skyguy04
04-04-2019, 06:03 PM
I need some advice. On my model 4 912UL, I've got 914 carbs that have been re-jetted for 912 and the throttle position sensor has been removed. For all intents and purposes it has been running fine for 200+ hours except for within a narrow rpm range of 2100-2700, especially when it's cold. When i'm warming up, i try to avoid that range. Once it's warmed up, it can hover in that area OK, but it's not as smooth as the rest of the RPM range.
This annual i'm determined to lick this problem. Working with an A&P, we've tried all sorts of things. We've...
checked float weights
checked float levels by measuring the bronze fork thingy to verify it's around 10.2mm when carb upsidedown
i've replaced all the rubber
upgraded to the new 893 115 fuel pump (originally had 893 110)
added a 4-way splitter on the pressure line to add an oraphice (tried .035 and .040)
added a pressure gauge to see what the fuel pressure is
tried a pressure regulator and set pressure to ~5psi instead of spec 7+ of the new fuel pump
The pressure regulator seems to have made the biggest difference. It runs fairly smooth (with mechanical sync only and no throttle cables attached yet) in the 2400 rpm range, which it has never done. My question is why? Why would Rotax spec the new fuel pump to put out 7.2 PSI when the Heavy Maintenance manual specifically states that the carbs expect no more than 5.8 PSI? Even with the return line and .035 oraphice, we're seeing pressure at the carbs of 6+ PSI. Our theory is that the carbs not holding back the fuel pressure from the pump, being forced to run rich, vibration/shaking ensues making the problem worse (especially on a model 4). LEAF tells me that they have an $85 treatment that they've had to do even on NEW carbs to make sure they can hold back the pressure of the new pumps. What the heck?
I don't like the pressure regulator solution, but it seems to be my only solution at the moment. If i stick with the pressure regulator, would you put it before or after the 4-way splitter and return line, or add 2 regulators after the 4-way splitter between the splitter and each carb?
Thanks in advance.
[EDIT] OH, before you tell me to just live with the shaking in the "bad" RPM band... I just put in an order for a new exhaust from Kitfox because of a crack i found due to this shaking. We've also found other signs of damage due to shaking that I've replaced or repaired. So it's just not OK for this engine to shake at any RPM band.
This annual i'm determined to lick this problem. Working with an A&P, we've tried all sorts of things. We've...
checked float weights
checked float levels by measuring the bronze fork thingy to verify it's around 10.2mm when carb upsidedown
i've replaced all the rubber
upgraded to the new 893 115 fuel pump (originally had 893 110)
added a 4-way splitter on the pressure line to add an oraphice (tried .035 and .040)
added a pressure gauge to see what the fuel pressure is
tried a pressure regulator and set pressure to ~5psi instead of spec 7+ of the new fuel pump
The pressure regulator seems to have made the biggest difference. It runs fairly smooth (with mechanical sync only and no throttle cables attached yet) in the 2400 rpm range, which it has never done. My question is why? Why would Rotax spec the new fuel pump to put out 7.2 PSI when the Heavy Maintenance manual specifically states that the carbs expect no more than 5.8 PSI? Even with the return line and .035 oraphice, we're seeing pressure at the carbs of 6+ PSI. Our theory is that the carbs not holding back the fuel pressure from the pump, being forced to run rich, vibration/shaking ensues making the problem worse (especially on a model 4). LEAF tells me that they have an $85 treatment that they've had to do even on NEW carbs to make sure they can hold back the pressure of the new pumps. What the heck?
I don't like the pressure regulator solution, but it seems to be my only solution at the moment. If i stick with the pressure regulator, would you put it before or after the 4-way splitter and return line, or add 2 regulators after the 4-way splitter between the splitter and each carb?
Thanks in advance.
[EDIT] OH, before you tell me to just live with the shaking in the "bad" RPM band... I just put in an order for a new exhaust from Kitfox because of a crack i found due to this shaking. We've also found other signs of damage due to shaking that I've replaced or repaired. So it's just not OK for this engine to shake at any RPM band.