May 2002
Electrical
Problems:
After
dealing with Murphy and his damn law…
The alternator whine, which
had returned was corrected by replacing the filter installed back in September
/00. I purchased the original from JC
Whitney for around $50 and it worked well until May /02 when it failed. The replacement is made by American
International and is a model S-15A CL.
It also works well, but cost considerably less at the local Canadian
Tire store (about $25).
The Revenge of Murphy:
I had
only flown about an hour since my last debacle when I noticed another “small
problem”…
My
next run-up showed a noticeable mag drop and some engine roughness on one
mag. Checking the plugs showed them to
be clean but it was obvious some arcing had been taking place at one of the
lead ends. This end had already been
replaced once and there was insufficient material remaining to repair it
again.
Since
the remaining leads were showing their age I decided to have a new set made up
and have the mag inspected. I was
planning to have this done later this year anyway since the engine was
assembled in 1983 and now has about 250 hours.
While
the mag was off I decided to do a 50-hour inspection. During this inspection I noticed the exhaust valves on the front
cylinders were still closing in and pulled the heads to investigate.
What
I found were small cracks beginning to form between the valves (a common occurrence
in VWs). It was also obvious the front
cylinders were running leaner than the rear ones. It appeared the valves were sinking into the head (which would
explain the gap closing).
Rather
than repair the old heads I purchased a set of premium heads from Steve Bennett
and had them machined for the Revmaster.
I liked the idea of the stainless valves and extra heat dissipation
capability of these heads.
While
the heads were off I also planned to hone the cylinders and re-ring the
pistons. To this end I enlisted the
services of a local engine shop, who it turns out are also familiar with
Murphy.
Back
to Steve Bennett for a set of CIMA 92 x 82 cylinder/pistons, 0.100” shims and a pushrod kit (required
since I am using the shims to lower the compression ratio.
Compression
Ratios:
My
engine was set up for a 9.3 : 1 compression ratio. Currently Steve Bennett is recommending a maximum of 8.0 : 1.
Examination
of my old pistons (front) showed signs of detonation (removed material). The causes of detonation are… too lean a
mixture, too low a fuel octane rating or an excessive compression ratio.
The
full rich mixture was set to about 125 deg F below peak as recommended
(measured at number 3 cylinder).
Cylinder head temperatures (again measured at #3) were well within
limits in all flight regimes. It was
possible to enrichen this mixture somewhat prior to the onset of engine
roughness.
Fuel
was exclusively 100 low lead.
Examination
of the plugs/pistons/cylinders suggest the mixture was lean despite the EGT
indications. This may be caused by
running the engine at “unfavourable” carb settings.
During
cruise flight I noticed that operating the engine at full throttle actually
gave lower egt and cht readings than at some lower throttle settings. In fact, reducing power from 3200 RPM to
2900 RPM would noticeably increase EGT and CHT. This would seem to indicate the needle profile of the carb is not
optimum.
When
we examine the way we set up the carb I believe we find the answer.
One
observes the engine reaches full RPM long before the throttle is fully
open. At the max RPM point the carb is
flowing as much air as the engine can, however the fuel orifice has not yet
reached its maximum size. As the
throttle continues to open the needle is pulled further from the orifice the
mixture becomes richer. Since we are
adjusting the carb for full rich mixture at full throttle it seems apparent
that the engine will run “lean” in the range of throttle settings between max
RPM point and full open.
I
plan to move the EGT probe to #4 cylinder (front) and adjust the full rich
mixture to 175 deg F below peak. This
combined with my reduced compression ratio (7.6 :1) should greatly reduce the
stress on the engine and extend its service life.
Trim:
After nearly 2 years flying
it was high time to get rid of the “flying dishwasher” look. It was also apparent that The Gadget was
hard for other pilots to spot and needed some colour to aid in visibility.
The trim is basic automotive
enamel (spray bomb). Masking was done
with 3M Fine Line tape. This stuff is worth
the extra money. Regular masking tape
will not give a consistently clean edge.
The procedure is straight out of Bingelis.
Lettering and logos were
obtained from a local sign shop. Applying vinyl is really quite easy and had
the fuselage stripes in my design been of equal width I would much rather have
used vinyl here as well.
Inside of the canopy and
turtledeck was painted with a simulated stone type paint and sealed with the
recommended clear coat. To date it has
proved reasonably durable.
