zerogravity760
Member
I bought an RV-4 last year with an O-360 A3A with an Ellison Throttle Body and a C/S prop. About 200 hrs on the engine.
I pretty consistently get an engine stumble during takeoff at full throttle. As soon as I close the throttle a bit, it goes away. It does not matter whether I have the electric fuel pump on or off, it still stumbles. The engine data shows a slight drop in fuel pressure when it happens (goes from around 3.5-4.5 down to usually 2.5ish but I have seen 1psi as well when the airplane was already hot) and the fuel flow is somewhere around 13-14.5gph, so definitely on the low side.
It doesn’t always happen and it seems to be worse on hot days. I don’t have any issues if I don’t fully advance the throttle on takeoff.
The stumble has also happened in cruise at higher altitudes when I go full throttle. Lately, it has also has started stumbling after pulling positive G. For example, I’d come out of a loop (no negative G whatsoever), be straight and level, look at my instruments, see the fuel pressure drop and at the same moment get a stumble. That issue seems to go away with the electric pump on.
I went through the entire airplane and couldn’t find anything obvious. Timing, spark plugs, etc. are all fine. No issues as far as I can tell with the fuel lines, vents, electric pump and the fuel selector was just replaced. Checked all the filters, nothing found. Fuel flow test on the ground resulted in about 21gph. I’m replacing the mechanical fuel pump right now, but am not overly optimistic that it will change anything, as the “old” one was replaced in 2021 and has about 100 hours on it.
The only thing I haven’t really gotten into is the TBI because I don’t know much about it. I went through the manual and made sure the idle mixture is set up correctly and the installation seems to be according to the manual as well. I’ve read through the posts on it here. Short of replacing it with a Carb, I’m out of ideas… Any help is very appreciated!!
I pretty consistently get an engine stumble during takeoff at full throttle. As soon as I close the throttle a bit, it goes away. It does not matter whether I have the electric fuel pump on or off, it still stumbles. The engine data shows a slight drop in fuel pressure when it happens (goes from around 3.5-4.5 down to usually 2.5ish but I have seen 1psi as well when the airplane was already hot) and the fuel flow is somewhere around 13-14.5gph, so definitely on the low side.
It doesn’t always happen and it seems to be worse on hot days. I don’t have any issues if I don’t fully advance the throttle on takeoff.
The stumble has also happened in cruise at higher altitudes when I go full throttle. Lately, it has also has started stumbling after pulling positive G. For example, I’d come out of a loop (no negative G whatsoever), be straight and level, look at my instruments, see the fuel pressure drop and at the same moment get a stumble. That issue seems to go away with the electric pump on.
I went through the entire airplane and couldn’t find anything obvious. Timing, spark plugs, etc. are all fine. No issues as far as I can tell with the fuel lines, vents, electric pump and the fuel selector was just replaced. Checked all the filters, nothing found. Fuel flow test on the ground resulted in about 21gph. I’m replacing the mechanical fuel pump right now, but am not overly optimistic that it will change anything, as the “old” one was replaced in 2021 and has about 100 hours on it.
The only thing I haven’t really gotten into is the TBI because I don’t know much about it. I went through the manual and made sure the idle mixture is set up correctly and the installation seems to be according to the manual as well. I’ve read through the posts on it here. Short of replacing it with a Carb, I’m out of ideas… Any help is very appreciated!!
