Engine runs but is hunting

scramble Tue May 22, 2012 2:22 am

Hi, My rebuilt BB 049 strats well, with prime, but runs erratically, the revs hunting up and down. 25% nitro fuel. Any thoughts?

John Goddard Tue May 22, 2012 3:48 am

Welcome on board Scramble.
First thing to look at using 25% have you got at least 2 head gaskets
In there? Golden rule is 1 per 10%.
Then depending on your funds.
Check replace head
Install a Bee rebuild kit, paying attention to the crank case gasket,
O ring/ there is an o ring between the tank and back plate isn't there?
And check the reed valve is seating.
In fact (pre coffee still Very Happy

) start by removing the needle valve
Taking the spring off trim a little bit of r/c fuel tubing and slide
That carefully over the needle. Gently screw it back in. This does
Two things one it stops your needle adjusting it's self, two it'll
Stop air leaks which is what your condition sounds like.
So needle.
If no good check o ring/you can make one from a sliver of fuel tubing,
case gasket and reed.
If funds allow buy a bee rebuild kit and new glow head
Which depending where in the world you are is about 10 bucks.
Good luck.
Ps
Is the fuel fresh? And make sure it's castor based
PPs
I'm sure as the Stateside guy awaken they'll add
More hints.

scramble Tue May 22, 2012 4:25 am

Hi, thanks for the prompt reply (I'm in the UK). I am only using one gasket under the head, so I will add one more. I will then add the small piece of fuel tubing over the needle. I will then test the engine before making any more changes. Thanks

Kim Tue May 22, 2012 5:58 am

Hey Scramble!

As John said, "Welcome to the Forum!"

Here's a photo showing an engine with the tubing installed as John suggested. A small washer, placed under the stem's lip will give it a bit more surface.

proctor Tue May 22, 2012 6:44 am

Welcome Scramble,
enjoy the forum.
Just one item to add to John's very sound dissertation.
I like to rub the crankcase (tank mating face) on a piece of
600 wet and dry stuck to glass. this removes the little raised
pimples where the tank screws pull out the aluminium round the
screw holes.
John

scramble Tue May 22, 2012 6:52 am

Hi John, thanks for that. I was careful not to overtighten the 4x screws during the rebuild, the rear face was smooth. I am now waiting for the opportunity to test the mods mentioned above so far. Best regards.

**RknRusty** Tue May 22, 2012 8:00 am

Sometimes the screws are a little too long and bottom out without fully sealing the crankcase, so check on that. When you take it apart to inspect it, remove the needle and make sure the sharp tip is intact. With the needle removed, force some alcohol or other solvent through the needle valve orifices. Brake parts cleaner is good for this. And make sure there is no corrosion or old fuel scum sloughing off and getting sucked in through the pickup tube. It's a good idea to remove the crankshaft and make sure the oil groove is clean. Too much friction in the crank tube can cause overheating and slowing as it heats up.

Even if the engine was new in the box, the packing oil is old enough to have congealed and needs cleaning before attempting to run. This is pretty much SOP when preparing to use an unknown engine.

Welcome to CEF.
Rusty

EDIT: I spoke too soon, I see you already rebuilt it. Can't hurt to double check some of that though. Use lint free rags like t-shirt material when cleaning in the tank area. The orifices are so small, paper towel fibers can give it hiccups.

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