|
Hey Kevin,
Before you go trying to diagnose the problem
piecemeal, please do a flow test AT THE CARBY. And before you do
that, go and do the sums so that you know how much you SHOULD have as
a minimum flow rate. FWIW, Air BP have a website, and the
relevant info about 100LL AvGas density is on there. See:
Seems that the relative density varies from a low
of 0.6582 to a high of 0.7238. Let's take the lower density. Each
litre will weigh 658 grams, which is around 1.45 Lbs. (2.2 Lbs to
the Kg). Taking the high, each litre will weigh 724 grams which
is around 1.60 Lbs. If you directly weight the fuel that flows into the
container (digital bathroom scalesa are accureate enoungh), then you don't
have to do as much math.
Again, I freely 'fess up that I
know next to NOTHING about Lycosaurae. But each Shetland Pony on any Otto
Cycle engine will need at least 0.55 Lbs of fuel per hour per BHP at
WOT, and there should be a 25% EXTRA flow margin for safety. SO do
the math, and do the flow AT the carby. This will tell you if you
are getting enough fuel AT the carby to produce the power of which the engine is
capable (all depends on the spec of your O 320 but they are somewhere in the
vicinity of 150 - 160. So let's say 150 BHP to make the math
easy.
150 x 0.55 = 82.5 Lbs per hour = / 60 = 1.38 Lbs per minute. Now one litre
weighs 1.45 lbs, so 1.38 Lbs is 1.38/1.45 = 0.95 Litres. So you
SHOULD have a flow rate of somewhere around 1.2 litres a minute (or
whatever that is in US quarts, pints, or US Gallons etc - we got
metricated downunder in the late '70s, and I've always been confused with
US gallons as opposed to Imperial (real blokes) gallons
anyway).
Then switch tanks and measure the flow rate from
the other tank. If all is well, look in the carby for a
restriction, if not, work back doing the easy things first like the
fuel filters, BUT do the FLOW TEST first!!!!! (Make sure you have a
GOOD fire extinguisher on hand, and an assistant to turn the pumps on
& off).
Make sure you also have a clear understanding of
the difference between causes & symptoms. Be systematic, change
ONE thing at a time and note the results. If for example, you switch
tanks, and the flow rate goes up, it means that there is a
restriction on the supply from the OTHER tank. If there is no
change, then EITHER both tanks are OK, or there is a similar problem
in BOTH tanks. Similarly, if you change the fuel
filter(s) - (do ONE at a time!), and the flow rate goes
up, it means that the filter was partially blocked. If nothing
different happens, then the fuel filter was OK.
If you start fiddling with fuel filters, back
flushing tanks etc, BEFORE you know what your flow rate is, and what
it should be, you will NEVER know what the CAUSE of the problem
actually is unless you can compare flow rate results with each systematic
change.
Moreover, if your fuel flow meter IS
reading accurately, and is on the low side as you suggest, the low
flow rate COULD (but not necessarily) cause the high CHTs because the
engine is running lean under load. Running full rich is just going to more
rapidly use up whatever fuel is getting into the float bowl. All the
more reason to do a flow test to verify the accuracy of your fuel flow
meter. However, you then still have to find out WHY the fuel flow is
low. Fix the flow and the high CHT problem will most probably
disappear, and your p[ower will come back. If they persist,
then you need to look for another cause(s).
Anyhow, that's about all I can suggest from
where I sit in the Antipodes.
Chers,
Leon
----- Original Message -----
Sent: Sunday, July 24, 2005 9:37 AM
Subject: [FlyRotary] Re: FUEL FLOW was
Re: [FlyRotary] Lycoming debugging test - -HELP!
thank you for the reply. I will
start looking into my fuel supply. I do have a flow meter but haven't
been watching it too much since I was so concerned with the very high CHT's
that I've been hitting. the mesh filter in the carb is good, brand new
in fact. I have two auto mesh fuel filters ahead of the facet
pump. they look clean thru the glass, but I will remove them and look
more closely. I have been reluctant to remove the wing tanks in order to
check the pick-up tubes since it is so much work, plus my tanks are quite
full. I suppose I could try to back flush them first with an air hose if
I am VERY careful. now that I think about it I don't think I have tried
switching tanks yet. that would certainly be an easy task.
