Vapor Lock Issues Resolved For Now

[FlyingMonkey]

On 4/26/2024 at 4:45 PM, Tom Baker said:

Andy, post a picture of the top of your engine showing the fuel lines.

Wilco, I'll snap one tomorrow.  I thought I had one but can't find it.

[Roger Lee]

Hi Andy,

This didn't happen until you changed hoses. So what is different? Plus the old hoses were in fire sleeve and these aren't. I bet the fire sleeve does help with heat absorption. May be worth a try to insulate the fuel hoses on top for a test. Header wrap won't hurt your exhaust pipes when installed properly. Also it isn't hard to tell when an exhaust pipe breaks because it snaps all the way across which would take the wrap with it. You could easily see it of hear it. Like your broken pipe. They break at dad welds, but the majority of the time they break due to stress from its mounting.

[FlyingMonkey]

4 hours ago, Roger Lee said:

Hi Andy,

This didn't happen until you changed hoses. So what is different? Plus the old hoses were in fire sleeve and these aren't. I bet the fire sleeve does help with heat absorption. May be worth a try to insulate the fuel hoses on top for a test. Header wrap won't hurt your exhaust pipes when installed properly. Also it isn't hard to tell when an exhaust pipe breaks because it snaps all the way across which would take the wrap with it. You could easily see it of hear it. Like your broken pipe. They break at dad welds, but the majority of the time they break due to stress from its mounting.

I agree that the probable cause is something with the new fuel system.  As I mentioned, I thing the new hoses might absorb and retain heat more.  To be clear they do have a fire sleeve, it's just integrated into the silicone around the hose.  But I do agree the old fire sleeves probably insulate the hose better.  The header wrap is seeming like an easy enough thing to try.  Shielding the hoses on top of the engine might be fairly easy as well.  I think it all comes down to how much work I want to put into resolving this in the Summer flying months beyond "put 100LL in it and forget about it".  I suppose I can just run 100LL for now and try some of these other ideas when the weather is too poor to fly.

I can also easily experiment with mixtures of mogas and 100LL to see if I can just dump a few gallons of avgas into it every tankful as an easy fix.  

[Roger Lee]

The fire sleeve in the new hose is thin and protects from fire, but it isn't as thick or insulating as the original fire sleeve. Like I said maybe just sliding some of the old fire sleeve on the top hoses may fix it. Make sure none of the new hoses aren't  too close to an exhaust pipe.  

[FlyingMonkey]

On 4/26/2024 at 4:45 PM, Tom Baker said:

Andy, post a picture of the top of your engine showing the fuel lines.

Here you go:

[FlyingMonkey]

I found the header wrap kit and stainless lock ties in my hangar, I'll bump that to the top of the project list.  When wrapping how much do you guys use for overlapping, about 50%?

[Tom Baker]

Well, I don't see anything glaring in your installation. Have you verified the orifice in the top banjo bolt is clear, and that air can flow through the line to the gascolator?

[FlyingMonkey]

31 minutes ago, Tom Baker said:

Well, I don't see anything glaring in your installation. Have you verified the orifice in the top banjo bolt is clear, and that air can flow through the line to the gascolator?

Well, it was clear when I installed it last year during the hose swap.  I hate to disassemble it and potentially induce leaks in that stack to re-check it, but I can if you think it's worth doing. Or is there an easy way to test that without taking it all apart?

I think I'm going to wrap the exhaust and then get some fire sleeve around those lines on top of the engine just for extra thermal protection.  When I do that I'll probably have to pull the lines and I can check that orifice then.

[Roger Lee]

Absolutely DO NOT over wrap by more than 3/8". 50% over wrap is way too much. Some header wraps have this marked with a thread on the outer edges. Over wrapping WILL cause toooo much heat retention and warp your exhaust pipes. This is why Rotax isn't a fan because people fail to do it properly.

Give me a call. I've done somewhere between 70-90 wraps and never had an issue.

[FlyingMonkey]

2 minutes ago, Roger Lee said:

Absolutely DO NOT over wrap by more than 3/8". 50% over wrap is way too much. Some header wraps have this marked with a thread on the outer edges. Over wrapping WILL cause toooo much heat retention and warp your exhaust pipes. This is why Rotax isn't a fan because people fail to do it properly.

Give me a call. I've done somewhere between 70-90 wraps and never had an issue.

Whoa, I didn't know the pipes could warp.  I'll call you when I get a chance and our time zones line up.

[Roger Lee]

Rotax even has some pictures of warped and collapsed pipes that also show the over wrapping. Stemmie motor gliders wrap theirs right from the factory and so do a few others.

Call anytime. It's 0730 here and I've been up since 0530. Normal for me.

[Madhatter]

17 hours ago, FlyingMonkey said:

Well, it was clear when I installed it last year during the hose swap.  I hate to disassemble it and potentially induce leaks in that stack to re-check it, but I can if you think it's worth doing. Or is there an easy way to test that without taking it all apart?

I think I'm going to wrap the exhaust and then get some fire sleeve around those lines on top of the engine just for extra thermal protection.  When I do that I'll probably have to pull the lines and I can check that orifice then.

I could guess that a hose could  be added to the bypass hose to a container to verify flow during a short ground run. But the container needs to be secured at a distance for safety. I have the teflon lines also but elected to eliminate the banjo fittings in favor of AN fittings. If you do don't forget to cap off the gascolator fitting

 

[Anticept]

One of the alternative ideas I have had to address this issue was to take a page from the injected engine and use a small header tank and fuel pump that goes all the way to a tee at the the mechanical fuel pump, and return to the header. For the same reasons as the injected engine: purge air.

Upside, don't have to worry about overpressuring the system and if the fuel pump fails, it could flow back through the return hose just fine.

Downside is a lot of extra weight and complexity, so I ended up abandoning it when I found facet fuel pumps.

However, I am starting to think about doing this on the ground as a test for vapor locking issues. Return line to a transparent tank with a valve, and see if i can get fuel issues to develop. Open the valve and flip on a pump... See if a bunch of vapor comes through.

I do notice that the sudden full throttle advancement and the pitch up on takeoff both trigger these alarms for a short time.