Ok cub fuel draw trouble

davidll1984 Fri Feb 19, 2021 1:24 pm

Y have m'y old ok cub engine runing and y found that m'y fuel tank mabe To hi in the fuselage Not sur if y point up engine lean To stal if point down ful ritch wi y cant Find Good tank position ?? The engine run very Good if its stay level y have try lower fuel tank change Not for Good Now cant point up just stal very fast and same if point down stal To ritch y use tube on the neadle valve is it posible spray bar Not pointing in Good direction ? Tink fuel tank Not correct To large ?Not sur

ffkiwi Mon Feb 22, 2021 3:32 pm

OK Cubs have always had a reputation for poor fuel draw.....compounded by a rather poor needle valve shape (rounded off end, not much taper..!)....whilst your problems COULD be due to the spraybar hole being incorrectly positioned-this is unlikely considering the spraybars are pressed in at the factory-and thus unlikely to rotate much-if at all, unless people have tried to remove and replace them-which does mess up the interference fit.

In practical terms-and ingnoring the needle valve issue itself-the Cub intake is a bit too large for the engine (or the spraybar diameter too small-which amounts to the same thing)-the first option I would take is to plug the intake slightly-using a piece of balsa shaped to fit either behind or in front of the spraybar....and see if this improves the fuel draw (it should). Second thing I would do is grind a decent taper on to the last 1/4'" of the needle-though you'll need a lathe or a dremel to do this accurately-you don't want an offset taper-it wants to be exactly concentric.

Final option would be to replace the entire NVA with a better quality one-sch as the unit from the MP Jet 061-which has a very finely crafted needle...

Cubs were-and are-very variable-due to the apparently non existent factory QC procedures-if you have a good one-its quite good...if you have a bad one-its often atrocious.

ChrisM
'ffkiwi'

dckrsn Mon Feb 22, 2021 4:08 pm

Another possibility.
you can press the old spraybar out, and replace it with a
fine thread(128tpi) one from a postage stamp backplate. It's
a slightly loose fit, but a little JB Weld will take care of that.
One of my .049Bs would hardly run before the retro fit, but
now it starts easily an runs like an anemic champ!

Bob

davidll1984 Mon Feb 22, 2021 5:25 pm

Bob it sound like Good option y was tinking wat type carb wil fit on the ok cub engine y like rc plane y have change the lenght of tube for longer and smaler help litle the engine run strong wit large 7x3 just engine get To ritch if point down and lean if point up now the engine keap runing y want him To run solid like cox engine posible that it wil never run constant rpm if posible To use carb To get correct fuel seting tink its posible just nead Find smal carb like the asp wasp .061 rc y dont want To damage the engine it has very Good compression. Run strong on the bench wel in m'y hand now Very Happy

its Perfect weight power ratio for the plane y build wit y wanted big réserve of fuel cant place fuel tank lower y wil try mod the neadle tip litle se if it help ??Not sur Huh...

dckrsn Mon Feb 22, 2021 6:09 pm

David, the needle taper fix will definitely help, bit it's
a delicate job getting the taper just right.
I hope it works, but if it doesn't, PM me and I will
send you a fine thread needle and spray bar if you
want to try that.

Bob

davidll1984 Mon Feb 22, 2021 6:42 pm

Thanks bob I'll try modify and if it Not helping y wil try use the parts u have I hope to find the solution y realy want To use that litle engine

Oldenginerod Mon Feb 22, 2021 7:17 pm

Be very careful not to taper the needle too far along as you will find that the needle will bottom out on the threaded end and not allow you to shut the needle off fully

dckrsn Mon Feb 22, 2021 7:25 pm

Oldenginerod wrote:Be very careful not to taper the needle too far along as you will find that the needle will bottom out on the threaded end and not allow you to shut the needle off fully

Yep, true dat, Rod. Taper in small increments.
Bob

roddie Mon Feb 22, 2021 8:14 pm

Hi David, Cool to see you working on an airplane!! (although your recent truck project is AMAZING!) Chris (ffkiwi) mentioned the size of the carb/venturi being a bit large for that engine. A needle mod may well help.. but reducing the opening for the air-intake is an easy thing to try (as Chris mentioned.. using a little wedge of balsa..) The intake-air's velocity should theoretically increase.. which should assist drawing fuel.

