slow down faster). On the third or fourth attempt, the engine abruptly quit,
and the plane cartwheeled down the runway. The repairs took me a few
weeks and then I was off again. The plane flies like the proverbial homesick
angel, and I was having a great time—and then the engine stopped dead
again (in level flight) and the plane went straight down into a swamp. After a
couple of days of hunting, we eventually found the plane, and miraculously,
there wasn’t a mark on it. The engine was a bit wet, and the bulrushes had
cushioned its fall. We put the plane back on the bench, dried out the engine,
and fired it up. It ran perfectly for more
than 30 minutes, still on the same tank
of fuel. I had some discussions
with the folks at Evolution, and
all they could suggest was
opening up the needle a
couple of clicks. I am afraid
to fly this plane again as
I feel I cannot trust this
engine, and when the
power goes out on this airplane, it
glides like a rock!
—Dick Parkes, Kamloops, BC, Canada
Answer: Dick, gasoline engines are
far more critical in their mixture-adjustment range than glow
engines. You were probably
just setting the engine too
lean and should have used a
richer mixture than the fellows
at Evolution told you. You also
say that you dried the engine off
and flew it again and that it ran OK on the same tank of fuel. Even so, I would
be suspicious of water having gotten into the fuel. Try using some fresh fuel
and setting the engine richer.
UI have an older Super Tigre 51 that had only been run for a few minutes
years ago, then loaded with after-run oil and stored away. I have a building
project in mind that calls for an engine this size. Yesterday morning, I gave
the engine some short 30-second runs. Initially, the cold compression was
good but after the running is now very low. The engine uses piston rings.
Any idea why the compression would drop like this when cold?
—Dirk Dedoes, Fountain Valley, CA
Answer: Dirk, several things come to mind. Did you check the head screws
for tightness? This should always be done when a new engine has been
run for the first time. The aluminum head and cylinder expand, stretching
the steel screws, resulting in the head loosening. The same thing should
be done following an excessively lean run. When you say it is an older
engine, how old is it? The past couple of years, Super Tigre engines have
been made in China. I get quite a few letters complaining about the soft
compression, which usually gets better after the engine has seen some
running time and the rings are seated; this might be what’s causing
the problem. Also, after the engine has been out of use for a while, the
after-run oil may have congealed or thickened, sealing the compression.
After running, the thicker oil will be flushed away. I would be willing to bet
that after getting more running and air time, the rings will seat and the
compression will improve.
That does it for this month, gang. We’ll be back in the February 2018 issue,
beginning the 49th year of writing the column.
While working on this month’s column,
I received word that Frank Bowman,
who has been making aftermarket
piston rings for 37 years, has decided to
retire. I have always found Frank’s rings
to be equal to or better than original
factory rings, especially some of those coming from China.
However, we have good news as Frank has sold his business
to a fellow named Bjorn Baal, and since the first of the year,
he has been training Bjorn on the art of making rings using his
Logan lathe, which he also sold to Bjorn. Frank is still serving
as an adviser if Bjorn has any questions. Frank has always
had rings available for just about any make of engine, and if
not, he would make them if you sent him the engine’s piston
and sleeve. I am assuming that Bjorn will be doing likewise.
You can contact Bjorn at firstname.lastname@example.org or
delivery problem but had no improvement on the top end. I’m using Wildcat
15% nitro with 20% oil and castor. I’m curious if it’s possibly a timing issue,
although I’ve removed the cam cover and the timing mark aligns with the
pushrods as per the instruction manual. Perhaps it was mismanufactured?
I read and enjoy your column monthly and recall one that addressed “valve
float” but really doubt this is the issue since it’s a brand-new engine. I would
appreciate any advice you might have to offer.
—Bob Davis, Volunteer Aeromodelers, Knoxville, TN
Answer: Bob, you sure have a puzzle that I am not sure I have an answer for.
The only thing I can come up with is the cam timing is off, as you suspected.
Remove the rocker-arm covers and, with the piston at top dead center on
the over-lap stroke (i.e., when both valves are partially open), lay a small
ruler across the rocker arms. It should be approximately level or just a hair
off. If not, the punch mark has been punched in the wrong place. I would
appreciate hearing what you find.
Response: Mr. Lee, I did the troubleshooting suggested in your email, and
sure enough, at TDC on the over-lap stroke, the valves were askew about
10 degrees. I reset the cam gear so that the valves were level at TDC and
reassembled the head. I ran the engine on the aircraft and got a slightly
rich-running rpm of 9700. Bottom line—the timing was the issue. The
factory punch mark on this engine is now a 12:00 instead of 1:00 as it came
out of the box. I’m surprised it ran so well “mistimed”; the only indication of
a problem was the low output. Many thanks for your Engine Clinic column
and sound advice. I’ve read your column religiously for many years and,
after reading your email, remembered you had offered this technique/
procedure for timing a four-stroke in one of your previous columns. Age is
taking a toll on my memory!—Bob Davis
Reply: Thanks for the follow-up, Bob. Evidently the instructions should
have read “straight up” rather than aligning with the pushrods. Thunder
Tiger four-stroke owners, take note!
U Being disgusted with glow engines due to their unreliability, I decided
to switch to more reliable gas engines. I completed an older Carl Goldberg
Ultimate 10-300 model and installed a brand-new Evolution 15cc gas
engine. It started well, and I broke it in according to the manufacturer’s
instructions and specified fuel mix. The first flight went amazingly well until
the landing. I couldn’t get the plane down on the runway as I had set the
idle a little too high (I had assumed that a biplane would have more drag and
Like other gas engines, the Evolution 15cc
powerplant may require a richer mixture
than the manufacturer recommends.