What is The Rule of Thumb for Fuel Line Routing?

[Kafin Noe’man]

Hi everyone,

I think I have just found out the reason why I've been struggling a lot setting up my P40 with an LA46 on it.
It seems my fuel system is the main culprit here.

First, there's a leak on my fuel system.
On the field, I was trying to block the overflow vent, but the engine stayed the same, not going lean nor dead when I did that which was a clear indicator that I had a leak. Back home, I found out a pin-hole leak on the cap that's capping the overflow tube.

Second, fuel lines routing.
My current fuel lines routing causes the in-line filter to touch either the engine crankcase or muffler.
I think this has caused a heat and vibration problem that leads into fuel foaming that ended up with me always having a lean setting up in the air no matter how high or how low RPM I set on the ground.

So, on this thread, I'd like to ask the rule of thumb for the fuel lines routing.
I know that it should be as short as possible, and my experience has taught me to keep anything out from touching the engine or muffler especially the filter.

I have tried several combinations (please find the attached photos, hopefully the resolution is clear enough), and would like to ask you which one has the possibility to work the best?

On those 6 combination I tried, I have managed to put the in-line filter away from touching the crankcase or muffler, but I still can’t avoid the silicone lines form touching it. Is it okay if the fuel lines touches the crankcase or muffler?

Looking forward for your input.


Best,
Kafin Noe'man

[Dan McEntee]

   I think you have figured some things out on your own. Experience is a wonderful teacher!  The first thing to remember is that the hottest parts of the engine are the head and the exhaust, with the exhaust leading the way I think. If you really want to know how your engine is running, monitor the exhaust temps!! 1) I try not o ever let anything in the fuel circuit touch any part of the engine, especially the exhaust. This means fuel filters and fuel line. keeping the fuel line as short as possible is important, but a little extra slack to keep it cool is worth a bit extra where needed. 2) For capping fuel tank tubes or fill/overflow tubes, I use short lengths of fuel line, and plug the tube with one or two BBs, like you shoot from a BB gun. I also use the plastic colored pegs from kids Lite-Brite toys, as they are easier to see if dropped in the grass. I replace the tubing whenever it looks like it's getting sick looking. Fuel line itself gets replaced and regularly and checked often. 3) A big reason to learn how to solder is that you can make your own metal tanks, and modify commercial metal tanks to get the tubes located where they will do the job the best. The tank in your photos has the pick up tube in an underslung location that helps with fuel routing, and I route the fuel line under of behind the muffler, as close to the engine mounts as I can. You can use the head and washer of a mounting bolt to anchor a wire loop to pass the fuel line through to keep it away from the exhaust/muffler header. I like to put 2 copper tubes through the fuselage of profile models, up near the top of the fuselage, one for fill/vent and one for over flow. The fill/vent is in front and bends 90 degrees forward into the airflow/prop blast, and the overflow is about 1/2" behind it and is kept straight. Leave about 1/2" protruding on the outboard side. You connect the tubes on the tank to these tubes with lengths of fuel line. This makes for easier fueling and less likely for any fuel to syphon out of the tank. You can still move pressure lines around as needed, and will work for standard vents and uniflow.    And you are correct in testing your system by closing the off the vent or pinching the pressure line. You should get a change in RPM pretty quickly, and if held long enough, will kill the engine. You are on the right track, and just like everything else with stunt, practice with this process will help you figure out the best route for each airplane. I try to make every airplane with the same fuel routing as close as possible to to keep things simple and easy to troubleshoot.
  Type at you later,
  Dan McEntee

[Dave_Trible]

In your case the worst thing I see is the unfortunate location of the fuel pick up leaving the tank.   It would be far better if that tube exited the tank below the exhaust of the engine closer to the fuselage.  Then you'd have a straight shot.   I don't believe silicone tubing will transfer engine vibration into the fuel system but the filter might to a small degree.   In my engine cowlings most often the filter rests against the engine and yet not a problem.  Your main culprit here I think is simply that profile fuselages are notorious vibrators and simply dealing with that can be a pain.  I have tried a series of very large profile with up to .61 size engines and just gave up on anything much above a .40 or LA .46 which by their nature can run in a two cycle semi-scream anyway when you can't get the vibration down.

