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Carb ice on takeoff, or fouled plugs or?

A

AKClimber

Registered User
Juneau, AK
The last two takeoffs from icefields, I've experienced loss of power and rough running engine about 5 seconds after applying full power.
The first time (at 5,000', 40 degrees F), I thought I was too lean, so I stopped, pushed full rich, full power and took off without issues.
The second time (at 6,000', 32 degrees F), I leaned for altitude prior to landing, then dragged the snow a couple of times and no issues (even when applying full power on takeoff).
After shutting down and coming back after a couple of hours of human powered skiing, I start up and takeoff, engine starts running pretty rough, loss of rpm, I apply carb heat (no change), I push in full rich (no change).
I stop and do a run up (no change in rpm on right or left magneto), and try to clear up the possibly fouled plugs. The second takeoff attempt was normal.
Do you guys think it was carb ice, or fouled plugs, or perhaps something else? Note that the takeoff from the sea level airport was normal.
This has never happened to me before even in similar situations, and I haven't changed my operating procedure. I'm taking it as a sign to change to wheels.
Thanks in advance for any insight.

BTW. It's a Lycoming O-360
 
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The engine may have just loaded up and had a bad stumble when cold. Small continentals do this but I just flew an O-360 cub yesterday and it did it too. Sitting there idling on a ~55 degree day doing regular checks after startup and when I put the whip to it the thing just stumbled and coughed. Pulled the throttle back to idle and then slowly advanced it and all was well. My brother (who flies it mostly) said it does that if you idle under 1000rpm for too long. Just loads up and needs to be cleared out.
 
Long shot, but you could have an exhaust valve hanging intermittently. Has the Lycoming valve guide SB been done ever? I think it’s recommended every 400 hrs.

Rich
 
Richgj3 said:
Long shot, but you could have an exhaust valve hanging intermittently. Has the Lycoming valve guide SB been done ever? I think it’s recommended every 400 hrs.

Rich
Click to expand...

Hmm, I'll have to check the logbooks - interesting thought, and consistent with the engine behavior.
 
It could be several things, but, I would look close at the exhaust valves. They can be sticking or miss seating. Pull the valve covers if they have a lot of gray blow by buildup on the exhaust valve suspect a worn guide allowing the valve to miss seat. Borescope and see if you have any burnt valves. Also do a mag check at 2400 RPM. DENNY
 
I had what I thought was intermittent carb ice for months, and finally bent a pushrod and broke a cam follower due to a stuck exhaust valve. Lyc O-360. I had NOT done the recommended 500 hour valve guide reaming...hard way to learn a lesson!
Lou
 
Thanks for all the advice!
I checked the logbooks and the Lycoming SB 388 has never been done. The engine has about 1,200 hours SMOH.
Looks like I'll be doing that first. I am afraid to even start it now!
If the SB 388 is mandatory, why is it not an AD? In other words, why would the mechanics miss this mandatory SB, three times now?
 
My guess is some guys never look at SB’s. Others think it’s too much trouble and would rather wait until something shows up as a symptom. The reason I’m aware of it is that a good friend A&P owns a Comanche and does everything by the book. When I had my Comanche, no mechanic ever mentioned it to me.

Another example is a Cessna SB to check for wear on the flap bracket attached to the flap. I just learned of this old SB from the 170 forum. Everybody knows about the flap track and roller check, but not everybody knows about this one. I guess it’s easier to research AD’s than SB’s

Rich
 
AKClimber said:
Thanks for all the advice!
I checked the logbooks and the Lycoming SB 388 has never been done. The engine has about 1,200 hours SMOH.
Looks like I'll be doing that first. I am afraid to even start it now!
If the SB 388 is mandatory, why is it not an AD? In other words, why would the mechanics miss this mandatory SB, three times now?[/QU
Soooooo the reason is is not done is it is a SB AND NOT MANDATORY!! Depending on the aircraft and how it is flown/CHT/ECT it is up to the IA to determine if it should be done. I think valve guide wear is caused by more than one factor and temp is one of them so if you have no clue as to what the CHT'S of all the cylinders are then you don't know what you don't know. Who was taking care of the plane? How is it flown? Is the baffling correct? Does it have 4 cylinder EGT/CHT? Does the pilot have the money to pay for the test? Lots of factors involved in doing or not doing the SB!! I know of no IA's that do the SB at 400 hours unless the plane has issues and only a few that even look for the issue below 1,000 hours. I think the 400 hours is for helicopters by the way.
DENNY
Click to expand...
 
