Roger Lee Posted February 12, 2012 Report Share Posted February 12, 2012 Hi all, HELP! I need a little help. Someone just called me and said that when he pushes the nose over the fuel starves a little and the engine coughs with no change in the throttle setting. When he pulls back and flys flat and level or climbs it's just fine. Any ideas. I'm stumped on this one. Link to comment Share on other sites More sharing options...
Tom Baker Posted February 12, 2012 Report Share Posted February 12, 2012 How hard is he pushing over? Link to comment Share on other sites More sharing options...
Jim Posted February 12, 2012 Report Share Posted February 12, 2012 My first thought was the Merlin engine and Miss Shilling's Orifice. Going into a steep dive would lift the carb float and needle valve from its seat, flooding the intake with fuel. If this is what's going on, the fix would be to not dive so hard. Link to comment Share on other sites More sharing options...
Roger Lee Posted February 12, 2012 Author Report Share Posted February 12, 2012 I just got off the phone. It is when he pushes it over and gets a slight negative G. It doesn't happen on a normal descent. Still this shouldn't happen. Link to comment Share on other sites More sharing options...
Runtoeat Posted February 13, 2012 Report Share Posted February 13, 2012 Jim from the Left Coast may have hit on something. I had an opposite situation from the engine missing the other day during a power on stall practice session. I was at full pitch up to get to stall while under full throttle. I got a strong fuel smell during this which I had not gotten before and which has not happened since. At the time, I suspected that I may have had fuel overflow from the carbs due to the extreme pitch up angle preventing the floats from shutting off fuel. Could it be that the pitch down in your case might also cause the floats from shutting off fuel flow and allow flooding of the engine as Jim suggests? Or, this pitch down may prematurely shut off fuel and cause fuel starvation? Link to comment Share on other sites More sharing options...
FastEddieB Posted February 13, 2012 Report Share Posted February 13, 2012 Could it be that the pitch down in your case might also cause the floats from shutting off fuel flow and allow flooding of the engine as Jim suggests? Or, this pitch down may prematurely shut off fuel and cause fuel starvation? I'd think the latter. The floats float. With negative g's they'd lift up in the bowls, shutting off the flow of fuel into the bowl. Now, the fuel already in the bowl would do the same, floating up and exposing the fuel pickup to air. This is just my take, and by no means authoritative. I'd think it would pretty much have to happen given the operating principle of our carbs. Link to comment Share on other sites More sharing options...
Roger Lee Posted February 13, 2012 Author Report Share Posted February 13, 2012 Hi Eddie, You account sounds good. If anyone else has any ideas I'm all ears! Link to comment Share on other sites More sharing options...
bitten192 Posted February 13, 2012 Report Share Posted February 13, 2012 Negative Gs, gas goes up, float will "float" to top of liquid which is down (just like level inverted flight). eb Link to comment Share on other sites More sharing options...
Roger Lee Posted February 13, 2012 Author Report Share Posted February 13, 2012 I think you guys are right. It would be nice to see if this is repeatable. If a couple of you guys want to test this then give it a try. Push the nose over at your regular rpm and see if the engine coughs within 2-5 seconds. Link to comment Share on other sites More sharing options...
FastEddieB Posted February 13, 2012 Report Share Posted February 13, 2012 As a data point, both the Citabrias I owned had carbs and the engine would quit the instant negative g's were applied. For sustained inverted flight you need to have fuel injection, though I vaguely recall something called "pressure carburetors" that might avoid the problem.* Bear in mind, this should not happen in a stall where you just let the nose fall through. Negative g's are a different animal, requiring a definite push and is usually accompanied by cr*p coming up off the floor. BTW, in about 260 hours I've never pulled negative g's in my Sky Arrow, so I can't say from experience what does or doesn't happen. *special oil systems are also required for sustained inverted flight. Link to comment Share on other sites More sharing options...
Jim Posted February 13, 2012 Report Share Posted February 13, 2012 I've been thinking about this way too long. At zero g, everything in the carb floats because there's no gravity. No gravity, nothing to hold the needle valve against the seat. Carb floods. OTOH, if negative g forces the needle valve against the seat, stopping fuel flow, it's probably not a problem because there's a reservoir of fuel in the bowl. Alternate explanation is that the fuel passage to the main jet is unporting because of the fuel in the bowl moving around. Note that Rotax specs -0.5g for 5 seconds max in the installation manual. Link to comment Share on other sites More sharing options...
FastEddieB Posted February 13, 2012 Report Share Posted February 13, 2012 Note that Rotax specs -0.5g for 5 seconds max in the installation manual. Speculation mode>on Unless they give a reason, my guess would be oil pressure concerns. Link to comment Share on other sites More sharing options...
josjonkers Posted February 14, 2012 Report Share Posted February 14, 2012 I think you guys are right. It would be nice to see if this is repeatable. If a couple of you guys want to test this then give it a try. Push the nose over at your regular rpm and see if the engine coughs within 2-5 seconds. Yep, I have experienced this but in my case it is more like the engine missing a few beats rathethan the 2 to 5 seconds. Pretty repeatable too! Link to comment Share on other sites More sharing options...
FastEddieB Posted February 15, 2012 Report Share Posted February 15, 2012 Yep, I have experienced this but in my case it is more like the engine missing a few beats rather than the 2 to 5 seconds. Pretty repeatable too! I think you may have misunderstood Rogers's comment. It was that the engine would "cough" within 2-5 seconds, not that it would "cough" for 2-5 seconds. Right? Link to comment Share on other sites More sharing options...
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