Dave

[Ken Culbertson]

This is the setup I used on my LA46 profile back in the pre electric days.  I never had any real luck with metal tanks on the LA and never had any problems with the clunks.  The only filter I ever used was the two ended inline type.  If memory serves me they were called "crap traps" or something like it.  Dave is correct, the pickup is in the wrong place to get a smooth shot to the venturi.  The straighter you can make the fuel line the less problems you will have but it is not as important as the pickup in the tank.  Vibration is not much of an issue in the fuel line itself.  More of a foaming in the tank issue.  If the motor vibrates a lot you may need to find a sweet spot where you can use a slightly out of balance prop or 3 blade to balance an out of balance motor. I also never had any luck with low pitch high RPM settings.  Probably all those years running Fox 35's that spoiled/ruined me.  If all else fails, try a decent 11-6 running just a hair below the 4-2 break.

Ken

Opps, that is an LA 25 not the 46.  All is the same except the prop.  A cut down 10-6 to 9 1/2" would be my choice or a 9-6EW if you could find one.

[Colin McRae]

Kafin, definitely not good to have the fuel filter touch the engine as others have already said. On some of by models the tubing itself might touch the engine somewhere, but not the aluminum filter. I have not seen an issue if the silicone tubing touches the engine. I try to avoid it, but not experienced any issues.

I don't have the skills to build my own tanks, so I usually use Brodak tanks, and sometimes Sullivan clunk tanks.

On my P40 ARF (with Evolution 36 engine), I use the Brodak oval profile uniflow tank (see note below!!). This particular tank design has the fuel pickup closer to the fuselage allowing the tubing to run under the engine between the engine and the fuselage. In the picture, the fuel filter is 'close' to the exhaust, but it does note actually touch it on my model. My engine runs fine. As long as it does not touch it, the significant air flow prevents any heat transfer.

One important thing to note: The stock Brodak oval profile uniflow tank has the fuel pickup on 'top' of the tank. That does not work well for a profile application. Brodak will custom-fabricate a tank for you is you want. Obviously, a bit more expensive. I asked Brodak to take their standard oval profile uniflow tank and simply move the pickup to the bottom which places it closer to the fuselage.

BTW, the Evolution 36 is roughly as powerful as your 46LA and I run a MAS 11-4 prop at a ground rpm around 11,400.

I also run muffler pressure on all of my OS-LA engines irrespective of size.

[Lauri Malila]

I wouldn't use the transparent CrapTrap in the feed line, especially not with small/small consumption engines. With slow flow rate, they have a tendency that air bubbles get "stuck" inside, causing an uneven engine run. You should try something as small as possible there. Randy or Jett sells the very small ones.
CrapTrap works fine in the replacement air line, and in filling bottle. L

[Colin McRae]

If I'm not mistaken it looks like Kafin is using a Dubro fuel filter which have a relatively large diameter. 

I use the Brodak filter which is about as small as they come nowadays (diameter-wise).

[Lauri Malila]

If you look closely, the first 6 or so pictures show the CrapTrap (Sullivan..?), and the rest have some aluminium one. L

[Colin McRae]

If you look closely, the first 6 or so pictures show the CrapTrap (Sullivan..?), and the rest have some aluminium one. L

I see now

Looks like he has tried both.

[Motorman]

I route my fuel line so the (dubro aluminum) filter does touch the muffler on the inboard side to keep the fuel line from touching/melting. Been working great for years like that on several planes.

You can put a piece of 3/32" music wire in the (copper) feed tube and bend it for a better angle. Also, from your pics, bend your vent tube higher so it's not down in the wake of the engine cylinder. 

