One Black Pipe

[davevramp]

I have a 45 170b that I think the carb is not in the shape that I should be.
I could not shut down the engine with the mixture control. The same thing happened A few months ago and went away now its back Also on final with throttle at idle it would after fire out the exhaust several times.
When I pushed Her into the hanger I saw that the left exhaust pipe was sooty black and the other was light tan.
Any ideas?
Thanks In Advance
Dave

[Bruce Fenstermacher]

Just a wild idea.

Check to make sure your Primer is not leaking allowing fuel to be drawn in to the intake keeping the engine running after you've pulled the fuel mixture.

If I recall correctly the primer is on the left side intake which is why that exhaust side could be very rich indicated by the black soot and the other side brown the way it should be.

While I can't say with authority, I've been told that backfiring can be cause by intake leaks so I'd look over your intakes for them. At least that's what I think I remember.

[HA]

what Bruce said. check your primer, o-rings could be leaking so fuel would be getting into the intake even when you don't want any.

carb probably isn't the problem, it would affect both sides equally.

sometimes backfiring at idle is caused by intake leaks, so check those over too. gaskets at the cylinders, or those old, brittle hose connectors. worst case leaking valves, can find that with a compression check.

Hans

[davevramp]

Does the primer discharge into the left intake manifold and the vacuum gauge tied in to the right intake manifold? I am on the road and do not have my manual with me.
Thanks
Dave

[cessna170bdriver]

Does the primer discharge into the left intake manifold and the vacuum gauge tied in to the right intake manifold? I am on the road and do not have my manual with me.
Thanks
Dave

The primer does discharge into the left manifold. On an original installation, the vacuum gauge would have been tied to the venturis. The left and right manifolds are identical parts, so the right manifold fitting (1/8 pipe) is normally plugged. The right manifold can be modified with a larger fitting to provide a backup vacuum source.

Miles

[Bruce Fenstermacher]

Miles I think Dave was thinking manifold pressure but typed vacuum gage. Any I remember the primer going to the left side and the manifold pressure going to the right.

[GAHorn]

The reason engines "backfire" when they have a leaking intake manifold is due to excessively lean mixture. This is contraindicated with a sooty exhaust pipe on that side.
Engines may "afterfire" if excessive fuel is in the exhaust system, tho', and that does lead to question the primer.

[davevramp]

The primer was not locked. It did make the left exhaust pipe black.
Fuel burn over 10 gallons /per hr. The next time out I flew 2 hr and burned 26.4 gallons. Both exhaust pipes were light tan In color. This was killing me. The last thing to inspect was the carb. The float was non metallic soft and chaining shape. Installed the “kit.” Next 2 hr of flight under 9 gallons per hr.

thanks
dave

[GAHorn]

The primer was not locked. It did make the left exhaust pipe black.
Fuel burn over 10 gallons /per hr. The next time out I flew 2 hr and burned 26.4 gallons. Both exhaust pipes were light tan In color. This was killing me. The last thing to inspect was the carb. The float was non metallic soft and chaining shape. Installed the “kit.” Next 2 hr of flight under 9 gallons per hr.

thanks
dave

Before I could finish reading your entire response... I got to the "flew 2 hr and burned 26.4 gallons." and I immediately blurted out "(expletive)" as I envisioned a carb-float stuck open!
Great catch!, Dave. Yes! Anytime qty indicators drop more rapidly than usual is a fine time to land and check it out! A stuck carb float is a big leak that is hidden from the cockpit!

Although it is sometimes smelled in the cabin as a faint gasoline-smell, it's unlikely a pilot will make the connection unless he notices the qty indicators dropping faster than usual. It's usually suspected to be a dry-rotted vent line hose, etc. A former member had called me to complain about inconsistent fuel burns, and at first thought we'd suspected leaking fuel caps. (He did find the red-silicone check-valve missing from his right fuel cap.) But upon replacement of that cap, he still had varying fuel burns from flight to flight.
Then one day, on an overnight at an outlying field, he arrived to find the grass dying and strong gasoline odors around the airplane. The "oily looking" grass beneath the cowl was a clue. He'd lost about 10 gallons overnight he calculated, but the carb was not leaking at the moment.
He refilled and things were fine so he flew home and after engine shut down the carb was pouring fuel! He'd had that gas-smell again on final approach.
Hope this is a heads up for everyone.

(Additional Suggestion: I store my airplane overnight ...and all the time when in a hangar.... with the fuel valve in the OFF position.)

[davevramp]

The carb never leaked gas when it was not running. The aircraft is hangered and I could smell the sumps leaking when I had an o ring problem. I have put 2 new gas caps on and removed both red check valves. I am thinking of replacing the one in the right tank. It just looks real small for a vent hole.
Ya it was getting expensive, and frustrating, and time consuming. We should be flying these birds not spending the summer working on them.

thanks
dave

[dacker]

A pilot's worst fear... fire in flight! In the Navy helicopters I flew once upon a time, we were allowed leaks of certain fluids in varying amounts, transmission fluids maybe a few drops per minute, hydraulic fluids maybe a few gallons per minute (OK, not really), but fuel leaks were zero! If fuel lines or anything else was leaking, or fumes were smelled in flight, then the aircraft was down (grounded) until repaired. Don't mean to jump on the old soap box, but there is no normal reason that we should accept smeling fuel fumes in flight.

By the way Dave, why did you remove the check valves from your fuel caps, they are there to prevent siphoning from the fuel tanks?
David

[davevramp]

I have a 54 170b and it came with the little periscopes on the gas caps.
My system is only vented through the fuel caps. When I install the new caps using the proper part number, I experienced a lot of pressure build up in the fuel tanks. The little check valves were not letting the pressure out when the gas expanded. Having a 140 been through the gas cap stories I removed both check valves. If I get a little nerve up I will replace one of them

[dacker]

Dave, it sounds like the other vent is plugged then. This is the vent on top of the fuselage, the little tube sticking up just a few inches behind the windshield. If you have your parts manual you can see it in the schematic. As a general rule, if a correct part doesn't work as is, then you have another problem somewhere else. I have two recently installed vented caps on mine and they work beautifully, complete with an even fuel burn from both tanks.
Hope this helps,
David

[davevramp]

No vents on mine 1954 170b

[GAHorn]

Dave, all 170B's are supposed to have the gooseneck vent. If yours has been removed, was it documented and approved? Has your airplane had it's vent moved to the lower left wing ala later aircraft? If so, was that documented?