Carb Ice - Continued

[sully]

I also considered the primer. I usually just put a plug in the intake spider to eliminate the primer.

Just another thought.

Sully

[otrcman]

Maybe I'm so stuck on lean running that I can't see anything else, but all that you describe from today's flight says "too lean" to me.

Do you know if unlocking your primer lets a little more fuel leak into the induction system ? Leaving the primer unlocked has always been an easy fix for me on a cold, balky C-85 while taxiing. Once warmed up for a few minutes, I can lock the primer prior to takeoff and the engine will run fine. If so, you might try flying with the primer unlocked to see if the problem goes away.

The other test you could do would be to stroke the primer slowly while the engine is running rough. At least that would tell you if the engine is running lean versus all the other possibilities.

Dick

[Scott]

Nice installation.
Got to be something simple.

You should have a "bounce" on your mixture control when it's at full rich ie the arm on the carb should be up against the casting (not just the gasket, sometimes difficult to see so use a feeler gauge or strip of paper) before the mixture control reaches full travel. Check this with the engine in the problem power range too.

with the engine at the trouble rpm slowly lean the mixture, you should see a small increase in rpm then a decrease. If just a decrease engine is too lean, excessive increase- engine too rich. You'll likely see the former. The reason the engine clears up with carb heat is just that the heat results in a richer mixture.

once your sure the mixture control is rigged correctly, the next bet would be an intake air leak though I would think this would cause problems at lower rpm too. There are methods to pressure test the intake system.

worth checking your idle mixture too. It impacts the first 1/3 or so of throttle maybe more given you're near sea level. With the engine at idle slowly lean the mixture, you're looking for a slight increase in rpm before the decrease. The official number is 50 rpm rise (I prefer less, especially near sea level but any rise is good for our purposes here).

it could be contamination of some kind in the carb, but the fact that it clears at full throttle I would think suggests the carb is ok.

still thinking...

[PA1195]

All good suggestions but who's doing the maintenance? Surely that person owns the problem?

Edit: partial throttle issues fixed by WFO tend to exclude the main jet orifice as Scott notes. They can include the atomizer nozzle and bowl vent system. But that's if the carb is the source and not some nefarious issue. There's likely a transition in air/fuel that's apparently a cause. Put maintenance in the plane and go fly. If they won't or can't diagnose then find someone who will fix the problem.

Gary

[Robert Lees]

I'm glad that Kerbs is approaching this issue methodically (and safely): that's the way to do it!

Keep a record of your flights, adjustments etc

[Kerbs]

Continuing the saga... I finally made it out to the airport late int he day. I spoke with my IA at length. He things it may be something in the carb related to the enrichment circuit/jet. He could not remember when the carb switched from the idle circuit to the enrichment circuit on a c85, but it does make sense. I flew for about 20 mins catching the last light. Prior to, I drained the carb to see if I could clear anything that might be plugging the needle. I thought I was off to a good start and after climb out throttled back to 2,100rpm. It let me fly there for a bit and then started giving me the same issues. I tried cracking the primer as well and pulling it back slightly to open it up but to no avail. I will try to fly again (time and work allowing) ad report back what I find. If I don't have some more success, i think we are going to pull the carb and check to make sure there are no blockages, etc. I may try to remove the primer from the manifold and block it with a plug just to be sure that I do not have an induction leak there.

[PA1195]

We're hoping for your success Stu. You must be frustrated but please be careful in case the engine decides to shutdown. With the primer unlocked it should suck enough at the spider to flow excessive fuel providing the intake is leak tight and manifold pressure correct in my experience. I can make the engine run a bit rough if unlocked. You could also try a few prime strokes when cruising to see if the excess fuel causes a running change in rpm and running. It typically slows from a shot excess fuel then recovers.

Another reason for a MP gauge is monitoring intake and cylinder seal. It should read local air pressure (altimeter set to "0" feet) when stopped. After startup they read low and a tight engine 6-700 rpm will be near 10" after ~30" before start (depends on elevation). In cruise it depends on prop length and pitch vs rpm, but mine will not exceed 24" at 2400. This feature has nothing to do with potential carb problems (except icing) but can detect intake leaks and worn engine parts.

Same for a multi-point EGT gauge. If they read ok at lower rpms but increase temps markedly in cruise as the throttle is advanced where you're experiencing roughness you'd know there's an air/fuel ratio problem. Near full throttle they should drop in temp due to added fuel by design of the carb or fuel injection.

Edit: You mentioned if briefly before but does the engine run well wide open once you passed the rough rpm range? If running improves at full throttle total fuel flow to the carb or internally via the float components isn't the major issue. The part throttle fuel flow may still be a concern so clean the sources. Manifold vacuum decreases at full throttle and so would the overall effect of any intake seal leaks.

