?Sticking Valve or Carb Ice?

[fangzz]

It's been a great summer for flying here in N.E Washington State. Haven't done as much 'backcountry' stuff as we'd have liked , but we've gotten some good local area flights in. Sightseeing. In the process, what I thought was carb ice, I now believe is a valve that's threatening to stick.

My bird is grounded until I figure out what is causing what I will call 'an in-flight engine malfunction.'

A sticking valve didn't seem possible in my mind since the 0-300a only has about 250 smoh. Using Exxon Ultra 20w50. Spin-on filter. Mogas, except on xcountries, no additives. I lean aggressively (ground ops too). The situation has always occurred in flight and has resolved before landing, and operation was normal til shutdown, and is normal on engine start and takeoff. So, I'm still not 100% certain it's not carb ice. But, there's been no visible moisture, low humidity, temp/dew point spread has been wide, and...I exercise carb heat on a regular basis. And, I think that might be the problem. I may have carboned up the system by being overly diligent with the preventive carb heat, application every 30 min for about one minute.

This has happened three times in the last 10 hours or so (increasing frequency). Maximum rpm is about 21-2200. Mags check good (good ground checks also). Each time, I've thought 'carb ice' and pulled on the heat and kept it on til resolution, limping for home. Thinking back, I believe it happened once last winter, but it was after passing through a light shower. Once on the trip out from Florida. And tree times more recently and of longer duration. Onset is immediate, 1-2 seconds. No gradual degradation of rpm. After 5-10 minutes of degraded engine operation, with mild to moderate vibration, it resolves. Resolution is not quite as immediate as onset, perhaps 3-5 seconds back to full rpm. Until the last occurance, it had always been immediately after a power reduction from cruise. The last occurance was the only one associated with advancing power. VERY inconveniently, it was on go-around from a low approach to a backcountry strip on a mountain, and I had had carb heat engaged, as always when flying a pattern to any strip. So, it happened AFTER carb heat had been engaged and then de-selected and power advanced. That's what clicked the light on in my head. A sticking valve never had even occurred to me. Since then, I've read some of our tic170a threads and pretty much decided it's a valve. After only 250 hours. Imagine that. I've grounded the 170 until this is resolved. If this happened late on takeoff roll, it would not be good.

Unless one of you aviators can conjur up what the cause of this power loss is, my thoughts are running toward pulling all the exhaust valves and cleaning them and the guides using the 'rope trick' with each, one at a time, versus pulling all the cylinders to do it. Do you have a feeling on that? I might be able to determine one or more sticky valves with a compression test, but I would be suspecting another cylinder to follow suit at another, possibly even more inconvenient time. Is my logic ok on that.

Anyone? Thanks for your help.

[marathonrunner]

I would ask that as well how long in years since overhaul. Also does it initially run rough on start up? If it is a valve you can just do the wiggle test one cylinder at a time using the rope trick. I have slos seen a loose valve guide do the same thing. Hate those annoying sometimes happens and you can not quite put your finger on it. Good luck and do let us know what you determine. It is always good to know for when it happens to someone else.

[Bruce Fenstermacher]

Drew asked me this question in a PM. You will see I encouraged him to ask the question of the forum. I'm posting my answer here so others can read it and maybe jog a thought or two from the experienced noggins and it will be here for others like Drew to review down the road.

• Hi Drew, I don't mind answering privately but in asking in private you have deprived yourself of one of the biggest benefits of a forum and that is reaching a lot of different experience. Sometimes folks don't want that benefit and would rather focus on one opinion.
  
  First my immediate reaction is I'd look a few other places than the valves and here is why. A sticky valve generally will not show itself in cruise flight. It will be stuck open at start and the engine will run rough and your not likely to fly it. It may only be sticky and show as a slightly rough engine when the engine is cold but clears as the engine warms and expands. Your just not likely to be flying along happy and the valve start to stick and the engine run rough.
  
  Now if you told me you ran the engine hard and it was hot and you pulled throttle back to idle and dove several thousand feet into a field shock cooling the engine and upon throttle advancement a valve was stuck I'd believe you because I've done that.
  
