C-145 (O-300A?) Heavy Leak/Drip & Tach Question

[ptporebski]

Hello All:

Two questions for the experienced personnel in the association:

1.) My 1959 C-172 has always had a heavy drip upon shutdown since I bought the plane in 2009. The dripping fluid is the viscosity of oil but it is a blue-green color. It is not the color of the engine oil rather more like a concentrated blue such as 100LL. However, it is not the viscosity of gas. It does seem to be coming from the carburetor. I wipe the bottom lip and inside of the cowl after flight and it is heavy with this. The nose strut and steering damper will also be coated with this fluid. I first noted it by seeing the drip on the nose wheel tyre. Today I primed the engine and taxied about 3/4 of a mile to a friends house in the neighborhood. Upon shutdown (by mixture cut-out) there was a small puddle of this blue-green fluid puddling on the bottom cowl & tyre. Crapped up his newly refinished taxiway and was rather embarrassing!

I have questioned my IA and he tells me it is the carb float bowl or jets leaking. I had him change out the carb for a rebuilt Marvel-Schembler unit. He also replaced the fuel line hose this past annual. I note that the outside of the carb always seems kind of wet with this fluid and the bottom of the air box where the round opening is will have drips of this fluid. My IA tells me that all aircraft engines drip and this is normal. I don't believe the amount of drippage that I am seeing is normal.

I have asked about this before (in 2011). Since then I have had the A&P (aka as my IA) replace/overhaul the carb as mentioned above, repeatedly check the lower and upper plugs to no avail. The engine probably has 350 to 400 hrs now since a field overhaul by P.O. that receipts show totaled about $25k with all new Millennium cyls. It doesn't appear they cut corners. The engine runs fine - it just dumps this fluid on shutdown and during flight. BTW - oil usage is about one qt every 4 - 5 hrs if at full power and about every 8 hrs if loafing at 2300 - 2100 rpm.

Do other members of the association see the same on their 170's with the C-145 or O-300 engines? Does anyone have any thoughts on this?

2.) The tachometer has recently begun to "swing" back and forth when I change throttle settings (approx +200 rpm. It also does that to a lesser extent in cruise. Any idea what is happening here and more importantly what needs to be done to fix it? Is it a matter of lubing the spring drive tach cable? I seem to remember doing something like that to my childhood bicycle speedometer.

[voorheesh]

What you are describing is not normal. You say it happens on shutdown (only?). Is there evidence of this fluid under the fuselage, aft of the engine, indicating it is leaking in flight? Do you smell fuel? Does it smell like oil? You should get a mechanic who is experienced with these engines to troubleshoot this. It should not be too difficult to identify the type of fluid followed by a thorough engine/fuselage wash and a ground run. If the leak persists after the ground run, it should not be difficult to id where it is coming from. Some oil leaks are really hard to track down but if the quantity of fluid is as you describe, I would think a good mechanic should be able to figure it out. Leaks like this are a potential fire hazard so get it looked after. You will probably hear more advice on this unusual condition and you should be able to get it fixed. Your tach problem needs to be checked by a mechanic also. Good luck.

[Bruce Fenstermacher]

Your tach is a very expensive, but not necessarily better made, bike speedometer. The needle is attached to a piece of steel which is turned in one direction by the magnetic field of a magnet rotated by the tack cable. The needle movement turns in a delicate bearing. The rotation of the needle is biased in the opposite direction by a spring.

The tack cable itself can act like a spring as it is turned. As the cable sheaths wear or gets dirty the end of the cable is held back and the cable winds up to a point the tension overcomes the drag and the cable unloads. When this happens the magnet bounces the needle against the spring which bounces it back against the magnetic field and so on. Dirt or wear in the needle bearing in the tack can do the same thing.

Many times removing the tack cable from the outer sheath and cleaning it with solvent then lubing the cable can fix a bouncing tach. BTW engine oil in the tack cable can cause bouncing as the engine oil is to thick. The lube for the tack cable according to Keystone Instruments should be plain old petroleum jelly.

Now the fluid you see is most likely 100LL. The blue tint is the clue as there really isn't any other fluid that would look that color and also the fact it is all around the carb. Gas will evaporate and dry quickly, oil will not. That is how you can tell the difference. A few drips from a carb is common occurrence. A continual gas wet carb to the point it stains your tire and the ground, is not.