Have you considered a sealed balloon type tank? I'm not meaning a pressurized feed. A toy balloon is used. There is no venting. The air in the balloon is evacuated prior to fueling; using the same syringe as to fill it. You make an internal fuel pick-up using some silicone fuel-line, with multiple "nibbles" taken-out.. for fuel to enter.

You could run as much fuel.. as the model's payload permits.. and maybe a few cc's more. The fuel-payload's location is less critical.. which is another plus for your application.

davidll1984 Tue Feb 23, 2021 12:27 am

That's right I hadn't thought about this ballon trick tink its Good way As I had already chosen the tank I didn't think of try that tink y wil try as soon as y Find the correct ballon size y want large réserve To fly rc at least 10minute and glide after engine stop Not sure for the matériel To use is milar or nitril fuel résistant in long terme as once the tank instaled wit the plane finish I can no longer access this place To try change the tank position y have take the plane To piece the top fuselage was close finish now its posible To make change Not sur how To build som sort of access To fuselage

The plane is old y have made that litle plane in 2010 tink Not sur its one of the first y have build after m'y crash wit sport atv Yamaha 450 its how y came back To the hobby it was painful To build fractured back two fractured vertebrae y start build after long Recovery of m'y accident Unable to work several hours on the model I must say that it is far from being well built Laughing

and its wy y just never finish that one it has lost its top wing rear tail ruder wheel the model Should look like this

davidll1984 Tue Feb 23, 2021 12:43 am

Y want To build one plane from cinéma Howard Hughes build plane for speed of biplane take its top wing of rinforced the wing

Is Somting facinating This man fascinates me with The largest plane made of wood

Oldenginerod Tue Feb 23, 2021 1:02 am

dckrsn wrote:
Oldenginerod wrote:Be very careful not to taper the needle too far along as you will find that the needle will bottom out on the threaded end and not allow you to shut the needle off fully
Yep, true dat, Rod. Taper in small increments.
Bob

Also, I should mention that a lot of the .049 Cubs had a venturi insert which presses into the case. If you have a case machined for the insert and the insert is missing, the venturi bore is waaay too big. These inserts choked the venturi way down to about 1/8" and were commonly fitted to the Comet models.

davidll1984 Tue Feb 23, 2021 1:37 am

My larger than .049 my is .074 yes the ventury look large for the size y tink of use carb if posible Intake mesure 5.11 mm y have .7 carburateur wil mesure its ventury m'y cox 074 carb look smaler

It mesure 4.10 on the smaler side 4.17 on the larger side Intake side so now y tink of insert Cooper tube inside top ventury try fix the carb if posible the .7 bugy engine carb wil posibly fit Huh...

Oldenginerod Tue Feb 23, 2021 5:34 am

davidll1984 wrote:My larger than .049 my is .074 yes the ventury look large for the size y tink of use carb if posible Intake mesure 5.11 mm y have .7 carburateur wil mesure its ventury m'y cox 074 carb look smaler
It mesure 4.10 on the smaler side 4.17 on the larger side Intake side so now y tink of insert Cooper tube inside top ventury try fix the carb if posible the .7 bugy engine carb wil posibly fit

David. Your engine, being the .074, never had a veturi insert. The original OK insert that I mentioned was only used on the .049A & .049B.
The early .049 & .074 had a possible manufacturing "oversight" in that some of the crankshafts were never finished correctly. I have actually found this on one of mine. The intake port in the crankshaft was never finished correctly on some. It just had a relatively small round hole drilled in the crank, far smaller than the size of the venturi diameter, but there was a rectangular outline defining what the intended port shape and size should have been. Opening this port out with a Dremel tool to the larger outline will significantly improve the running of the engine. It not only improves gas flow but alters port timing as well.

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