----- Original Message -----

From: Leon Promet

To: Rotary motors in aircraft

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 -----

From: kevin lane

To: Rotary motors in aircraft

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?

Kevin Lane  Portland, OR
e-mail-> n3773@comcast.net

----- Original