I'll spare you the long, drawn out details, but the story goes like this.

I bought the '73 185 out of Idaho last year, and we've been getting acquainted off and on since the fall. You know how you get in a new or different machine and your gains are turned up a bit because you're trying to figure out what sights, sounds, smells and feelin's are "normal" and what just doesn't seem right?

Yeah. That's where I'm at.

So I've had this "miss" coming from the engine for a long time. It's not like a fouled plug type of miss, just a little pop. A puff.

It happens at idle and at higher cruise power but the sound seems to go away at full power. Probably because at full power the shock waves from the 86" MAC at 2850 RPM are making so much awesome "look at me we're leaving planet Earth" noise that the problem just automagically goes away.

I've also got a fluctuating fuel flow indication that seems to be tuned in time with the "pop". It only bounces around by +/- 1 gph or so, and it's irregular, just like the popping sound.

On top of that, the manifold pressure at idle is about 14" at ~600-700 RPM, which seems a little high for idle manifold pressure compared to other airplanes I've flown with big bore Continentals.

Further, when I lean the engine out to book values, 50 deg F rich of peak on the first cylinder to peak, the fuel flows are about 2 gph higher than what the book publishes for a given altitude and power setting.

Mags check fine; happens on either mag with no effect switching between the two or selecting both.

I've engaged my crack team of forensic airframe and power plant magicians and we've started with the simple stuff. Rechecked the timing on the new mags, cleaned and gapped the plugs, soaked and cleaned out the injectors, checked the continuity in the harness and switches, replaced the original flux capacitor.

Just kidding on that last part. The original flux capacitor was just fine.

I've been looking for induction leaks; checked all the clamps and looked for any staining or blow by on the induction tubes. Nothin'.

And then I happened upon this little detail.

There is a drain line installed in the balance tube that vents to the atmosphere just behind the throttle body with some other drain lines. The really exciting part (I've been at this a while fellas, so spare me a bit) is that there's no check valve or sniffle valve installed in that line. So in effect there's a gaping hole in the induction system.

The IO-520D parts catalog doesn't show a drain line installed in the balance tube, just a plug. I don't see anything in the 185 parts catalog regarding the installation of a drain line, much less one with a sniffle valve in it.

So I have to wonder why someone would put a drain line in a taildragger with this type of induction setup when any fuel in the induction is going to flow down hill to the air box, and not the balance tube.

Any of you guys running an IO-520 with a drain line in the induction balance tube? Or am I the only lucky guy flying around in an airplane with an induction leak designed into the system just to #@$^! with me a bit...

Anyway, the drain line is capped and I'm waiting for this next blow of snow to pass through Kansas to go run it again. Hopefully I'll eradicate the pop and I can get on with spending time and money on something else.

I'd bet coffee and a doughnut you have a float kit, and I seldom put my donuts at risk!

The front manifold drain is added to float equipped aircraft so that when you're levelled out on floats, not tail low, excess fuel still has a place to go. You'll find it on the page with all the other float plane details near the back of your parts manual.

Doubt it's the source of your issue. If you're scrutinizing fuel flow, and it seems high, low ambient temperatures could be why. You're drawing so much dense air that it takes more mass of fuel to match the greater mass of air that is entering.

Looking for induction leaks is one thing. Pressure testing for them may reveal what the naked eye missed. Clean, or better yet, new vacuum. Plug the induction. Pressurize the exhaust. Spare with soapy water.

Also, verify fuel system set-up by following Continental's procedure. It's in their master service manual now. Used to be a separate service instruction letter. Free on their site.

I'm not the most well versed with 520s (read not at all) but that seems odd. Any chance that any of that drain plumbing is leaking and sucking air?

Also, don't give up in ignition just yet. I only have one datum for this scenario but years ago I had a 300 hour mag that was causing a hiccup. It was timed just right, plugs were clean and gapped. It turned out that it needed new points and condenser. That solved the issue and smoothed things out.

Pinecone wrote:I'd bet coffee and a doughnut you have a float kit, and I seldom put my donuts at risk!

Tell me where you'd like me to mail your coffee and doughnut. (I hate parting with my doughnuts.)

Float kit!

Wouldn't having the induction system open to the atmosphere (albeit a small hole) pose a problem, especially when it's sucking in that really heavy, cold air? Seems like there should be a check valve in line with that drain. I'll have to research the parts manual in the float kit section; thanks for pointing that out!

