Hello all,
Today during my first landing/touch n go I moved the throttle very fast from idle to full. This resulted in a hesitation/stumble followed by the engine coming to a stop. I was able to coast/taxi clear on to the taxi way. I restarted the engine with no problems and moved out to an unused part of the taxiway.
I was able to duplicate the problem each time I rapidly moved the throttle from idle to full open; Stumble----stop. If I moved the throttle slowly I could develope power. It showed just slightly above the "No takeoff" area on the gas gages so I refuled thinking this could be an issue. Topped off the tanks and taxied out. Same problem.
As I could attain full power by smoothly pushing the throttle open I took off and climbed to 4K MSL. The engine ran great all the way up. I stayed over the field and rapidly retarded the throttle from full open to idle. After slowing I moved it from idle to full open...ran fine to problems at all. I repeated the several times including setting up for a landing/go around while at altitude. Again the engine ran normal.
I set up for an engine failure pattern and landed no problem with power on. On the ground I was able to replicate the problem again!
Temp today was 50F with light winds, clear skies ect. I flew the aircraft two weeks ago for an hour after it came out of MX to replace a dead generator with an Alternator conversion. No problems noted then. I was out today to test the Alt with everything up an running and leaks; it all works great.
So, what's happening here? Is the engine "loading up" when moving the throttle too fast? I can't remember my techniques from before on throttle movement and I'm starting to doubt myself on what I'm "supposed to do" regarding throttle movement.
Thanks again for the help....Cheers,
Lee
Engine Stumble/hesitation/failure during touch n go
Moderators: GAHorn, Karl Towle, Bruce Fenstermacher
Engine Stumble/hesitation/failure during touch n go
Lee Collins
1951 C170A
N1733D
1951 C170A
N1733D
Hello, Lee!
You are describing what may be a malfunction of the "accelerator pump" that is incorporated into your carburetor.
From the TCM overhaul manual, X30013,pg 47 :
"The accelerating pump lever has 3 holes into which the upper end of the accelerating pump link may be fastened. The outer hole...gives the longest stroke or maximum accelerating fuel. The lower hole No. 1 ...gives the shortest stroke, or minimum accelerating fuel, and the upper hole No. 2, ...provides a medium supply of accelerating fuel. The normal position is the No. 2, ...or medium... however...for hot...high conditions the No. 1 (shortest) stroke may be necessary to prevent heaviness or slowness on acceleration. No. 3 hole (longest stroke) may be required in extremely cold weather."
The pump-linkage resembles a heavy-gauge wire "bail" that is connected to the throttle lever and slips or "hooks" into the upper-end of the plunger via a rotating plate with the three holes in it. By removing a small cotter, and repositioning the bail, the different holes may be selected. This is on the left side of your carb.
Lee, ...it's also very possible your accelerating pump has failed or is clogged completely. In 99% of all airplanes the medium hole/stroke is all that is required. The accelerating pump may be repaired without too much trouble. It is identified/repaired thru a small plate-with-a-dome, held to the lower/left side of the carburetor by two small screws. If the plunger seal/cup has failed, or if the ports have become clogged, it may be possible to clear and/or replace the plunger without too much trouble.
Otherwise, you may find it best to remove the carb completely to do the job on the bench.
Do not make any other adjustments/changes to your carb if it's otherwise been functioning properly.
(Be careful not to use hard/sharp instruments/tools in order to avoid any damage to interior ports or scratches in the pump bore. Ordinary carb cleaner aerosol spray may be useful in cleaning out the ports. Compressed air is also sometimes helpful.
The pump itself resembles a long shaft with an upside down cup at the lower end. It merely pushes extra fuel into the carb throat during acceleration. The cup may be made of either rubber or leather, and can be replaced as an individual repair part. I've seen several pumps that have experienced cup failure that subsequently display the symptoms you describe. (At the risk of starting an unrelated argument,...can you spell "autogas"?)
You are describing what may be a malfunction of the "accelerator pump" that is incorporated into your carburetor.
