ilikeplanes

There I was, maiden flight for my CG 'koi that has been in the works for six years. I was confident that my engine would run great because I ran several tanks through a month ago with excellent results. The Webra was a kitty cat and ran perfectly. Now, my engine was completely inside the round 'koi cowling and the low speed needle could not be adjusted and the high speed needle could only be adjusted through the front hole in the cowling while the engine was not running.

The engine started on about the fourth flip. Idled fine and ran up good but seemed to sag a bit. Shut the engine down open high speed needle 1/2 turn. No change. Repeat process until needle was ready to fall out of carb. Still no change and engine still sags. So I take off with a saggy engine for the very first flight. Half way around the field my engine quits and I make a perfect three point dead stick landing.

Start fiddling with the engine in earnest on the ground. Engine gets extremely hot in cowling. Could this be the problem? Remove cowling and try to tune engine. Same results as before. I can fly around the field but engine sags badly on vertical.

Now five people are involved and no one can figure it out. Air leak? No, did pressure test. Junk in carb? No, washed it out with fuel. Faulty fuel tank and plumbing? No, tank pressurizes and feeds fuel in every attitude with no bubbles.

Finally, I and a young but enthusiastic and experienced flyer notice that at w.o.t. the low speed needle is still in the spray bar. Is this normal we wonder. Other engines seem to have a small gap here. So, we open the low speed needle until a small gap appears between the spray bar and needle. Start engine and it runs great and can be tuned from blubbering rich to saggy lean. We use a tach to tune to peak-300 rpm. Engine runs good in the air and all is well.

The moral of the story? I don't really know. Even with the excessively lean low speed setting, the idle and transition were great. Excellent, in fact. I guess I learned that if you seem to have lost your baseline, and adjustments don't respond, open everything rich and start over. Oh yea, engines get hot in unbaffled round cowlings.

djlyon

Yup, what you went through is common with any twin needle carb. In an effort to get a very good idle to keep your airplane from running away on the ground we often make the perfect slightly rich idle adjustment which very often winds up with the low speed needle interfering with the high speed adjustment producing exactly the condition you got. The fix is only lean the low speed needle enough to get good transition. This will normally leave the mixture quite rich at idle and notproduce any interference with fuel flow from the high speed needle.

ShempHoward

My Feedback: (3)

You have plenty of air coming into the cowl but how about exiting the cowl??

ilikeplanes

Ten 1-1/4 inch holes in the belly pan.

MAJSteve

My Feedback: (-1)

Sounds like you may need a baffle in the cowl to direct the air from the non-cylinder side to the cylinder side. Did you mean 10 x 1 1/4" holes? That's certainly enough but without the baffle, the air will take the easiest route out - which is opposite the cylinder head, then down and out.

I'd be inclined to make a baffle around the head that takes all the air possible and forces it by the cylinder head and out a hole around the cylinder head. Won't look as nice but I'd prefer an engine that didn't get hot enough to seize.

ilikeplanes

I am planning on adding some type of baffle. Do I need to cool the pitts style muffler or just the engine cylinder. Big hole in the side of the cowling? Only If I get desperate.