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fuel pump replacement and return line

PostPosted: Tue Jun 07, 2016 2:22 pm
by KevinC
Greetings all, it has been a long time. Tough 3 years but now I'm ready to get back to flying!!

I am doing my annual condition inspection and have decided to do some of the Rotax rubber replacement (fuel/oil this year), carb rebuild (had floats fail) and the fuel pump replacement per service bulletin. Back in '06 when I was building many Rotax users including me did not plumb in a return line. I have been researching the pump change and have concluded that with the new pump I really need to have a fuel return. I'd really rather not go through the firewall if I can avoid it. Heck, I hate to break into the system at all - it has worked flawlessly for 450 hrs - but the float failure and gas all over the exhaust manifold have 'motivated' me... :shock:

My system is gravity feed to a gascolator mounted low on the firewall. I do not have a boost pump. Some other designs (Zenith, CT) have used the unused 1/8" primer "hole" on the top of the ACS gascolator as a return point (through a restriction orifice fitting).

I am interested in any experiences, opinions and thoughts from this esteemed group.

thank you

Kevin
#103

Re: fuel pump replacement and return line

PostPosted: Tue Jun 07, 2016 9:00 pm
by danerazz
If a return is really needed, I would like to see it go somewhere vapor can be vented off and there is enough volume to cool the returned fuel; either through just cooling from the return trip through the lines or the fact the return goes to a suitably sized vessel like a wing tank or the header. This would be how high volume/pressure pumps for fuel injection and pressure carbs are generally set up, but not generally carbs.

Disclaimer: I am not familiar with the actual return requirements of the rotax pump. Assuming it is still a float carb (Bing?) it would still be a very low pressure. If you are running auto gas it would obviously have a higher propensity to have vapor issues than avgas.

Re: fuel pump replacement and return line

PostPosted: Wed Jun 08, 2016 9:03 am
by alan
Hi Kevin.

This is my sulution; put some salt on it. :-)

I have no gascolator. I put an "X" fitting in place of the "T" distribution block between the carbs and put that "T" inline just outside the firewall. Apparently there is a purpose built "X" fitting from Rotax that has a place to install the restricter that you "must" install in the return line. I used a guide bushing from my MIG welder. The size was only a couple thousands larger and has worked perfectly for a couple years now.

Warning! If install your restricter inside the return line as I did, you MUST clamp it in place with a hose clamp as the return pressure is great enough that the line will swell and allow significant bypass.

As far as venting goes, the carbs vent any air that gets into the lines quite well, as long as you don't have any big vertical loops in your fuel lines. Going back to your header tank wouldn't help that anyway.

Since you have a gascolator installed, that would be an even better place to route the return line. I also installed a quality 100 micron, cleanable, fuel filter after the return "T".

Enjoy...I know I do. :-)

Alan

Re: fuel pump replacement and return line

PostPosted: Wed Jun 08, 2016 5:08 pm
by Wes
Kevin,
Welcome back! I missed your updates.
That's the problem when we are out flying or life gets busy, we disappear from the forum ;-)

My Highlander (now at 453 hrs) has always had a return line and a "boost pump". I routed the return line from an orifice'd fitting on the fuel distribution block on the firewall back to the header tank. I made the orifice out of a rivet pressed into the AN fitting and drilled with a teeny-tiny drill to Rotax spec (approx. .020, I think). I couldn't figure a good way to have the return end up in a wing tank.
This scheme seemed to work fine until one day I switched off the fuel for a test (during annual) and found that fuel was still making it's way to the engine via the return line! Duhh! So I installed a check valve in the return line to fix that problem. That's the current configuration and all is well.
The boost pump and return line, to me, are important. The pump allows me to purge fuel through the system for test purposes and when the engine is still very hot for a restart. In flight, the return line allows a continuous flow of cooler fuel to the carbs to prevent vapor problems. Probably not such a major problem in your cooler part of the country, but welcomed down in 'Sunny' (make that sweltering) Florida!
Let us know how it goes----

Wes

Re: fuel pump replacement and return line

PostPosted: Tue Jun 14, 2016 4:24 pm
by KevinC
Curiously, all of the advice (Lockwood, Rotax mechanics) I've had on using the restricted return flow with the new fuel pump has been related to the increased pressure the new pump has and overpowering the float valves - "and, oh-by-the-way, it'll help you with vapor lock"... I have not experienced vapor lock with my Highlander, and I've not heard any stories of problems from others. I use almost exclusively non-EtOH mogas and I live in a plenty-hot area.

I have made some decisions and will post pix/results as I go.

K

Re: fuel pump replacement and return line

PostPosted: Mon Aug 22, 2016 11:07 am
by KevinC
I wanted to post a follow up on this. I used the Aircraft Specialty hoses to replace my fuel (and oil) lines and ran the fuel bypass line into the top of my gascolator. It works great! So there you are - no need to run a line back to the tank. I have pictures and would be happy to answer any questions if able.

Dang it feels good to have my plane back!

k

Re: fuel pump replacement and return line

PostPosted: Mon Aug 22, 2016 9:16 pm
by john2
KevinC wrote:I wanted to post a follow up on this. I used the Aircraft Specialty hoses to replace my fuel (and oil) lines and ran the fuel bypass line into the top of my gascolator. It works great! So there you are - no need to run a line back to the tank. I have pictures and would be happy to answer any questions if able.

Dang it feels good to have my plane back!

k


Kevin,
Thanks for the follow-up and confirmation, I have mine plumbed the same way. Also used Aircraft Specialty oil hoses, looks like really good stuff.