecessary variables.
Be advised that the fuel distribution problems in the manifold of the
0-235 make leaning to uniform temps virtually impossible unless one is
using fuel injection (which is uncommon and unlikely) or a newly designed
twin carb setup using external manifolds Ala Rotax 911. This problem is a
consequence of design flaws that do not make themselves as apparent in
conventional a/c whose weight and dirty airframe often result in using
full power from the O-235. Long's and vari-eze's are clean machines and
can maintain flight practically at idle thrust, low power settings. It is
precisely at these lower power settings where proper fuel atomization is
nonexistent and, as a consequence, balanced fuel distribution suffers.
EGT instrumentation will help you avoid problems but you will need a
probe for each cylinder (one, on the so-called "got cylinder" is not
enough) Ideally, readouts on all four cylinders will be simultaneous, not
selected by a switch. Be prepared for some frustration when trying to
adjust mixtures at anything less than WOT.
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