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Permalink Reply by Matt Pope on July 13, 2025 at 12:21pm

Just something to consider - if it was a one-time burble and the problem seems to have been resolved, and it's an aircraft that had already run optimally for a reasonable number of hours, I would weigh the risk of attacking the fuel system, because any time you open up a fuel system you're introducing the risk of a maintenance-induced failure, which is a significant cause of accidents. If you start adding more hoses and fittings and drilling holes in fuel tanks, there's the potential for leaks and contamination and other unforeseen consequences. Also, it sounds like you're not certain the fuel system caused the issue, so it might be a lot of work and added risk for nothing. 

All that said, if you determine the return line is causing problems and you decide to go ahead with the mod, I would plumb it into the tank at least a couple of inches above (or far away from) the fitting that feeds the gascolator so you won't be sucking hot fuel or bubbles toward the engine, and install it low enough that it won't be aerating the fuel when you're down to your minimum reserve level (the way aiming a garden hose into a bucket of water stirs air into the water). I'm not familiar with the 601, but if there isn't already a suitable port in the tank, your options will be pretty limited without some major work. There's a fitting called a "universal fuel tank return fitting" or a "weldless fuel return fitting" (not designed for aircraft) that you can install if you drill a hole in the tank, but you need to thoroughly clean out the tank before and after installing it: https://www.youtube.com/watch?v=y-56VABLtCM 

Cheers!

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Permalink Reply by Brad Jacobson on July 13, 2025 at 2:05pm

I see that the fitting you linked to (which does look like a great design) suggests that it be installed from the top. I will have to see if this is possible. Thanks again!

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Permalink Reply by Brad Jacobson on July 13, 2025 at 1:35pm

Thanks so much for this very thoughtful reply Matt!

The previous owner never experienced this problem and wasn't aware of a problem like it prior to him. The engine has about 1200 hours on it. He however only put E0 fuel in it (that was what was available at his airport) and prior to him he said the owner was fanatical about avoiding ethanol...perhaps there was a reason for that. Either way the fuel was what changed prior to the problem and replacing it with E0 fuel seems to have made the problem go away. Also the day in question was fairly warm (30°C).

To my way of thinking having the return line plumbed into the gascolator is like hardwiring the exact problem you mention would be created by putting the tank return too close to the port that the gascolator draws from, ensuring that if there is any vapour present it will be recirculated with the warm fuel from the return.

I appreciate the potential for creating new problems, and I'm not super thrilled about drilling a hole in the bottom of my fuel tank😉 though I will certainly have a look at that video you linked to. I think the rest of the mod would be fairly straightforward, putting a plug in the top of the gascolator and routing the return line through the firewall under the fuel tank. On initial inspection it appears as though the underside of the fuel tank is the only accessible side without removing the tank. It is a 16 gal tank located between the instrument panel and the firewall with the fuel line connected in the center of the tank through a lovely welded fitting and proceeding through the shutoff valve to the gascolator located at the lower left side (facing forward) just forward of the firewall.

Once again I thank you for your thoughts and remain very open to more as I try to think this through.

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Permalink Reply by Brad Jacobson on July 13, 2025 at 1:47pm

I see that the fitting you linked to (which does look like a great design) suggests that it be installed from the top. I will have to see if this is possible. Thanks again!

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Permalink Reply by Matt Pope on July 13, 2025 at 3:10pm

Hmm, yeah, I'd be really surprised if a fundamental flaw in the fuel system design took 1200 hours to show up, unless maybe it's never flown on hot days before and/or you're flying out of a significantly higher elevation airport than the previous owners. To me, it seems much more likely that you had some bad fuel, or that air got in somewhere along the line from a new crack or an improperly torqued fitting. Depending on what type of fuel lines you have, they can have a limited lifetime, and if someone was working on them or wiggling them recently (during your pre-buy, for example), that could maybe cause an old hose to crack in such a way that air can get sucked in at full power under some circumstances. But since changing the fuel seems to have fixed the problem, I'm guessing that was the issue.

I'd check the ages of the fuel lines and if they're the rubber Rotax ones then I think they have a five year lifetime, and also the rubber components inside the carbs have a five year lifetime, so if those things haven't been replaced in the last five years or so, I'd do that before messing with anything else.

Cheers

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Permalink Reply by Jim and Amelia Isaacs on July 13, 2025 at 4:30pm

…and the Installation Manual will provide the answers to your questions, have your engine serial number available and download it at Rotax-owner.com.  In the manual you will find the sentence “The installation of a fuel return line is mandatory.”

 

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Permalink Reply by Brad Jacobson on July 13, 2025 at 9:05pm

Thanks again folks! Much appreciated! The fuel lines are 3 years old all installed as per Rotax, insulated against heat. There is a fuel return line but apparently it was common practice for awhile to route the return line to the gascolator which to me makes less sense than going back to the fuel tank...especially after having my issues post takeoff the other day 😉.

It is possible Matt that I am over thinking this as the new fuel really seems to have solve the problem, I took it up again today and it was just as hot out and it ran like a Swiss watch. I'm just not at all wanting a replay of the other day.

Thanks again!

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