From the TCM overhaul manual, X30013,pg 47 :
"The accelerating pump lever has 3 holes into which the upper end of the accelerating pump link may be fastened. The outer hole...gives the longest stroke or maximum accelerating fuel. The lower hole No. 1 ...gives the shortest stroke, or minimum accelerating fuel, and the upper hole No. 2, ...provides a medium supply of accelerating fuel. The normal position is the No. 2, ...or medium... however...for hot...high conditions the No. 1 (shortest) stroke may be necessary to prevent heaviness or slowness on acceleration. No. 3 hole (longest stroke) may be required in extremely cold weather."
The pump-linkage resembles a heavy-gauge wire "bail" that is connected to the throttle lever and slips or "hooks" into the upper-end of the plunger via a rotating plate with the three holes in it. By removing a small cotter, and repositioning the bail, the different holes may be selected. This is on the left side of your carb.
Lee, ...it's also very possible your accelerating pump has failed or is clogged completely. In 99% of all airplanes the medium hole/stroke is all that is required. The accelerating pump may be repaired without too much trouble. It is identified/repaired thru a small plate-with-a-dome, held to the lower/left side of the carburetor by two small screws. If the plunger seal/cup has failed, or if the ports have become clogged, it may be possible to clear and/or replace the plunger without too much trouble.
Otherwise, you may find it best to remove the carb completely to do the job on the bench.
Do not make any other adjustments/changes to your carb if it's otherwise been functioning properly.
(Be careful not to use hard/sharp instruments/tools in order to avoid any damage to interior ports or scratches in the pump bore. Ordinary carb cleaner aerosol spray may be useful in cleaning out the ports. Compressed air is also sometimes helpful.
The pump itself resembles a long shaft with an upside down cup at the lower end. It merely pushes extra fuel into the carb throat during acceleration. The cup may be made of either rubber or leather, and can be replaced as an individual repair part. I've seen several pumps that have experienced cup failure that subsequently display the symptoms you describe. (At the risk of starting an unrelated argument,...can you spell "autogas"?)
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons.
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons.
George,
Thanks for the post, very interesting as would this seems to fit the problem. I'll take a look on Sunday and hope something obvious sticks out.
The airplane came from south FL where it lived for 25+ years. Is this something that a cold wx snapp or ops could have brought about?
thanks a million,
Lee
Thanks for the post, very interesting as would this seems to fit the problem. I'll take a look on Sunday and hope something obvious sticks out.
The airplane came from south FL where it lived for 25+ years. Is this something that a cold wx snapp or ops could have brought about?
thanks a million,
Lee
Lee Collins
1951 C170A
N1733D
1951 C170A
N1733D
The change to a climate of drastically different conditions might explain why a previous locale did not indicate a problem, but I suspect not. As I said, the medium position works very well in most airplanes in the world. (But clearly, there's a reason the other positions are provided for.)
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons.
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons.
Smooth throttle operation is a hallmark of a pilot who plans ahead and cares for his equipment. But the very purpose of the accelerator pump is to accomodate a "slam acceleration" and if properly adjusted should work. There is no "choke" on these carbs tho', and the engine must also be properly warmed before flight. A "cold" engine will not respond well even with a properly adjusted accelerator pump.
Remember that this is an updraft carburetor. A flood of fuel sprayed into an idling engine that is cold, may not properly vaporize and may run back down (since the incoming air may not be sufficient to bring liquified/non-vaporized fuel against gravity) and a "back-fire" thru the carb, damage to the air filter, carb-heat butterfly, and fire damage to the filter, may be possible. This can also happen to an engine which has been flooded prior to start by excessive use of the accelerator pump spraying raw fuel into a non-running carb. The fuel will drain downward, and if a backfire occurs, a fire may damage the filter, etc. (KEEP CRANKING to "suck" the fire up into the engine. And....you DO have a fire extinguisher handy at all times and inflight don't you?)
Any engine that has suffered backfire thru the carb should be inspected for such damage.