Dubro makes two size filters. The big one and the smaller one are both the exact same size inside so just use the smaller/cheaper/lighter one. Was very disappointed on that as I wanted a bigger screen for more flights between cleaning.

Don't use the crap trap for anything you want to run right.

Fuel foam comes from having the vent tube too close to the pick up tube inside the tank.

Route your tube so there's no stress on the spray bar connection and your filter can't swing out and flop around.

Make sure your spray bar discharge hole is pointing straight down the venturi.

Flush out the entire fuel system after every flying session.

Hope that helps,
MM

[dave siegler]

for me, I would put the filter closer to the needle valve and have some bend radius for the line going to the tank.  I route under the muffler. 

so tank long line filtershort as practical to the needle valve.  I think that will help 

[MikeyPratt]

Karin,
Most of the photos showing the instaliaion should work, some are better than others,  the problem is your cold solder joints, they will leak air every time.  The solder must flow evenly into the joint, if not they leak.

Later,
Mike

[Ken Culbertson]

Don't use the crap trap for anything you want to run right.

Could it be that I run most everything on muffler pressure that I have never had an issue with the "Crap Trap".  I seem to be in the minority.   Still better IMHO, run a closed system (pressure) on the plane and a pump on the fuel can with no filter on the plane's fuel line.    Double filter what goes into the tank.  If you fly over grass/dirt (I don't) then make sure your venturi wears pantyhose.

Ken

[Brett Buck]

Don't use the crap trap for anything you want to run right.

   Oh, another mistake I have been making for, oh, 40ish years now. Maybe that's why my engines always run like crap.
 
     Someday, I will learn all this important wisdom.

     Brett

[Lauri Malila]

 I don't like such absolute comments either, but there is some truth in it. As I wrote earlier, they have caused issues with lower fuel consumption and are one of those things to check before starting to add head shims, for example.
Probably not an issue if you burn 6-8oz of fuel per flight, but you never know.. L

[Brett Buck]

I don't like such absolute comments either, but there is some truth in it. As I wrote earlier, they have caused issues with lower fuel consumption and are one of those things to check before starting to add head shims, for example.
Probably not an issue if you burn 6-8oz of fuel per flight, but you never know.. L

   Flow rate is certainly not an issue. I was only running about 6.2 ounces of 15%, which is on the low side for current piped IC systems. I have had exactly zero problems with these filters on anything from a 15LA (or even wimpier, McCoy 19 RR) to an 88. The only issue I have ever seen is that every once in a while, 2 examples over 40ish years, they were assembled with the outer hard plastic "tucked under", i.e. folded, when the pushed in the end cap. This caused it to leak air around the end fitting.

    This drove us crazy for a while at the 95 NATs. Bruce Perry just couldn't get a consistent run all week. Finally, he fired up up in is "RC Club dork" giant standing toolbox/holder that we had all be making fun of all week, upside down with the cowl off. Fires it off, in seconds I notice a stream of bubbles out of the intake side of the fitting. Folded under.  Replaced it, ran fine the rest of the week.

    Point being, that and one other similar incident over half a century, with virtually every competitor using it with no problems for years, tells me it's not a big problem. and it basically impossible to clog up with even the slightest care with fuel handling.  The alternative is the Dave Shadel teeny-weeny take-apart filter, which is great for fit, but has to be treated very carefully to avoid leaks, and if there is any contamination, it clogs up. 

    Brett

[Reptoid]

Many of us have been using this filter for over 20 years for both stunt airplanes with conventional tanks, and high pressure bladder fed systems with excellent reliability. They don't come apart and don't leak. To clean you just backflush. I've only seen one fail in all those years and it had been subjected to much abuse on combat planes. They are very compact and weigh almost nothing and the barbed ends hold fuel line well

https://www.fourmostproducts.com/our-products/fuel-fittings/

Lower right of page #157 filter

[EricV]

   Flow rate is certainly not an issue. I was only running about 6.2 ounces of 15%, which is on the low side for current piped IC systems. I have had exactly zero problems with these filters on anything from a 15LA (or even wimpier, McCoy 19 RR) to an 88. The only issue I have ever seen is that every once in a while, 2 examples over 40ish years, they were assembled with the outer hard plastic "tucked under", i.e. folded, when the pushed in the end cap. This caused it to leak air around the end fitting.

