Using a Jett 50 FIRE on a delta I am constantly leaning out in flight. There are no bubbles in the intake line, and the tank is at approx. the level of the carb. First I was setting it 500 below peak, then 1000, etc. Each flight I reduce RPM to hand launch, then quickly open full. Within seconds it is full RPM (Approx 18,800 with an 8.8 X 8) and no smoke. If I go vertical it will die quickly. Having done that a few times I no longer have an interference fit, though lots of compression. The pipe length is 14.25", as set by factory. I'm at a loss for things to try now. The problem may have begun when I discovered that a loose needle valve was letting air in. It has had over a gallon through it, and during break-in it did not exhibit this problem. Is it time to send it back or have you an idea to try something?

Hi,

Thanks for writing.

There is likely nothing truely wrong with the engine.

What you described is not uncommon.... we get it every so often from the whiplash and dust guys....and you sorta have it figured out. You are getting bubbles in the fuel.

More accurately, the fuel is foaming. Tell me what aircraft, what type of fuel tank you have, and how it is installed.

Our first rule here, is to put the engine in a test stand. Put a tach prop on it (same prop, clip 1/4" off of each tip, balance it... dont fly with it). Run the engine. If it runs, behaves, needles well, and gets up to RPM properly, there is nothing wrong with the engine. Although you have it broken in, and have run it in a stand before, please do this (it will be the first thing Dub will ask if you send the engine for service). Letting the engine run three or four times for about 3 minutes each will help recover your seal. Start running it rich, slowly approach peak rpm. When you find it, back off 1/2 a turn. Let it run there.

Of note, the Jett remote needle valve should not let air into the system. The remote needle has no rubber seals. Even with the packing nut loose, to the point that the needle will turn by itself due to vibration, the needle has sufficient internal threads that it will not leak. The nut is primarily to adjust tension on the needle.

Following the test run....

Let me assume for a moment that you checked and replaced all of the fuel lines, and the clunk line inside of the fuel tank, and there are no kinks or restrictions.

You said you didn't see bubbles in the intake. That was on the ground. You are probably correct there... or the bubbles were tiny where you might not easily see them at that point. What is likely happening, is that after you launch the plane, the engine is unloading (it hits a higher RPM than you will see on the ground), and that is causing an airframe vibration which is creating fuel foam.

There are two ways to prevent fuel foaming
1) ensure that the tank is wrapped in foam, and ensure that no part of the tank is touching the airframe.
2) use a bubble-free fuel tank. One of our bubble-jett tanks, or a Tetra tank, or which ever fits best in your application.

Fuel foaming caused by high RPM and high vibration frequency is something that is unique to high performance engines.... most sport pilots will never experience it. It can drive the average modeler nuts, but it is just part of the hobby that is 'learned' by experience.

What you described was one of our biggest problems with Q-500 and QM40 racing. I've had my share of zeros and blown plugs over the years, because the fuel tank somehow slipped out of the foam and touched the fuselage side or firewall. Engine needled great on the ground. As soon as the plane made it to turn 1 and the engine unloaded, the engine leaned out. Required immediate shut down, or the engine would be ruined. To solve this, EVERYONE in 428 and 422 racing uses bubble-free tanks. No foam problems, because there is no air that can mix with the fuel It also provided a consistant needle setting to the very last drop.

That inconsistant needle is another way to toast a good engine. With a regular tank, when you are down to about 1/3 fuel, many engines begin to lean out. It has a tiny bit to do with where the fuel level is in relation to the carb.......but it has a LOT to do with the clunk bouncing around out of the fuel, and if there was any small amount of foam in the fuel.....that is what the engine tries to run on.

Let me know how you make out.

Bob Brassell

[email protected]

Thanks for the comprehensive answer. I learned some things and you reinforced other points. The airframe is a Laser Arrow, Sullivan 8-oz. slant tank wrapped in 1/4" foam pad, not touching the airframe. I am planning on moving it to my Whiplash after a gallon of fuel or so. In expectation that I will never succeed to reduce bubbles in either craft I will now move to the bubble-jett bladders. I want to do large loops and I can't afford to burn it up.
Three questions arise:
Does the bladder tank use the pipe pressure nipple? Can it touch the airframe? Does it require any padding at all? If not then it will fit in a tighter space. I wish they were available in 8-oz instead of 5-6oz.
I will do as you suggest and put it back on the bench and do the three 3 minute runs. I don't understand what it means to lose the seal, nor can I imagine how it gets restored. I would like to have my interference fit at the top restored. I gave me the feeling that I would have maximum compression. I did have one run where I saw fuel exiting between the top of the sleeve and the bottom of the head. a retorqing of the head stopped that. I will let you know how my move to bladder tanks works out. The Whiplash will need one for sure as the tank touches the airframe.
I understand your point about tightening the needle valve collar. In my case I assure you that the steady stream of bubbles stopped once I tightened it. Murhy's Law proven once more!
Roland

Hi,

sorry for the delay...we were off at toledo

The bubble jett tanks do not require extraordinary packaging.... usally just some foam or something around them to keep them from rubbing/wearing on any structure.