Gary

[Kerbs]

I am not sure if I have made progress but thought I would provide an update. I have spent a fair amount of time with my IA the last two days checking and double checking things. We have not seen any indication of leaks in the induction system. He thought it might be a carb issue so we double checked numbers and everything else with the carb. At my IA's suggestion I called Marvel today and talked to one of their techs. He was pretty helpful and spent a fair amount of time with me. Here is where it gets interesting.

The tech at Marvel said that they have seen this issue on less than a dozen C75 & C85's over the last ten years or so. Basically where it runs really lean in the mid power range. He asked if I had superior cylinders which I do. Apparently, after the AD in '95 Marvel switched to a one piece venturi and a different nozzle in their carbs. These generally work really well, however, there have been a handful of C75 & C85 motors with superior cylinders that do not like the newer one piece venturi. The cure ultimately for the others that have experienced this is to send the carb to Marvel and have them swap out internals to the original two piece venturi and corresponding older style nozzle.

Before I went and pulled the carb, he suggested that I try to move the accelerator pump arm from the middle notch to the furthest out, and then make sure I have about a 50-55 rpm rise at intake cutoff. I tried these changes today and flew for about 30 mins with no improvement.

I also stopped by our aero museum on the field (they are fantastic to work with) and ran it by them. They were equally as confused and pretty much ran through the items we already discussed. They were a little surprised by the solution from Marvel but said they have come across weirder stuff. They also mentioned that they have sent a number of carbs in with issues that come back checked out and working fine after.

I am going to call Marvel back in the morning and report my progress. I am guessing the next step will be to send my carb into Marvel.

[PA1195]

Hmmmmm...thanks for the info and update Stu. Yea best to let them have a look at the carb....there's probably some old employee in the basement that fixes stuff like this when no one else can. Perhaps the Superior cylinders are really that and need special carb parts to work.

Did Marvel overhaul your carb or was it someone else? Hope it gets fixed so you can enjoy the airplane.

Edit: Figure 4 here shows the pump options and why: http://www.insightavionics.com/pdf%2...b%20Manual.pdf We leave them max except in the heat of summer on some engines (Lycomings on floats) then go one hole less travel if they do a rich stumble.

Gary

[Kerbs]

Its been over a month but I finally have an update. After going through nearly everything with my IA and checking many of the helpful suggestions offered up on here, I pulled my carb and sent it in to Marvel. They had it about 3 weeks and then sent it back. The remarks on the 8130-3 say " This assembly was repaired and tested IAW Marvel-Schebler Aircraft Carburetors, LLC specifications. Complete details held on file in reference for job order above." No other information on what was repaired or feedback.

I got the carb back on the plane this weekend and had just enough light to do a long run up and then test flight around the pattern. I need to get some more time on it weather providing this week, but it appears the issue of the engine running lean is not longer an issue. The plane ran fine in the same mid power range of 1900-2300 that had been problematic and was smooth. I will follow later this week when I get a bit more time in it. We have a system moving in that is suppose to drop 3-6" of rain in the Olympics so it may be a few days.

I am planning to Marvel and see if they will send me the notes on file. Not knocking Marvel, they were really good to deal with and fixed it with no expense or return shipping charge. I would just like to know what the issue was so I can share the info with the tribe in case someone else runs into it in the future.

More to come.

[PA1195]

Thanks Stu for the update. The "FIX" would be worth knowing. But maybe they have a lean-run component setting problem and are dealing with it one at a time?

Gary

[Robert Lees]

Thanks for the update. You will probably not get a written response. Sometimes it's worth phoning them up and saying "it all works great now, can you tell me what the problem was?"

And they may well tell you verbally, but most overhaul companies keep their written records to themselves...I guess for liability reasons.

Rob

[PA1195]

Sometimes accessories can have their data plate revised during service to track any modifications. Did you notice any change to stamped letters or numbers after the repair or revision? Might be hard to tell unless there's a copy from earlier. They may have changed the float setting or components, main discharge nozzle design, or venturi setup to alter the fuel flow.