    This drove us crazy for a while at the 95 NATs. Bruce Perry just couldn't get a consistent run all week. Finally, he fired up up in is "RC Club dork" giant standing toolbox/holder that we had all be making fun of all week, upside down with the cowl off. Fires it off, in seconds I notice a stream of bubbles out of the intake side of the fitting. Folded under.  Replaced it, ran fine the rest of the week.

    Point being, that and one other similar incident over half a century, with virtually every competitor using it with no problems for years, tells me it's not a big problem. and it basically impossible to clog up with even the slightest care with fuel handling.  The alternative is the Dave Shadel teeny-weeny take-apart filter, which is great for fit, but has to be treated very carefully to avoid leaks, and if there is any contamination, it clogs up. 

    Brett

I'll pile on here and add that I ran CASES of fuel, (8 ounces at a whack, through craptrap's on a PA75, 6.5 ounces a whack on the PA65) with never an issue that was quality of filter or fuel flow related.

Install them in the right direction so fuel hits the convex side, don't kink your bends... and I make a sleeve at the NVA out of a band of fuel tubing cut a little wider than an O-ring for extra grip to the NVA, I don't like zip ties. Use the good Pink Prather tubing if you have it.

Zero issues literally over thousands of flights.

I briefly tried the Master Airscrew filters because they were cheap and smaller (lengthwise, but fatter diameter) on recommendation from a friend to fit a classic plane, but they would fall apart at the seam. I had to wrap them with a strip of silkspan and epoxy them, but it wasn't worth the effort, and you had no visual on filter status because it is not clear.

So... in my opinion, Craptraps rock! If you really are worried about fuel flow, they do come in 2 sizes, there is a large craptrap (with 5/32" ends)...

EricV

[Dan McEntee]

    In my experience with Sullivan/Pylon Brand products since I started to fly stunt competitively in the mid 1980's, I have found their quality control to not be the best in the world. I lost 3 models along the way using brand new Sullivan/Pylon lines that one broke up in the crimp at the handle end. I quit using crimped lines of any kind after #3. The filter subject started about the same time. I remember many guys here in the Midwest having issues with them leaking air. I remember seeing several that you could rotate the fittings on each end. The solution at the time, and I may have read it in StuntNews,  was to heat the plastic tubing with a heat gun or a cigarette lighter to reshrink the tubing. That was a very real issue at that time, and it may have been a regional, limited time things but it did happen. Their flying handles could have been better also. Some of the crimps on those cables looked really chintzy, and the one C shaped handle with the adjustment lug that makes 3/" or so hops that won't stay where you put it I'm sure has cost some people some airplanes!! Some one here on the list had a sketch or a very easy mod for those that uses a wire terminal section imbedded in the big groove in the back of the handle. The cable is long enough to cut one end off, install the connector lug and secure it, pass the cable through it to the other end of the handle and remake the loop. Now you have a handle that is adjustable in as small amounts as you need. their stuff has always been pretty expensive and they have a really limited line to offer also. They might be getting by strictly on electric starters motor sales!!
  Type at you later,
   Dan McEntee

[Motorman]

LOL who knew there was much love for the crap trap. The one I had the ends loosened up and leaked first time I used it. I didn't buy another one. Did they improve the design in later years? If you guys got them to work then God bless but, I'll take the Dubro. It's got O-ring seal, you can take it apart with two prop wrenches, If you loose the screen a chainsaw carb kit screen will work and it's hard anodised in a few colors. I like it, it's been dependable on grass fields.

MM