Zzz wrote:Also, don't give up in ignition just yet. I only have one datum for this scenario but years ago I had a 300 hour mag that was causing a hiccup. It was timed just right, plugs were clean and gapped. It turned out that it needed new points and condenser. That solved the issue and smoothed things out.

Maybe, but these are brand new mags (installed before it left Idaho). So not likely, but if I don't get anywhere with an induction leak I'll start scrutinizing the mags again.

Pinecone wrote:
Also, verify fuel system set-up by following Continental's procedure. It's in their master service manual now. Used to be a separate service instruction letter. Free on their site.

I would second a check of this for sure as u never know if it has been done properly and rarely done by mechanics when installing new engines it seems. If it was a fuel set up issue it would be pretty obvious with inability to lean in cruise or super high fuel flow and having to lean too much. If in cruise u have to run full rich I would say high end fuel pressure is off for sure. How does it lean?

My computer chair bet would be an ignition problem. Infant mortality of ignition parts is very common and should not be ruled out due to being new. Intake leaks cause continentals to run terrible at low power but don’t show up at higher power settings especially intermittently. Are you just mag checking on the ground at 1700 rpm? When u switch to RH mag in cruise and run for 5 mins do you still get a miss? What about LH?

PAMR MX wrote: When u switch to RH mag in cruise and run for 5 mins do you still get a miss? What about LH?

This is exactly how I deduced that mine was a mag issue and not fuel.

PAMR MX wrote:If in cruise u have to run full rich I would say high end fuel pressure is off for sure. How does it lean?

Intake leaks cause continentals to run terrible at low power but don’t show up at higher power settings especially intermittently. Are you just mag checking on the ground at 1700 rpm? When u switch to RH mag in cruise and run for 5 mins do you still get a miss? What about LH?

Don't have to run full rich in cruise and it appears to lean out just fine. The #1 and #2 cylinders peak (EGT) first, and the other 4 cylinders are about 100-150 deg F richer than the #1 and #2. Makes plenty of indicated fuel flow at takeoff power.

I've checked the mags on the ground at idle, at 1700 RPM during the normal mag check, and in flight at cruise power. But I have not run them for as long as 5 minutes.

I'd risk a bunch of doughnut's and bet that its your mags/ignition. I'd what swap them to test it. Not a huge job and will give you some piece of mind.

Jim

If your intake is sucking cold dense air at wide open throttle (WOT) on take-off, it would seem your EGT's would be going through the roof.

I stopped up at the airport today and one of my friends was working on the install of an overhauled IO520D on a 185 on amphibian floats. We talked about the cross over drain, he had to install it as the airplane came to him for the engine change without the drain tube installed when he removed the engine to send it out. He told me that the actual hole in the cross over tube inside the welded on boss that the 45 degree elbow screws into was only about .040 in (forty thousands) of an inch. I did not verify this but I trust what he says. Would not suck much air in that case. We have seen three 185's in the last few months without this drain line installed so mechanics must be missing that it is part of the float kit at engine changes.

Tim

Try cleaning your injectors.
The stock fuel flow gauge is just a pressure gauge so it wont tell you if fuel isn't getting to the engine.

Also check and clean the fuel screen on the engine. Lots of mechs forget to clean it I have found several plugged. Although i would guess this isn't your issue in your case. just a good thing to do.

Intake hoses and gaskets can also be problematic usually at lower rpm

One time many years ago a company I worked for had a 210 that had an intermittent miss. we tried everything to find it ( injectors, mags, plugs etc) but no luck.

It only had about 60-70 hours to tbo so the boss just ordered a new engine. While swapping out parts I found a number 8 machine screw with a nylock nut and flat washers attached that had apparently been rattling around in the intake it was all rounded off and I guess it was getting sucked up to a valve once in a while and causing a miss, It was a little too big to go through but could have been disastrous

Have you checked the gaskets where the induction tubes attach to the cylinders? I’ve seen these gaskets shrink and start to get sucked in on a newer factory reman i0-520-d with low time. Also, check your fuel pressure and flow iaw the tcm sb. It could be a combination of induction leaks and incorrect fuel system setup. Does your 185 have the high pressure fuel switch that kicks on at full power?