For a whiplash, you can order a 6 oz Bubble Jett PC tank (pole cat design). It fits perfectly in the whiplash. We sold all we had of those at the toledo show.

On the needle.... I goofed. The packing nut does provide for a good part of the seal. If it is pretty loose, it will allow for a leak. Leaving it a bit loose to adjust the needle does not in itself cause a problem. You are right... keep that nut as snug as possible, but it is generally not necessary to tighten it to the point so the needle cant be moved. ( I think where I was going with my previous post, is there are no rubber seals to leak as with other types of needle valves..and I got lost in my own brain clutter )

The piston fit from the factor is always tight. Usually too tight, and it becomes optimal over time. What I meant about the fit coming back, is that in some circumstances, if you let the engine unload, the proper heat cycling can help the parts fit better. If parts are damaged, nothing will fix them, but in most cases I've found success with ABC and especially AAC set ups. The cold compression is a general indication of how the fit is. However, what you want to check is the hot seal. See how the engine fits after running and it is still warm. If the RPM's are not there, after all is said and done, you may want to send the engine back to be checked.

Loose heads happen sometimes. Fortunatly you found that. Over the weekend, we looked at an engine where the head came loose. Engine ran weird, so the guy changed the glow plug. In doing that, the sleeve and head button ended up rotated 90 deg. Engine ran like junk, and he didnt know what to do. Simple fix.

Bob

Bob,
Things are looking up. I installed the 6oz Jett tank in the Arrow as you suggested, and plan to do the same in my Whiplash. NO MORE BUBBLES!
I thought from reading tech notes that I should set the needle a few hundred RPM below peak and launch. The result was a lean engine quitting three seconds after launch. I ended up setting the needle very rich so that upon launch it was not on the pipe, plenty of smoke. I think it was like 13K or such at launch. After launch it would quickly get on the pipe and scream like I hoped it would after all my efforts (and expense). It now hauls buns with the 8 x 9.75 but I wish I knew where to tach it for repeatable peak performance.

Im glad you got things worked out.

When dealing with speed, there are a variety of factors that can cause problems. Im glad you got the fuel system issue worked out, that is always a killer.

Usually one would not have to back off quite that much. Usually about 1000 rpm down is audibly rich, but still with a 2 cycle sound. It is likely well rich, but still running at 14K.

On the QM engine, the ground peak is about 25K, we launch close to 22K, and in the air they hit about 28K.

What you described sounds like you are on the edge of the pipe tuning. If you requested "fast" setup from Dub, that is likely the case. You may never see 'peak' rpm on the ground, because the pipe only stages completely when the prop unloads. The prop can not unload on the ground, and you will see a false peak. That is how FAI engines are set up. The needle is a bit of an educated guess. Some of the really fast guys in QM and Q500 do the same thing these days when using very heavy pitch props... the muffler never stages, so the needle has to be set by feel, and a previously known needle position using a slightly smaller prop.

You may want to play around with a few different props. Some with give you better speed, some will prove to provide a more user friendly installation.

As the engine gets more time on it, it will accelerate up to RPM more easily, and you will notice you can lean it out a tad more. But, if you can keep it rich, that is always a good thing. Sometimes 'peak performance' is a less than 'max performance'. Keep the engine rich, live with a few MPH slower, and the engine will last and run like that for a long time.

Interesting note....... you mentioned you had to back off to 'very rich' at 13K rpm...... there are some "40"s out there that couldnt get a 8.8x9 to that RPM in the first place

Bob,
I will try different props as you suggest. In fact I just received some 8.8 x 8.5 and 8.8 x 8.75. The pipe is 14.5" manifold to tip as Dubb suggested. I think I'll leave it there as a constant for now and play prop first. I appreciate your guidance as I'm the only "speed freak" out here with no one to exchange knowledge with. Here's a photo of me in the cow pasture getting ready for another adrenaline rush.
Roland

Thanks Roland......
Nice picture.

just to confirm..... Dub provided the pre-tuned exhaust system ?

Bob

That's correct, shipped with 50 FIRE and a note "14 1/2" from tip to face of case. Q500 props tuned 18K-20K.