High fuel flow/ Rich Mixture

I have been having high fuel flows on my RV-12 during the first 25 hours. They have been 6.2 gph at 5000 rpm economy cruise, and up to 8.2 gpm at takeoff climb power. To get 5.5 gph I would have to cut back to 4400 rpm or so.

So today I changed my oil and my sparkplugs are uniformly dark, just like a moderately rich engine. Both sides were the same color, so the carb balance was not the problem.

Today after seeing that 8.2 gpm (and yes, my flowscan is calibrated pretty well) I asked my CFI/A&P what could be wrong. We started thinking the gas level in the carbs could be high from too much fuel pressure, it would act like heavy floats because it would take more pressure (higher gas level= richer main jet) to seat the needle against high pressure, where a low pressure fuel system would cause a lower (Rotax normal)fuel level in the carb.

Then I remembered I flew a Tecnam Sierra in which you used a electric pump for takeoff. ( and landing) Then the rich mixture is good detonation insurance as well as mechanical pump failure insurance.

So I got back in, climbed to altitude, and pulled the fuse. Fuel consumption went to zero while it ran off the excess level of gas in the bowls, and then it stabilized at normal Rotax fuel flows.

When I put the fuse back in, the flow went to 10gph, alarms sounded for a bit, while it filled the carbs up to the artificially high level.

I tried it back and forth several times, same results.

Does the Rotax factory test engines with the mechanical fuel pump?

I think Larry Geiger had a good idea with the switch.

Please correct me if I am wrong about a technical detail above.
Ken at Vans said the Rotax bulletin didn't cover our engine S/N but it would seem to be physics.

My fuel pressure is 2.7 psi with the mechanical pump and 6 or so with both.

John

John, have you set the "K" factor in the Dynon yet? Most, if not all of what you describe is normal except for the flow rate numbers. The "K" factor needs to be adjusted to meet the actual burn rate determined by how much fuel you put in. Keep track of the flying time and the amount of fuel added, then adjust the "K" factor accordingly. I would give you my "K" factor number, but since I shut the electric fuel pump off (decreasing the rate of flow) my "K" factor number would not help.

In shutting off the second pump in flight you are going to see what you describe even if the system is working fine. This is the system adjusting to the different pressure / flow rate. Not a problem.

Thanks larry

Yes, my K factor is good. If I take off with 20 gallons and fly 2 hours, I might have 7.5 gallons fuel left and that agrees with the computers fuel remaining within .2 gallons or so.

I'm not saying everyone should put a switch on their fuel pump. I'm saying if you have dark brown spark plugs it might help. Maybe I just happen to have a higher pressure facet pump than the norm.

I do think the Rotax service bulletin implies this could be a problem, though.

John,

While Larry's idea is a good one and works for him, most of us are too lazy to put in another device. That said, my electric pump runs continuously and my fuel flow has averaged just over 5gph for 98 hours of flying. Unless you have a bad electrical pump (what does your Dynon show when you turn it on before you start it?), you might want to continue searching for the problem. I believe most flying 12's are not duplicating your results. I know the factory one runs continuously.

High Fuel Flow

What is your EGTs at these high power setting. There is a Mixture adjustment on each carb. When you set up your carbs for the first run did you adjust the Carb Mixtures per the Rotax manual. As I recall it was screw in the Mixture screw to the stop then one an a half turns out. If you EGTS are cool then your mixtures maybe rich. John

RV12 N1212K