Kamikaze_K

OK, I have an OS 60FP with an air bleed carb in a Goldberg Cub. I have a pitts muffler on it with one of the pipes plugged to increase fuel pressure. I can get it to run OK when the plane is level and when it's pointed straight up, but when I point the nose down a few degrees, it runs rich, the RPM decreases, and the engine dies. I just can't seem to get this engine to run right. I realize nobody likes air bleed carbs, and a lot of the FP series wasn't that great, but I got this engine free and I'd like to make it work if I possibly can. I don't know much about glow engines; I'm mostly an electric guy, so any help would be greatly appreciated.

w8ye

My Feedback: (16)

First of all... Welcome to RCU.

You may have a tank height problem?

Is you engine mounted upright, on its side, or inverted?

bkdavy

My Feedback: (1)

Have you tried it with both pipes unplugged to reduce tank pressure? Another trick, but only as a last resort, is to try drilling out the air bleed hole slightly to increase air flow? Does it exhibit this behavior at full throttle or idle? If its happening at idle, try leaning out your low speed screw (make adjustments of 1/8 turn or less at a time) and readjust your high speed needle. Check it again. Is it worse or better?

Brad

Kamikaze_K

Hey thanks.

I don't know, you'll have to tell me. The bottom of my tank is about 2 1/2" down from the centerline of the engine's crankshaft. Oh, and the engine is mounted on it's side.

Yup. It used to lean out horribly when you pointed it straight up. Then one of the guys at my field suggested that I plug one of the pipes. Now it runs fine when you point it straight up, but it still has the problem of dieing when you point it down.

It only dies at idle, but it does run rich at full throttle. BTW, do you turn the screw in or out to lean it out? See, I wasn't kidding, I really don't have a clue.

Sandyt

Turning the screw clockwise richens and vice versa. Make sure that the screw is half way across the air bleed hole for your initial setting. Once the hole is completely covered by the screw, or completely open, then further adjustments will not do anything. At least until the screw falls out

Sandy T

w8ye

My Feedback: (16)

The centerline of the tank should be as close as possible to the centerline of the needle valve.

bkdavy

My Feedback: (1)

Does it exhibit this behavior if you set your idle just a little bit higher?

I would definitely try leaning out the low end slightly. The hazards of a lean run are not significant at idle speeds. The hazards of the engine cutting off due to a slight nose down attitude are far greater.

Having said that, I went back and re read your original post. You say that when you point the nose down a few degrees, the RPM increases, and the engine dies. That would indicate that with the nose pointed down, the mixture is getting LEANER (rise in rpm is generally related to the mixture getting leaner) before it dies.

Now there is an alternative explanation, and that is that your low speed needle is already set too lean. You describe your tank location as "the bottom...is about 2-1/2 inches below the engine centerline of the engines crank shaft. That means, that if it is approximately 2-1/2 inches high, the TANK CENTERLINE is below the needle valve, probably on the order of 1 to 1-1/2 inches. Pointing the nose down places the tank centerline closer to the needle valve, resulting in a reduced negative pressure differential, increasing the fuel flow, and causing a richer mixture. The only time this would cause an increase in RPM is if the inital low speed mixture is set VERY lean. This could be indicative of an air leak around any of the O-rings, a loose or leaky seal around the engine back plate, or even a leaky exhaust port.

You didn't describe how the engine performs when its level for either the pinch test or the punch test. On the pinch test, with the engine level and idling, pinch the fuel inlet to the needle valve. RPMs should increase slightly before the engine stalls. If it just stalls without increasing RPMS, the low speed mixture is too lean. For the punch test (also known as transition test) with the engine warm and idling for about 20-30 seconds, punch the throttle to full. If the engine sputters and dies, the low speed needle is too lean. If the engine sputters, and then goes to full throttle, the low speed needle is too rich. Note the subtle difference between sputtering and dying, vs. hesitating and then going to full speed.

Brad

Kamikaze_K

Well i meant to say that the RPM DEcreases when you point the nose down. Also, last night when I opened up the plane, I found that the vent line from the muffler had broken off the fuel tank. (he he... oops) So maybe I'll try it now that everything is set up properly. Thanks for all the help guys.

NM2K

My Feedback: (14)

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The fuel pressure is different from holding it on the ground with the nose pointed down than when flying with the nose pointed down (less). The latter should permit the engine to run better when flying because the descent of the model reduces the amount of fuel pressure that would normally tend to make your mixture too rich. This is pretty typical of engines, whether using airbleed or two needle carbs.

Airbleed carbs are not bad. They just require a different way of thinking about them in some instances. Additionally, there are two types of airbleed carbs. There are true airbleed carbs and then there are two needle carbs utilizing an airbleed trim. The latter are just as good as other two needle carbs. However, the regular true airbleed carbs can do a good job too, once the engine is broken-in.

All glow engines, because of their variable timing, are sensitive to mixture adjustments when they are new and without much running time. If you can manage to get them through the break-in period without overheating them (too lean), eventually they will mellow out and not be so sensitive to their carb settings, both low and high speed. Just be patient and get some running time on the engine and ensure that it doesn't run too lean at any time. Patience will deliver you a fine running engine.

Don't hesitate to come back here and ask questions, or just to shoot the bull about running engines. Good luck and good flying.