Remember that this is an updraft carburetor. A flood of fuel sprayed into an idling engine that is cold, may not properly vaporize and may run back down (since the incoming air may not be sufficient to bring liquified/non-vaporized fuel against gravity) and a "back-fire" thru the carb, damage to the air filter, carb-heat butterfly, and fire damage to the filter, may be possible. This can also happen to an engine which has been flooded prior to start by excessive use of the accelerator pump spraying raw fuel into a non-running carb. The fuel will drain downward, and if a backfire occurs, a fire may damage the filter, etc. (KEEP CRANKING to "suck" the fire up into the engine. And....you DO have a fire extinguisher handy at all times and inflight don't you?)
Any engine that has suffered backfire thru the carb should be inspected for such damage.
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons.
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons.
I haven't had a chance to to go back out the aircraft yet and take a look at the carb/bail area. But George's post brings up a question in my mind...
Did I secure the primer after starting?
From his last post, I would assume that a unsecured primer would allow a constant flow of fuel into the carb area and perhaps cause a similiar problem...no?
I'm normally smooth with throttle inputs, but after this I've begun to question myself. It's one of those things you do without thinking after you solo and when something happens you think "How do I do this?". There still remains that in the event of a "unplanned" go around where engine response must be now, a throttle moved rapidly is a given.
My plan is to visually inspect the areas George pointed out and to double check the security of the primer. The highs this week are creeping back into 70's so this will be a good time to test the cold wx theory.
Thanks to all once again for your help and input.
Cheers,
Lee
Did I secure the primer after starting?
From his last post, I would assume that a unsecured primer would allow a constant flow of fuel into the carb area and perhaps cause a similiar problem...no?
I'm normally smooth with throttle inputs, but after this I've begun to question myself. It's one of those things you do without thinking after you solo and when something happens you think "How do I do this?". There still remains that in the event of a "unplanned" go around where engine response must be now, a throttle moved rapidly is a given.
My plan is to visually inspect the areas George pointed out and to double check the security of the primer. The highs this week are creeping back into 70's so this will be a good time to test the cold wx theory.
Thanks to all once again for your help and input.
Cheers,
Lee
Lee Collins
1951 C170A
N1733D
1951 C170A
N1733D
I don't think an unlocked primer would contribute to stalling. But I'm not sure what a primer leaking air into the system might do. Certainly any kind of air leak in the induction system would contribute to a too-lean condition that might result in similar stalling-symptoms as we're discussing.
On another angle to the issue of a carb-inlet fire.... I seem to recall an old AD (on Cessna 210's ??) that addressed fires in the induction system that could damage an air filter and result in engine failure. Does anyone else recall anything about it? (Seems it had to do with damage to the air filter that caused mechanical failure of the filter element, and subsequent collapse and blockage of the induction system. In any case, it's always a good idea to inspect the area carefully after a backfire, fire, and at regular inspection intervals.)
On another angle to the issue of a carb-inlet fire.... I seem to recall an old AD (on Cessna 210's ??) that addressed fires in the induction system that could damage an air filter and result in engine failure. Does anyone else recall anything about it? (Seems it had to do with damage to the air filter that caused mechanical failure of the filter element, and subsequent collapse and blockage of the induction system. In any case, it's always a good idea to inspect the area carefully after a backfire, fire, and at regular inspection intervals.)
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons.
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons.
-
Jr.CubBuilder
- Posts: 517
- Joined: Thu Aug 12, 2004 10:33 pm
Hmmmmm......I'm thinking about this one. Wouldn't an unsecured primer result in a dribble of fuel out the primer nozzles in the intake manifold? It seems to me that if the primer plunger is back out of the lock position the two checkballs should unseat due to suction from the manifold and then fuel would siphon from the primer system? Oh well, either way it should be in and locked after engine start.gahorn wrote:I don't think an unlocked primer would contribute to stalling. But I'm not sure what a primer leaking air into the system might do. Certainly any kind of air leak in the induction system would contribute to a too-lean condition that might result in similar stalling-symptoms as we're discussing.
When starting my J3 in cold weather, I leave the primer unlocked, which makes the mixture richer because fuel can be sucked through the primer into the intake manifold. If I don't do this with a cold engine, it will start, but it won't keep running. An open primer pump works just the same as a manual choke control.