I believe that my
fuel flow meter was showing 8.5+ gals/hr at full throttle. now that I
think about it that may be low, and 10.5 more typical.
thanks for your
help. we are having absolutely beautiful weather here and I was so
anxious to do some flying, but....at least I can hang out at the airport some
more. it's nice to find someone who doesn't immediately say "I'd get rid
of that electronic ignition to begin with"!
to answer Tracy, yes
I have EGT's, shows low 1100's since I am running full rich, I have
manifold pressure but never looked at it(I'd make a hell of an experimental
test pilot-wait, I am one, oops). in my defense, I have been nervous
running my CHT's up past 400 degrees in order to replicate this problem, since
I just had three cylinders welded because of who knows what? I've been
thinking about attaching my leaf blower to my plenum to help cool things on
the ground runups. it produces 220mph wind, or so the box
says.
----- Original Message -----
Sent: Saturday, July 23, 2005 2:48
PM
Subject: [FlyRotary] FUEL FLOW was Re:
[FlyRotary] Lycoming debugging test - -HELP!
Hey Kevin,
Check your fuel FLOW RATE, especially at
your normal climb angle (on the ground). It sounds like it's losing fuel flow on take-off - the carb
is using whatever fuel is in the bowl, and then
running lean. You might have to jack the fuselage to the required
angle in case there is something in the tank that is occuding the
pick-up(s). However, if you can reproduce it on the ground doing
a run-up, it definitely sounds like lack of fuel FLOW (not pressure)
to me. If the engine will maintain 1700 RPM, then it is getting
almost but not quite enough fuel FLOW at WOT.
Various things come to mind.
FIrstly, (and most easily), a partially blocked fuel filter
(just because it's new doesn't mean it'll work!!). So change that
first. Use a different brand to eliminate faulty batch
manufacture.
I once had a mysterious problem with an
RX4. Chased it for weeks. Turned out to be a bee in the fuel
line. (I'm serious - the guy who owned the car used to work for me and is
still around and will verify this!!). Anyway, Jack's pride &
joy would work just fine around town. But out on the open road,
it would run out of fuel in 3rd gear (usually when overtaking). Turned
out that a bee (very dead at this stage) had got into the fuel
tank. (The problem occurred soon after Jack filled up at some outback
petrol station when he was on holidays). The bee immediately got
stuck in the fuel supply line. At normal cruising flow rates,
there was sufficient fuel to run the car. On hard acceleration, the
bee's wings & body would PARTIALLY block the supply line in
the tank. We only found it when we removed the tank, replaced it
with another one, and cut the old one open. You just wouldn't
read about it in Ripley's or the War Cry!
I've also had pieces of fuel gum (and one
instance a piece of cardboard) block the fuel pick-up. When fuel sits
a long time at a bowser storage tank, especially if there is water at
the bottom of the tank, it will form a sheet of gum at the
fuel/water interface, almost like condom latex, (but nowhere
near as strong). This film of gum gets broken up the next time a
tanker does a dump, and bits of this will very effectively block a
fuel pick-up, or cripple a fuel filter.
I've seen it happen on several occasions over
the years. It was particularly prevalent in the early '70s during
the phase out of "standard" petrol before the introduction of unleaded into
this country (all rotaries used to run on standard not super - which was
heavily leaded back then). I found out the hard way soon after I had
purchsed my RX2 Coupe in 1971. Filled up at an "out of the way" petrol
station (where the fuel sales volume on standard was virtually nil),
and immediately had problems. After eventually getting back to
Sydney, we quickly diagnosed it as a tank problem. Even steam
cleaning the tank didn't fix it. The tank was replaced, and we
cut open the old one, and there was all these bits of gum, just
like a thin membrane tyhat had been cut up.
So the place I'd be looking is in your fuel
SUPPLY system, as it seems you have done everything else except check
fuel FLOW rate. Can't guarantee that this IS the problem, but
the quick and easy way would be to check your open flow rate at the
carb, and then compare it with somebody else's.
Alternatively, you can work backwards as you know that the engine
requires at least 0.55 Lbs per hour per BHP at WOT. (First convert that
to litres or pints or quarts per minute and you can direct measure it,
or run the fuel into a can for 1 minute and then go and weigh it).
Fuel flow rate should be at least 25% more than the max required at WOT so
that your carb doesn't run dry.
One other area of the problem could be the
incorrect needle/seat arrangement on the carb, or a partially blocked
filter at the banjo - I'm not privvy to what carb you are running so
this is conjectural as I plead total ignorance of Lycoming carbs.
