Originally published February 1998
(Our regular readers of The Leading Edge will remember past articles where Brian Martinez, Q-200 builder, suspected his Ellison Throttle Body Injector (TBI) was the reason his engine just wouldn't put out the power he expected. Turns out it wasn't the TBI at all--here's the rest of the story to set the record straight…)
It is just as I told you before. There was never anything wrong with the Ellison TBI. The TBI masked a fuel header tank venting problem. It goes like this:
1. Brian builds a Q-200 which is damn near per plans except for changes documented and pioneered in the Quickie Builder's Association newsletter. Per plans includes drilling a vent hole in the fuel filler cap on the side of the fuselage. Per plans includes a plastic connecting hose between the header tank and the vent tube which is bent forward, pitot style, to gather dynamic pressure to pressurize the fuel tanks.
2. Brian installs a Ellison TBI because it is simpler, lighter, and doesn't have any ADs out against it...without ever running the Continental O-200 with a Marvel Carb for comparison (first masking mistake).
3. Brian runs the engine statically and runs taxi tests to just about takeoff without any engine problems showing up.
4. Brian goes flying one day and upon reaching flight speed and climbing out finds that there is "just no more power"...no more throttle and it won't indicate more than 120 mph.
5. The masking increases:
a. Someone thinks its the prop...Brian borrows a different prop and buys a different prop with no difference in performance.
b. Someone thinks its the engine air inlet...Brian rounds the inlet more with no change in performance.
c. Someone says that they don't trust the Ellison...Brian calls Ellison and they say that the air may be turbulent and the fuel pooling in the manifold spider. Ellison also asks whether there is enough pressure from the header tank to give good flow. Brian assures Ellison that the header tank is kept full. Brian looks at Ellison's hypothetical fuel system schematic and it looks like his. Brian modifies the airbox and builds a second slightly larger airbox as a comparison. Results are the same and Brian doesn't trust anything anymore.
6. John Sharp says install a manifold pressure gauge and think about installing the Marvel Carb.
a. John Sharp makes a quick check of the timing and says it looks fine, but finds a slight leak on the manifold pressure tap from the manifold spider.
b. Ground running comparisons of the Marvel versus the Ellison seem to show that the Ellison has less Manifold Pressure at full throttle. Brian assumes the Ellison has a problem and installs Marvel for flight.
7. Paul Fisher e-mails Brian about the problem and asks about the fuel system configuration. Paul says, "my airplane looks like this, what does your's look like?"
a. Brian's Q-200 has the vent hole in the filler cap, Paul's does not. The hole which is on the filler cap on the side of the fuselage under an access door will suck head pressure out of the fuel tanks.
b. Brian does some further checking after Paul's hints and discovers that the plastic tubing connecting his header vent tube to the total pressure vent is kinked and discolored from the fuel vapors. Brian replaces the vinyl tube with a polyflow tube.
8. Brian goes flying and performance is dramatically different. True air speed is 180 mph. Slower than some, but something to work from.
9. The light goes on in Brian's head. There was a venting problem.
a. The Ellison was dependent on good pressure from the fuel system, whether pump fed or gravity fed. The header tank could not pressurize correctly due to a blockage in the vent line caused by the kink and the loss of existing pressure due to the hole in the filler cap. This would account for the loss of power during climbout.
b. The Marvel ran better during static runs because it has a float bowl carb and is not as sensitive to fuel pressure when its not going fast. The Ellison doesn't have floats. You have to shove fuel into it, gravity or otherwise. However, without the vent fixes, the Marvel would have run badly also.
10. Brian tests the theory. He pulls the Marvel, reinstalls the Ellison and goes flying. The Ellison runs smoother, has better throttle response, and shows every bit of performance that he saw flying with the Marvel and a good venting system. Ellisons are a good thing.
The moral to this story is:
a. Listen very carefully when you talk to yourself.
b. Don't be fooled by masks.
(Fuel vent problems are not an isolated incident--Scott Horowitz had a similar yet different problem in his Tri-Q-200, which turned out to be his fuel vent system acting as a siphon and starving the engine for fuel pressure. The fuel vent system is an important part of the fuel system--don't blow it off!)
Contents of The Leading Edge and these web pages are the viewpoints of the authors. No claim is made and no liability is assumed, expressed or implied as to the technical accuracy or safety of the material presented. The viewpoints expressed are not necessarily those of Chapter 1000 or the Experimental Aircraft Association.
Revised -- 20 September 1998