What fuel tank arrangement do you have?, one tank or two? in any case, get a 5 gallon bucket and see how much fuel drains from the gascolator, should be more than one gallon in 6 minutes ( = 10 gal/ per hour) try each tank or switch valve from right to left etc. Just make sure you have enough fuel getting to engine during takeoff. This is just one easy check point, there are other ways to check. For gravity fed fuel systems, if you get an obstruction in the line, it can restrict fuel flow, down to an unacceptable amount for full power operation. I like to see about a gallon in 3 minutes fuel flow.
 
Cub-tx said:
What fuel tank arrangement do you have?, one tank or two? in any case, get a 5 gallon bucket and see how much fuel drains from the gascolator, should be more than one gallon in 6 minutes ( = 10 gal/ per hour) try each tank or switch valve from right to left etc. Just make sure you have enough fuel getting to engine during takeoff. This is just one easy check point, there are other ways to check. For gravity fed fuel systems, if you get an obstruction in the line, it can restrict fuel flow, down to an unacceptable amount for full power operation. I like to see about a gallon in 3 minutes fuel flow.
Click to expand...
Sec. 23.955 — Fuel flow.

(b) Gravity systems. The fuel flow rate for gravity systems (main and reserve supply) must be 150 percent of the takeoff fuel consumption of the engine.

PART 3—AIRPLANEAIRWORTHINESS—NORMAL, UTILITY, ACROBATIC, AND RESTRICTED PURPOSE CATEGORIES
3.434 Fuel flow rate for gravity feed systems. The fuel flow rate for gravity feed systems (main and reserve supply) shall be 1.2pounds per hour for each take-off horsepower or 150 percent of the actual take-off fuel consumption of the engine, whichever is greater.
 
All previous threads true, and allot depends on fuel tanks, fuel valves, fuel screens, any of which can have an obstruction, be partially opened or closed. The plumbing can vary along with number of tanks and fuel line routings. Of course you want to exceed the full power fuel consumption fuel flow by gravity alone. That would be checked at the fuel line to carburetor inlet location. However, my first quick simple method to determine if I have enough fuel at the gascolator at the firewall, is to use the quick drain opened into a bucket and time it. I open, close and measure each tank valve individually, no, this is not the official exact method, but it will give you insight to investigate further if needed. Check your individual engine make/model and size for full power full consumption ratings to compare against. Up stream fuel problems, can vary, but common items include, a old glob of EZ-Turn, Fuel Lube, that has been over applied and balled up in the line. Or any other thread sealer or tank sealers. Or the fuel valve doesn't open fully. The finger strainers inserts in some fuel tanks will also hide obstructions.
 
Several years ago I bought a Super Cub that had been recovered in Alabama. I experienced very similar symptoms to what the OP posted. The engine would run fine on the ground no matter what attitude we placed it in, nose up or nose down on a bank, but would occasionally falter at full power on takeoff. Long story short, it turned out that the fuel caps had probably been left off during the recover and the wings left outside under an Alabama pine tree. A few pine needles found their way into the fuel tanks and slowly degraded over time. The little pieces of pine needles eventually found their way to the fuel selector valve and created what looked like a miniature beaver dam. The engine requires slightly more fuel when taking off than if you do a full power runup on the ground. Checking the fuel flow as mentioned above did not show the required 150 percent. Started tearing everything apart from the engine backward and eventually found the log jam. So I'd start by checking to see how much fuel flow you get out of the gascolator.
 

If I understand it right ? , the 400 hours for SB388C is if you have the older guides

652 Oliver Street Williamsport, PA. 17701 U.S.A.SERVICE INSTRUCTIONTel. 570-323-6181 Fax. 570-327-7101 www.lycoming.textron.com DATE:July 2, 2003 Service Instruction No. 1485A (Supersedes Service Instruction No. 1485) SUBJECT: Exhaust Valve and Guide Identification Procedure MODELS AFFECTED: All Lycoming reciprocating aircraft engines unless installed in a rotary-wing aircraft. TIME OF COMPLIANCE: Half way to recommended TBO or at 1000 hours of operation, whichever occurs first or earlier at owner’s discretion or anytime valve sticking is suspected. As a product improvement, Lycoming has introduced a new exhaust valve guide material with an increased chrome content for improved wear characteristics. The incorporation of this guide material has significantly reduced the amount of valve guide wear (bell-mouthing) seen by some operators of Lycoming engines. Improved exhaust valve guides were initially incorporated into some cylinder assemblies beginning inApril 1996. Since March 1, 1998, all engines, cylinder kits and spare exhaust valve guides shipped from Lycoming contain the improved material. Cylinder assemblies which incorporated the improved “Hi-Chrome” exhaust valve guides when the engine was shipped from Lycoming are identifiable by the letter “C” stamped inside a circle on the boss for thedrain back fitting. The individual guides made with the improved material can be identified by a 5° chamfer at the top of the guide (See Figure 1). Once guides made from the improved material are installed in all cylinders on the engine, it is no longer necessary to complete the mandatory 400 hour inspections specified in the latest revision of Service Bulletin No. 388. It is recommended that the inspection procedure from the latest revision of Service Bulletin No. 388 be completed at 1000 hours of operation or half way to the recommended TBO, whichever occurs first. (For recommended TBO, refer to the latest revision of Service Instruction No. 1009.) NOTE Prior to discontinuing the mandatory 400 hour inspections specified in the latest revision of Service Bulletin No. 388, you must confirm that all cylinders on the subject engine have the new exhaust guide
 
