Weird engine problems

One of my flying buddies and I have identical ARF Noblers with Brodak 40's. I mean same engine, same prop, same plastic clunk tank, plumbed the same, engine set far to the rear with shaft extension, everything except the stickers is identical. My engine runs spectacular but Bob B's runs like crap.

It is impossible to adjust the needle for a consistent 4-2-4. On the ground, It's either too rich or too lean. You start adjusting the needle in from 7K and all the sudden it jumps to 9k and stays there. Start backing it off and it will stay at 9k then suddenly go blubbery rich to 7k.

We have tried everything, swapping fuel, plugs and even pulled the whole nose apart at the field to check the tank pluming, filter and fuel lines. While we were at it we even pulled the spray bar and made sure it wasn't partially plugged. Sunday evening Bob pulled the engine out of the plane, put it back on his test stand and it ran great, needled just like it is suppose to so it has to be something to do with the fuel tank or vibration. The problem is a Brodak 40 doesn't really vibrate.

I have been unable to come up with an answer and really feel bad because I talked him into this set-up. Of course it didn't take to much talking after he saw mine fly but I still feel bad.

Any other ideas on what to look for?

Bob:
Swap powerplants entirely?

My guess is that the mounts on the shaker are less ridig than yours. "X" webbing might be the answer?

See Ya

Maybe the vent tube is too close to the pick up tube. When you get enough rpm the bubbles blanket the feed line and lean it out.

Giffy

LAST EDITED ON Oct-20-03 AT 08:25 PM (CDT)
 
Swap the tanks. If you still have the problem, see what is loose on his airplane.

Maybe his spinner is out of balance?

>Swap the tanks. If you still have the problem, see what is
>loose on his airplane.
>
>Maybe his spinner is out of balance?
>
>From somewhere near Parkville, Mo.
>William Crane
>AMA 6733

One more thing, is his tank wedged in solid? It could be vibrating and causing the bubbling from inside. Wedge the tank down solid, so the sides cannot vibrate and then see what it does.

Sunday evening Bob pulled the engine out of the
>plane, put it back on his test stand and it ran great,
>needled just like it is suppose to so it has to be something
>to do with the fuel tank or vibration.

Vibration!

>The problem is a
>Brodak 40 doesn't really vibrate.

Of course it does - just not that much. One of the tricky things about vibration and resonance is that the frequency matters a lot more than the amplitude. You might be able to put a Fox on the same airplane, and have no problems, even though the imbalance of the engine is far greater.

Paul and Howard had all sorts of vibration problems with the B-17. A15FP is a pretty darn smooth motor,and the motor mount/spar is a very stiff carbon pre-preg composite part, but with them all running at the same speed, they lined up with each other and drive various bits into resonance and completely screw up the engine run.

sounded good:o

What about the prop, is it balanced? If not try repositioning it 180 degrees from where it is now.

Switch engines.

If same, switch tanks.

Is everything tight on the offending Brodak?

Randy: Prop alone wouldn't do this. Currell

Currell

Don't bet on it.

Just to ease the tension. I gave him the prop he is running, it is a RevUp 11-5 cut from a 12-5, blueprinted and balanced. Identical to the one on my Nobler.

No tension Bob, Currell and I are OK, we jab each other for fun.

>It is impossible to adjust the needle for a consistent
>4-2-4. On the ground, It's either too rich or too lean. You
>start adjusting the needle in from 7K and all the sudden it
>jumps to 9k and stays there. Start backing it off and it
>will stay at 9k then suddenly go blubbery rich to 7k.

Sounds like the sucker is not broken in.

I would put it on the bench and scream it out with a 9" prop for a couple of tanks (if it behaved the same on the bench).

>
>Sounds like the sucker is not broken in.
>

That was my first thought, even though he did the recommended 20 runs on the stand and it has had at least 10 flights in the airplane. I suggested he stick it back on the test stand to see what happens. It ran great and needled fine.

Like several have said, almost has to be a vibration/resonance problem, last we talked he is going to go over the engine/tank installation again and make sure everything is secure.

Thanks for the tips guys..

It sounds like he is picking up a resonance in his tank. Above a set rpm the fuel begins to foam, which of course makes it lean out more. Setting it rich gets it below resonance.

Can you take the cowl off and look at the fuel line while the engine is running just to see if this is happening (or attach an extra long piece of tubing to the pickup and run it out the cowl exit hole and back to the spraybar. Check to see if you can see bubbles.