Engine Issues at WOT, Ellison TBI

Hawg Driver · #1 05-16-2022, 11:36 AM

Hi everyone,
Hoping i could pick the collective brain here at VAF about an issue that has me AOG. RV-4, O-320 (150hp), FP Prop, Ellison TBI.

Starting hearing a hiccup/surge during initial climb on the two previous flights, started a turn back towards airport and it immediately went away. I shrugged it off as some type of vapor lock issues (mistake) but engine ran fine afterwards so I continued on with continued monitoring.

On my last flight, on decent to landing, I pushed throttle up to WOT at an intermediate level-off, and the engine started to choke a bit - not rough, just felt like I was pulling the throttle back. Once I actually pulled the throttle back, the engine "caught" and started to run fine again. Rest of decent/approach/landing was fine. Fuel pressure was normal the whole time.

After fueling up, I took my time with a detailed run up. Each time I pushed to WOT, engine would die, and start running again when I pulled the throttle back. Fuel pressure stayed 4-5 psi...boost pump on/off, changing fuel tanks, all had no effect.

I've had this set up of TBI for 7 years, so nothing has changed recently...and I've put over 100hrs on the airplane since the start of 2022.

I got picked up from my that fuel stop and she's stuck AOG until I can figure out what's wrong. Visual inspection looked fine, no signed of fuel leak, air entering the system anywhere, including on the outside of the TBI.

My only "easy" thing I wasn't able to do was check the last-chance screen to see if somehow it is getting gunked up and won't allow enough fuel to pass at WOT.

Any other thoughts out there of where I should look?

Thank you, everybody.

Bicyclops · #2 05-16-2022, 03:02 PM

Can you download engine monitor data for when it happens? An airplane I know with O-360 and Ellison would run very rough on the go around of about the 7th touch and go. With data graphed on Savvy Aviator, we identified a significant increase in full throttle fuel flow that correlated with the oil coming up to temp. This Ellison is mounted right to the sump and I theorize that the increasing temperature is causing it to pass more fuel. Not sure exactly why. The data showed one cylinder's EGT dropping a lot when the roughness occurred. I think that cyl got too rich to run. A slight decrease in throttle clears it right up without giving up any RPM. Setting the mixture control slightly leaner might do it too if it doesn't increase CHTs too much. If you look at the Ellison, it delivers fuel through holes in a bar that are exposed by pulling back a throttle plate. This allows more air and more fuel into the engine. I'm curious how much fuel flow you're getting when your problem happens. If it is some outrageous amount for your engine, it would suggest an overrich condition. Black smoke would be a clue for somebody watching from the ground.

Ed

Hawg Driver · #3 05-16-2022, 03:48 PM

Hi Ed,
Thanks for the reply, don’t have a downloadable monitor, but I didn’t see anything jump oddly on the monitor when the engine starts dying (the egt decreases as engine dies.)

I did try leaning it out at high power in case it was getting too rich, that didn’t help. No smoke or smell that makes me think it is too rich, either.

With me having this setup for 7 years with no issues, I’m thinking it must be a failure mode of the TBI or fuel clogging down stream of the engine driven fuel pump/fuel pressure “T.”

rocketbob · #4 05-16-2022, 05:09 PM

My guess is you have a problem with the diaphragm, given the setup was working for years. I would take the regulator apart and overhaul it.

Lots of good info here...
http://www.miravim.org/4RE/EllisonDisassembly/
http://www.miravim.org/4RE/AviationT...lisonRotec.pdf

pitts flyer · #5 05-17-2022, 03:56 AM

Quote:

Originally Posted by rocketbob

My guess is you have a problem with the diaphragm, given the setup was working for years. I would take the regulator apart and overhaul it.

Lots of good info here...
http://www.miravim.org/4RE/EllisonDisassembly/
http://www.miravim.org/4RE/AviationT...lisonRotec.pdf

Many years ago I had an Ellison on my Pitts S1 (O-320). Ran good until one day it quit just after takeoff. Long runway so landed and did more ground checks. Would not run at full throttle. Sent the TBI into Ellison, and they found water in the diaphram in the regulator. Don't know how it got there, but removing the water and replacing the diaphram did the trick. Ran normally again after that. Rotec has also many problems with the regulator. I now shun them...
YMMV
Dave

pitts flyer · #6 05-17-2022, 04:41 AM

Quote:

Originally Posted by rocketbob

My guess is you have a problem with the diaphragm, given the setup was working for years. I would take the regulator apart and overhaul it.

Lots of good info here...
http://www.miravim.org/4RE/EllisonDisassembly/
http://www.miravim.org/4RE/AviationT...lisonRotec.pdf

Many years ago I had an Ellison on my Pitts S1 (O-320). Ran good until one day it quit just after takeoff. Long runway so landed and did more ground checks. Would not run at full throttle. Send the TBI into Ellison, and they found water in the diaphram in the regulator. Don't know how it got there, but removing the water and replacing the diaphram did the trick. Ran normally again after that. Rotec has also many problems with the regulator. I now shun them...
YMMV
Dave

Hawg Driver · #7 05-17-2022, 05:28 AM

Thanks for the posts everyone. I’ve ordered some diaphragms. It looks like a job I can handle on a ramp with borrowed tools. I’ll check the TBI fuel screen and we’ll and see what happens. I’ll keep you updated.

Thanks Again!

Hawg Driver · #8 06-26-2022, 10:33 PM

Replaced the diaphragm, aircraft still had the same issue, intermittently as before.

I used the links provided above, rebuilt the TBI, found no smoking guns. Cleaned the entire unit, cleaned out every hole in the fuel bar, cleaned out all screens and air filter. New diaphragm. Checked the fuel lines.

Ran the engine up again and it ran exactly as before. Runs fine up to about 2000rpm, above that it surges, will eventually dies at WOT. Fuel pressure remains constant. Does not matter if I have boost pump on or not, engine dies at WOT. Matters not if I am running off of my one electronic ignition or my one magneto, same effect.

Any ideas out there where else to look? No signs of fuel leak anywhere but maybe it’s sucking up air into fuel lines at WOT. Hate to start chasing parts ($$$) but I’m fresh out of ideas.

OKAV8r · #9 06-26-2022, 10:44 PM

Is it possible that a fuel connection upstream of the boost pump has loosened, or your fuel selector has started introducing air?
Does the fuel pressure ever become erratic?

Hawg Driver · #10 06-27-2022, 10:33 AM

I’ll start pulling up the floor to take a look at the selector and fittings next chance I get. My fuel pressure gauge is an old school mechanical gauge, I doesn’t appear erratic at all, but I wouldn’t be shocked if it would miss air being sucked in elsewhere.

Always open to other ideas out there…