The fuel flow issue is interesting - I will have to check into that.
Pulled the spark plugs today - all look good. I'll check the compression next, then fuel flow.

I'm pretty sure I have the older valve guides, except for one cylinder that had to have new guides installed about 6 years ago (I would expect the engine shop put the new ones in).
 
When it is stumbling give it a shot of primer and see if it catches. Easy way to see if it’s a fuel issue.


Sent from my iPhone using SuperCub.Org
 
Speaking of fuel flow, an interesting fact about the Cessna 170. There were three models 170/170A/170B, 1948 to 1956. The only one that was a rag wing was the 1948 C-170. It was also the only one with a engine driven fuel pump. It is mounted on the front of the engine case, old AC-Delco pump from automobiles. You can see it from the front inside the engine cowl opening. There was a specific reason that it was on there and was the only model Cessna 170 series that had it that one year in 1948. It was the first year that Cessna developed the 170, which is a outgrowth of the Cessna 140. Many borrowed parts. Fuel tanks being one of them. The 170 had the same two 12.5 gallon tanks, R & L, plus on additional 12.5 gallon tank giving it 37.5 gal. total fuel. The same diameter fuel lines were also borrowed from the 140. But the 170 had a 6 cylinder C-145/O-300 engine (145hp) with much higher fuel consumption than the 4 cylinder the C-140 had. In order to satisfy FAA certification, the engine driven fuel pump was added. The very next year starting in 1949, the Cessna 170A had increased size fuel lines, and eliminated the engine driven fuel pump. Also a metal wing.
 
Interesting - do you know the fuel line sizes? I did some research for an Exp I'm working on, and it turns out that CAR 3 is actually a little more restrictive than Part 23.

Re the original post - seems if the problem is occurring at altitude but not at sea level then blocked fuel flow would be unlikely. I wonder if water in the air filter from prior carb ice could be an issue. Reason I ask is that I have vivid memory of ice problems over the Juneau Ice Field on a CAVU day. Engine darn near quit when I pulled carb heat - fiddling with mixture and carb heat together resolved the issue, but I was looking down Herbert Glacier at Eagle Beach with considerable pucker factor!
 
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Cub-tx said:
....In order to satisfy FAA certification, the engine driven fuel pump was added. The very next year starting in 1949, the Cessna 170A had increased size fuel lines, and eliminated the engine driven fuel pump. Also a metal wing.
Click to expand...

I owned a 48 170 for a long time.
Another, perhaps more important factor in requiring a fuel pump was the fact that the fuel lines were routed down the forward door post.
If the airplane was in a nose high attitude, and unported a fuel tank outlet (as in being very low on fuel),
without a pump the only way to get fuel flowing again past that high point of the fuel system (at the top of the fwd door post)
was to push the nose down-- not always possible, and quite often very much counter-intuitive.
I knew several people who no longer had the mechanical fuel pump on their 48 mode, and (luckily) never had any problems.
 
Cylinder #3 had no compression - could hear and feel the air escaping the exhaust pipe.
The other cylinders were 74,77,77.
Guess I'll be sending a cylinder off to get new valve guides put in.
Thanks for all the thoughts and suggestion!
 
Now find out why #3 is a leaker? Valve cover removal reveals a stuck valve or ? Good you weren't needing the power. We pull the prop through before startup for a reason....checking compression and mag impulse coupling sounds. Kroil and tapping with a soft hammer on the stuck valve might let it last a bit longer but many recommend a thorough ream job.

Gary
 
SOOOOOOO now you have a 1200 hour motor with a valve problem on #3. What are the other cylinders doing??????? Is the problem a stuck valve or valve guide wear issue?? If it is Valve guide wear you may have others waiting to be an issue. Do the SB on the rest of the cylinders. DENNY
 
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