  Carb ice. Carb ice can be hard to detect and it is even harder to find evidence of it after the fact. You may not see an rpm drop before but you usually would. Upon carb heat application the engine usually coughs, runs rough and runs fine a few seconds later. On the other hand I once thought carb ice and pulled on the heat. It did not clear and I performed a precautionary landing at an airport under me. By the time I landing the engine ran fine and there was no evidence of it. We could never explain it and never found a problem. I suspect I really loaded it up with ice but it was all gone before I got on the ground. The carb wasn't even wet.
  
  I'd start with basics. What do your plugs look like. You could have a plug close to fowling with lead deposits but it clears itself. Are you running the engine to rich?
  
  Next time this happens switch to one mag then the other. If there is no change on one of the mags then it is either a problem with that mag or a plug the mag is connected to. I once thought I had carb ice but conditions were not right. On start up the next day I discovered a dead mag.
  
  I'd check the finger filter in the carb to make sure it is not blocked and I might even empty the carb bowl. On my second day of 170 ownership my new to me ship left me with partial power. Ran fine under 1800 rpm. Shock like the devil over it. The carb finger filter looked like someone jammed a cigarette filter into it.
  
  All of these things are not hard to do first before the valves. 250 hours is not to short a time to a stuck valve specially if the valve guide is on the tight side. But eliminate everything else first.
  
  Once you decide to go after the valves just do them one at a time and you might as well do them all. Then you will know. With the valve spring removed the valve should move in and out very freely and have a bit of wiggle. If it is wiggling the right amount you will think it to loose. If there is no wiggle the valve may be to tight.
  
  Good luck, I'll look forward to hearing if you find anything.

To expand on the mag issue. In my case the capacitor lead was broke clean off. Pretty easy to diagnos. If the capacitor lead was broke intermittently or the capacitor just bad the mag could come and go with symptoms being lower rpm and maybe perceived roughness.

[Bruce Fenstermacher]

Drew rereading your description I'm picking up on something I hadn't before.

Are you saying the max rpm the engine will obtain when this occurs is 21-2200 rpm? Sounds like your running on 5 cylinders if this is the case. And this is a pretty good clue because a single fowled plug or a fowled mag would not limit rpm that much. Restricted fuel or a stuck valve will.

You say it always happens right after power reduction and even your exception was after a power reduction to land but you may not have realized it until you applied power.
This is not likely a fuel restriction issue because power reduction reduces fuel requirements. So we may have deduced the problem to a cylinder and the single thing in a cylinder that can fowl and clear is the exhaust valve.

So if it is true you can not get over 21-2200 rpm and this happens after power reduction. I'm more convinced this could be a valve issue.

Do those other easier diagnostic things first like check your plugs and just the general overall condition of your engine.

IF this is a sticky valve issue it might be hard to find. One way would be to do a cold compression check rotating the engine slowly to TDC in hopes the valve would hang. Another way would be to fly above the airport and hopefully have the valve stick. Land immediately and on roll out jump out and feel the cylinders (or exhaust) for a cool cylinder.

Of course going on the theory this is a stuck valve my position is if one sticks all the others are likely and ready as well. Just clean all the exhaust valve stems. Along the way you might discover the one that was giving trouble and you will feel good about it. I'm thinking if you have a sticky valve but it works most of the time that it will be difficult to distinguish from all the others. You may get done cleaning them all and not know if you've done anything to improve the issue. In this case you will be a test pilot for a period of time.

Will you be doing this work yourself or hiring a mechanic?

[fangzz]

Bruce, thanks again so much for your quick thorough and thoughtful response. You brought up several questions, as did our other helpful responders. I'll try and hit them all:

Yes, Bruce, 21-2200 rpm, max. And rough. Some vibration.

Only 250 smoh. And that was performed in 09, so we've been flying 'er like crazy. A beautiful bird, lovely to look at and a pleasure to fly. Always reliable, and even now starts and takes off normally. This only happens after reaching operating temps, so far, not that it isn't waiting to happen earlier, which is one of the worst case scenarios I'm wanting to prevent. To take this a bit further, it's only shown up after reaching ops temps and having been been pushed with some full-throttle climbs for ridge or mountain passage, etc. It has been after getting nice and hot (never even into the caution zone, however) that it happens. And then, not always.

As I wrote, the mags checked good during a recent episode. Perfect ground checks. They are about 300 hours old. Checked at annual in April, by our peerless IA (a 140 and Stinson 10 driver).