Likely the float is set to high in the carb. You need to find a mechanic who is not afraid and understands how to make carb adjustments.

[rusty]

I just went through this same problem with my 1964 Skyhawk. The fluid is dripping out of the intake tube back through carb and air box. Replaced #5 and 2 bad valves. The RPM movement I had this problem too. MAG problem, Engine 0300-D. Hope this helps.

[DaveF]

I agree with Rusty, I'd check for worn intake valve guides. Oil leaking past the guides could run back toward the carburetor, picking up evaporated fuel residue, blue dye especially, along the way. Drop the induction pipes and look for oil up at the cylinder.

[c170b53]

See the post from GMAN and George's response. Likely oil but hopefully it's just a cylinder not the sump

[Bruce Fenstermacher]

How did you guys go from his IA telling him it is the carb leaking gas, to oil dripping down the intake and dripping out the carb?

Surely it is easy enough to tell the difference between oil and gas from the viscosity of the fluid dripping.

[rusty]

He is right saying that blue-green in color and thick like oil. That just what was leaking on my Front wheel pant. My IA called Friday and told me my intake was full of the blue-green dye from the 100LL. #5 Valves were bad, Intake valve was not letting the fuel in properly. Replaced #5 cylinder, with a total of 1050HRS SMO.

[cessna170bdriver]

How did you guys go from his IA telling him it is the carb leaking gas, to oil dripping down the intake and dripping out the carb?

Surely it is easy enough to tell the difference between oil and gas from the viscosity of the fluid dripping.

From the OP's second sentence: "dripping fluid is the viscosity of oil but it is a blue-green color". To me that indicates a mixture of oil and 100LL dye. I think his IA isn't really paying attention to what he's seeing.

[voorheesh]

It is difficult to understand symptoms from a post. The owner seemed to be asking if the condition that he described was "normal". His IA reportedly told him that some 'dripping" was normal. My advice is always to consult a qualified mechanic who is familiar and current on the equipment to get a reliable diagnosis and repair. I did not want to presume his IA was giving bad advice, but what he described is not normal and if his mechanic was telling him otherwise, he should get a second opinion. This forum is also a very valuable source of information and there are many more qualified than me to provide information and advice. All these replies are valuable and contribute to what the NTSB and the FAA describe as a positive safety culture. John Wrenwick alludes to that in a post over on the pilot forum where he speaks to the value of owner clubs. Bruce, George, Del and many others probably don't know how valuable their advice is.

I have seen too many general aviation aircraft flying around with leaks and stains with owner/pilots (sometimes renters) completely oblivious to the potential consequences. Recently, I have been trying to educate some flight instructors to take airworthiness more seriously and to practice what they preach. If you teach a pilot to preflight an aircraft by the book i.e. look for leaks, cracks, signs of trouble, should you proceed to fly after you discover leaks, cracks, signs of trouble? Or should you get a mechanic and find out what's up? Unfortunately CFIs don't get paid unless they fly and too many are looking the other way. New pilots will emulate their instructors which can lead to a deterioration of aviation standards. Those of us who care, need to counteract this trend and keep reminding our fellow pilots of the basics.

I also think the original poster here made a good decision to ask these questions and I hope we have been able to help him.

[Bruce Fenstermacher]

How did you guys go from his IA telling him it is the carb leaking gas, to oil dripping down the intake and dripping out the carb?

Surely it is easy enough to tell the difference between oil and gas from the viscosity of the fluid dripping.

The dripping fluid is the viscosity of oil but it is a blue-green color. It is not the color of the engine oil rather more like a concentrated blue such as 100LL. However, it is not the viscosity of gas.

Darn. It pays to go back and reread the original post. I missed the part about "viscosity of oil".

[GAHorn]

See the post from GMAN and George's response. Likely oil but hopefully it's just a cylinder not the sump

Wow, Jim! You looked hard for that one!

Here's the post to which Jim was referring:

http://cessna170.org/forums/viewtopic.p ... MAN#p51757

[blueldr]

That blue green drip must be a C172 thing. I never heard of it occurring on a C-170.

[rusty]

blueldr, The 172 has a birth defect!

[blueldr]

Oh.