However, all stock Mazda carbs have fine brass gauze mesh
filters, both at the banjo, and on top of the needle&
seats.
Additionally, in the racing 13Bs, an IDA
Weber would run out of fuel as the float bowl was just way too small for
anything over 250 BHP. We used to replace the 2.5 needle/seat assys
with 3.0 units. But we would STILL run out of fuel on long
straights. Extended float bowls were then added, which
did fix the problem, but welding the Weber material was
tricky, especially if it had been saturated with oil laced fuel for a
long time (sometimes the castings were slightly porous)..
My final solution was to add an EXTRA
float bowl (from an SU carb), with it's own needle & seat,
and a fuel return. We could then go back to a 2.5 Weber needle
& seat, as the 3.0 one was prone to flooding at idle and on the
over-run. (Nothing good has ever come of fuel dilition of engine oil in
a race engine). This fixed all fuel flow issues. Some time
later, affordable aftermarket EFI came along, and all this
passed into history. But fuel SUPPLY issues are even MORE critical
with EFI.
So these are just a few areas where you can
look. Be systematic, and start at the connection between the
carb and the supply line. You will then quickly eliminate either the
supply to the carb, or the carb needle & seat itself. If the
open pipe fuel flow is more than adequate, then I'd be looking inside
the carb for restrictions. As I have said, this is only a
guide. Feel free to contact me off line if you feel the
need.
Cheers,
Leon
----- Original Message -----
Sent: Sunday, July 24, 2005 6:33
AM
Subject: [FlyRotary] Lycoming
debugging test - -HELP!
I realize that this isn't exactly
the right forum, but you guys seem to have excellent debugging
skills. my local RV group is totally worthless ("I find the carnauba
wax doesn't show fingerprints as much as the....")(except Mike!) the
on-field mechanics are sick of me coming by.
my O-320 RV-6A
is having RAG (rpm attention getter) problems. it runs fine until
about 300' on take-off, then the rpm's drop to like 1700 (from
2200). switching mags to electronic ignition(Rose) makes no
difference. mixture no diff. pulling the throttle it will
seem to run fine at 1700rpm. I have 4 new cyls that I am attempting
to break in. ground run ups can only go like 2 minutes before CHT's
exceed 400 degrees. I discovered I can get the rpm drop on the
ground if I run it a minute or so full power. cht's will typically
approach 415 degrees when the drop takes place. I don't know if the
temp is significant or not, it does seem that way. I have a brand
new slick left mag. rose checked the electronic ignition and could
find nothing wrong, even put it in an oven to test. I installed a
new carb since the old one showed signs of leakage (and I'm at whits
end). no change. my fuel pressure has been showing 8psi this
last year, up from 6 psi when originally installed.(no idea why) I
installed a pressure regulator in-line which works like a charm. I
can dial whatever pressure I like. 2 psi makes no difference.
I bypassed the mechanical fuel pump and ran it just on the facet
electric. 2 psi, no difference. I discovered that you can run
the carb for maybe 30 secs with no fuel pressure while it burns off the
bowl gas. premier engines suggested induction leak making the engine run
lean. no leaks detected cold or hot engine. (pressurized intake with
vacuum, sprayed soapy water)
if the engine
didn't run right to start with then many explanations would hold.
but it runs fine for several minutes. it runs the same under mag or
electronic ignition both before and after the problem kicks in.
everyone really wants to blame the rose ignition, but I'm not seeing any
connection. with such a quick run-up my oil temp barely registers,
so I doubt parts are seizing. I have new mineral oil in it for
breakin right now. there is a bit of MMO in the fuel too I believe,
left over from a mechanics "what the hell" attempt from way back. my
cyls are all new, rebuilts, so no valve problems. I checked the push
rod lengths when I reinstalled them. the engine isn't missing
when the problems occurs, it simply won't run faster and it seems to
prefer the throttle pulled back to match the rpm it is putting out.
I can't figure out any more lean mixture scenarios to try. the new
carb runs just like the old one did. I recently replaced the mag
harness with a much newer, used one. I tested it and it was
fine. the plugs are all new, but again, no diff between mag and elec
ignition. if I flip the ignition to R and shut off the elect ig then
the engine quits as would be expected, no wiring problems.
fuel, spark,
air, timing, that's all there is. this damn lycoming lawn mower has
me stumped. my engine has been running fine up until this (hey, we
all have "cracked" cylinders, get out your microscope and look!).
you guys have any ideas?
----- Original
|