Gary

[lawheelock]

Kerbs,
This is long and may be boring, but here goes.
This sounds like my saga with my O-360 in my Mooney over a 5 year period! I would be flying along and get a "tick" feeling and then all was well. Many times, I could fly an hour or two and no "ticks". Finally in 2017 when over a 1000 miles from home, it got so bad and rough that I landed at a nearby airport. I was suspecting an intermittent sticking of an intake valve, or Lycoming Morning Sickness, though this was not on cold mornings or upon startup. Anyway, I pulled all valve covers and could see nothing. I pulled the mags and found one that had been arcing in the distributor plate area so much that it had eroded tha aluminum. The other mag had some oil in it from a leaking seal, though a mag check never revealed anything. Anyway, I bought Brand New Bendix mags, not rebuilts. Installed and did a test flight and all was well. Started home. Flew for two hours and thought I felt a "tick" once or twice. Landed for fuel and on takeoff it started running rough, but seemed to climb OK. I was going to get it home! So, kept climbing up to 10,500. During that climb, it would start to run a little rough and I pulled carb heat and it smoothed out. I then flew from airport to airport most all the time within gliding distance of the next airport while at 10,500. After a half hour or so, it never "ticked" again. Made it home after another fuel stop and climb back to 10,500. That was all in 2017. Never flew it much in 2018, but did sometimes feel the "tick". I gained confidence, though I never thought it was carb ice as I have had lots of previous carb ice experience in both my BC12D back in the '60s and my Franklin powerd Stinson in the 70's and '80s. Then this past summer, I started to Oshkosh in the Mooney. After an hour, it got to running very rough and landed at Kentland, IN where I met Chuck Classon a fellow 80+ old IA with an experience backgound very parallel with my own, an engineering degree, worked several years as an engineer and then got our A&Ps and IAs later in life, but involved in working on airplanes and engines for well over 60 years.
We brainstormed for hours and both were stumped by the symptoms. So we started trying to locate a problem. First disassembled all the valve springs and pushed the valves far enough into the cylinders to see if there was any buildups on the stems using a video borescope that I had. We found nothing. Searched for possible intake leaks or restrictions finally deciding to remove the carb and disassemble it which is no easy venture in a Mooney M20C. In the meantime I purchased a new float and float valve kit for the MS carb. Took it apart and replaced the float and float valve and when starting to re-assemble I noticed the viton tipping on the float needle valve was worn away a whole lot when compared with the new one. This was the first and only definite part problem that we had found in all this work. It did not make sense that this could produce all the symptoms and seem to be cured with carb heat. At no time was there ever any evidence that the carb float chamber overfilled.
After putting it all back together, have now flown it more than 10 hours with no "ticks". It still does not make much sense, but somehow this seems to have cured the problem. It will take quite a few more hours before I regain full conficenc in this 170 hour engine with brand new Lycoming cylinders at the time I personally overhauled it.

Now going back to my experience in the 70s and 80s with apparent carb ice in the Franklin powered Stinson with a MS carb. I never had carb ice as long as I could still get 80/87 except on days that one would expect carb ice or when flyng in the clouds IFR in winter conditions slightly above freezing. As 80/87 became unavailable and was forced to use 100LL when on trips, I started experiencing a lot of rough engine which carb heat fixed. When flying at home where I always used ethanol free auto fuel with the Peterson STC, I never experience carb ice, but if forced to use 100LL the ice symptoms returned. Again, can't explain it from an engineering standpoint as 100LL is supposed to have a more controlled vaporization rate whereas winter blend autofuel usually vaporizes easier and would be more suspect for proviging more carb throttle plate cooling than 100LL, but I observed tis over a period of several years and 100s of hours of operation..

Due to my advancing age and lack of ethanol free autofuel, in 2012 I bit the bullet and installed the O-360 that was originally in my Mooney in my Stinson. I have over 150 hours on it now since I did the major on it and no problems but I put all new parts in the carb at overhaul which I failed to do on the engine I overhauled for the Mooney several years ago.

You talked about sending your carb to Marvel and now it seems to work OK but you don't know what may have fixed it. They probably don't know either. They probably just replaced all the parts in it that could cause a problem including new float and float valve. If I had not been looking carefully at the float valve from my Mooney I would not know what may or may not have fixed it., if it is fixed. I do feel that this time it is fixed, but sine I winter in deep south Texas and summer in Indiana where I keep the Mooney, I won't get to fly it again until June. I continue to fly the Stinson down here in deep south Texas, but have had no carb ice experiences with it here.

My '46 Taylorcraft BC-12-D project in Indiana is the result of the previous owner experienceing carb ice in the A-65 and making a forced landing in a muddy cornfield. That engine had a legal MS carb on it. I am still working on repairing the wing that was damaged. I may disassemble that carb before running the engne again and replacing some parts, but I am convinced that it was real carb ice on that incident.

Hope this has not put too many of you to sleep with boredom, but sometimes things happen that we can never really totally explain what fixed the problem.

Larry Wheelock, A&P/IA, BC12D project, '46 108 Stinson with O-360 and '65 Mooney M20C with O-360

[PA1195]

This is one reason to install multi cylinder EGT/CHT instruments and keep track of what's normal and routine under similar conditions. Data spread on a given day or over time stuff. There may not be a pointing finger readily apparent but the info can start us down one or more paths to knowledge. And if you fly a Taylorcraft hide it in the glove box if it bothers you.

Gary