Best Regards,
John
Best Regards,
John
John Renwick
Minneapolis, MN
Former owner, '55 C-170B, N4401B
'42 J-3 Cub, N62088
'50 Swift GC-1B, N2431B, Oshkosh 2009 Outstanding Swift Award, 2016 Best Continuously Maintained Swift
Minneapolis, MN
Former owner, '55 C-170B, N4401B
'42 J-3 Cub, N62088
'50 Swift GC-1B, N2431B, Oshkosh 2009 Outstanding Swift Award, 2016 Best Continuously Maintained Swift
An open primer pump will only possibly allow additional fuel to enter the left induction manifold. A manual choke control places a restriction in the induction ahead of the carb venturi. Both result in richer mixtures, but technically not "the same". The choke places higher vacuum demands upon the carb venturi. The unlocked primer does not.jrenwick wrote:When starting my J3 in cold weather, I leave the primer unlocked, which makes the mixture richer because fuel can be sucked through the primer into the intake manifold. If I don't do this with a cold engine, it will start, but it won't keep running. An open primer pump works just the same as a manual choke control.
Best Regards,
John
Jr.cubbuilder's remark reminds me that primary flight students are taught early-on that the prestart checklists include confirming that the primer is closed and locked. I guess familiarity-breeds-contempt strikes us all when we fly our own planes all the time.
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons.
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons.
OK, poor choice of words, I guess. I should have said that on my J3, which only has one manifold, not left/right, the open primer pump works effectively like a manual choke control on engines that have such a thing. I leave the primer open while hand propping, then once the engine is running smoothly, I slowly close and lock the primer pump, similar to opening the choke on my snowblower once it's running well enough to keep going without it. I certainly didn't intend to mislead anyone into flying in winter with the primer pump open. 
Best Regards,
John
Best Regards,
John
John Renwick
Minneapolis, MN
Former owner, '55 C-170B, N4401B
'42 J-3 Cub, N62088
'50 Swift GC-1B, N2431B, Oshkosh 2009 Outstanding Swift Award, 2016 Best Continuously Maintained Swift
Minneapolis, MN
Former owner, '55 C-170B, N4401B
'42 J-3 Cub, N62088
'50 Swift GC-1B, N2431B, Oshkosh 2009 Outstanding Swift Award, 2016 Best Continuously Maintained Swift
When considering the effects of an unlocked primer pump, remember that the effect will be directly proportional to the manifold vacuum which is most pronounced at idle or with a closed throttle. A closed throttle on the landing roll may cause an overly rich mixture to load the cylinders and have to be overcome as the throttle is opened for the go around. Once the throttle is opened, the vacuum is reduced to almost nil and the fuel flow from the primer would probably almost stop.
The problem outlined above, however, sounds to me more like a "leaning out" due to lack of fuel from the accelerator pump.
The problem outlined above, however, sounds to me more like a "leaning out" due to lack of fuel from the accelerator pump.
BL
Pavewlc,
I had the same problem with my 170a with the C-145 and the MA3-sp carb. (I think I remember the carb numbers right). George is right on. At least in my case, the seal for the accelerator pump was not doing its job. Apparently it is very common for the leather seal to dry out or just wear out so it doesnt pump fuel. My engine ran great unless I rapidly (but smoothly) increased the throttle on touch and gos. When it stumbled I would back off on the throttle and it would run fine again. Getting the carb rebuilt was an other adventure for me. Hope that helps.
Kevin
I had the same problem with my 170a with the C-145 and the MA3-sp carb. (I think I remember the carb numbers right). George is right on. At least in my case, the seal for the accelerator pump was not doing its job. Apparently it is very common for the leather seal to dry out or just wear out so it doesnt pump fuel. My engine ran great unless I rapidly (but smoothly) increased the throttle on touch and gos. When it stumbled I would back off on the throttle and it would run fine again. Getting the carb rebuilt was an other adventure for me. Hope that helps.
Kevin