Since it comes and goes, but rarely, it's very hard to predict. Since I never even thought 'valve' until after last flight (when it happened in a horrible place (low alt, low speed, trees ahead)), I have always used carb heat immediately, maybe making the situation worse. The time before, it took 10-15 minutes to clear, as I said in the initial description, and I had heat engaged that entire period. It self-cleared when I was almost back home. Made a normal landing and taxi and shutdown. She dieseled a bit on shutdown, telling me I'd carboned up a bit on that long haul with c/h, and at low rpm (descent into homeplate).

This brings up a related question that I've never read anything about. Carb heat engaged. The idea is to have nice hot air through the carb, right, so I've lately been leaving the mixture leaned. Logically, I can only see that helping the situation, but much about flying often defies logic. Anyway, back to the main issue...

Yes, Bruce, unless I can find some obvious other thing (like your trash in your finger filter in the carb) I'm expecting to have to clean up the exhaust valves one at a time for me to have faith to make the upcoming 2000nm jaunt back to Florida next month. I've never done the rope trick, but this sounds like a perfect opportunity to learn it, under scrupulous supervision, of course. And, Bruce, like you, I will never leave home without a 'rope trick tool kit' in my traveling tool kit. So, would you please tell us again what YOURS consists of?! I'll duplicate yours.

[GAHorn]

It doesn't behave as if it's carb ice. It it were carb ice,...then carb heat would clear it up very quickly...and there'd be no immediate further need for heat application. Also, carb ice is not as common in these particular installations as it is in 4-cyl Continentals due to the oil-sump/intake-passageway above the carb mount, which keeps the carb warm (and helps cool the oil.)

This reminds me of two events I've experienced: 1) Leaking oil seals in my mags (caused intermittent roughness, although not nearly the loss of RPM described here), and 2) broken valve guide, which allowed a valve to seat most of the time...but as the valve rotated to certain positions, the engine would behave rough for about 5 minutes and then clear up again. (That was on a constant-speed prop installation (C-206) and therefore showed little change in RPM but noticeably rough.)
The valve could be wiggled excessively with the valve guide noticeable movement during the "wiggle" test with the valve in the open position. (Seated, the valve head kept the guide secured.)

[blueldr]

It might also be an intermittently "leaking" spark plug lead.

[Bruce Fenstermacher]

It might also be an intermittently "leaking" spark plug lead.

Yes but with one cylinder only firing on one plug the RPM wouldn't be limited as much as he is experiencing.

[fangzz]

this is great point/counter-point.

[FredMa]

If you are going to remove the valves and check them, you may want to have the valve springs checked as well if nothing is discovered with the valves. A weak spring will allow a valve to float and remain open. they are checked by compressing them to a certain dimension given in the overhaul manual and measuring the force applied to acheive that dimension. Any that are out of limits should of course be replaced. The good news is that they are very inexpensive as far as airplane parts are concerned. most people just replace them rather than trying to figure out which one is bad.

[canav8]

Has anyone suggested to this guy: See if the carb heat control is opening all the way open when deselected? It sounds like the carb air control cable might need an adjustment or the airbox has to much slop in the bushing area where the control rod passes through the outside of the airbox. The result is a door that vibrates and disrupts airflow through the airbox. I have seen that as a problem on airboxes. Doug

[Bruce Fenstermacher]

Could be Doug. But I wouldn't think a malfunctioning carb heat box would limit the RPM to 21-2200 RPM. This is an important clue.

The point I tried to make is to make sure the simple normal stuff is working as it should before digging for the valves.

[FredMa]

Bruce, I recall a guy in alaska some time ago with the same problem only worse. Due to the cold temps he was thinking it was caused by severe icing. It ended up be a cracked or broken butterfly valve blocking the intake when carb heat was selected.

[fangzz]

Thanks Fred, but onset is always BEFORE c/h application. C/h works great. Rpm drops to 21-2200 for no apparent reason (think I've ruled out carb ice, especially after George's nice input relative to the oil pan and carb installation). Then resolves itself after 5-10 minutes of very rough (like 5 cylinders) operation. I now don't think I was helping my situation with c/h application.

Next?

[ghostflyer]

Had a similar problem problem , and found a bad plug lead. I was out in the never never and a little worried. cowls were removed and the engine was run at night ?What a light show a plug lead had a small cut in it from the baffles. plug lead was about 25 hrs old. Moved plug lead away from baffle and wrapped band aids (from the medical kit on board) around the lead and flew home .That was 7hrs away. the engine was miss firing at 2000rpm .