If this is your first visit, be sure to check out the FAQ by clicking the link above. You may have to register before you can post: click the register link above to proceed. To start viewing messages, select the forum that you want to visit from the selection below. |
|
|
|
Thread Tools | Display Modes |
#1
|
|||
|
|||
Pawnee problems
On Apr 29, 11:13*am, "bds" wrote:
Our club has a Pawnee with an 0-540 that has been giving us fits for the last year or so. *We are having intermittent problems with power surges that can be significant and are sometimes even noticeable from the ground. *The problem will occur one day on one tow, and then sometimes disappear for several weekends. *One tow pilot seems to think that the problem seems to appear any time the airplane experiences noticeable negative Gs. In the past year or so we have replaced or rebuilt the following: Carburetor Ignition harness Fuel shut off valve Exhaust system Mags Spark plugs We have also cleaned the gascolator, checked the fuel tank for obstructions, and checked the vent system. The engine has about 1300 SMOH and early last season we did have a stuck valve in one of the cylinders. *We switched from auto fuel to 100LL at the beginning of last season and my recollection is that the problem might have started or gotten worse shortly afterwards. I am thinking that the engine is probably due for a top overhaul, but the problem doesn't sound like sticky valves because some of the other tow pilots say that the engine runs smoothly when the larger power losses occur. I have never experienced one of the major losses myself, but I have had a few of the 100 rpm drops which cleared up after about 15 seconds or so. When it happened to me I did notice a very slight engine roughness which does sound like it could be a stuck valve. *It also makes sense that if it is sticky valves that the problem would get worse running on 100LL. Any comments or suggestions on what to do next? *Two mechanics have looked at it and have not been able to resolve the problem as yet, and we need our Pawnee back on the flight line. *We are close to doing the top overhaul as a next step. Thanks, Bruce wing tanks or header tank? has there been any consistency in fuel level during these events? ive had engine problems in airplanes with header tanks at extreme nose up attitudes and low fuel levels. your "negative G" towpilot could be on to something. is there an engine driven fuel pump on the pawnee, even with a header tank? have you checked to make sure it is working? ive had a valve stick (on a cessna 150) and it was VERY noticeable, not "very slight" roughness. |
#2
|
|||
|
|||
Pawnee problems
On Apr 29, 10:13*am, "bds" wrote:
Our club has a Pawnee with an 0-540 that has been giving us fits for the last year or so. *We are having intermittent problems with power surges that can be significant and are sometimes even noticeable from the ground. *The problem will occur one day on one tow, and then sometimes disappear for several weekends. *One tow pilot seems to think that the problem seems to appear any time the airplane experiences noticeable negative Gs. In the past year or so we have replaced or rebuilt the following: Carburetor Ignition harness Fuel shut off valve Exhaust system Mags Spark plugs We have also cleaned the gascolator, checked the fuel tank for obstructions, and checked the vent system. The engine has about 1300 SMOH and early last season we did have a stuck valve in one of the cylinders. *We switched from auto fuel to 100LL at the beginning of last season and my recollection is that the problem might have started or gotten worse shortly afterwards. I am thinking that the engine is probably due for a top overhaul, but the problem doesn't sound like sticky valves because some of the other tow pilots say that the engine runs smoothly when the larger power losses occur. I have never experienced one of the major losses myself, but I have had a few of the 100 rpm drops which cleared up after about 15 seconds or so. When it happened to me I did notice a very slight engine roughness which does sound like it could be a stuck valve. *It also makes sense that if it is sticky valves that the problem would get worse running on 100LL. Any comments or suggestions on what to do next? *Two mechanics have looked at it and have not been able to resolve the problem as yet, and we need our Pawnee back on the flight line. *We are close to doing the top overhaul as a next step. Thanks, Bruce Going back to 100LL probably contributed to the problem. Lead deposits build up on the valve stems and inside guides causing them to stick. Unleaded fuels are one reason cars last so long these days. First check your engine for intake leaks (and for exhaust leaks while you're at it) The rubber hose sections on the intake runners are notorious for developing leaks. One way is to pressurize the intake system with the blow side of a very clean shop vacuum (new is good) and spray the outside of the engine with a soap solution looking for bubbles. If the intake system is tight, your suspicions on sticking valves may be correct. You'll need to read up on the infamous "Lycoming rope trick" which reads in part: "Insert about 8 feet of 3/8 inch nylon rope through the spark plug hole; then turn the crankshaft until the piston moves the rope snugly against the exhaust valve." See: http://www.lycoming.com/support/publ...fs/SI1425a.pdf This procedure allows the valves to be removed from their guides and cleaned without removing the cylinders from the engine. You should also clean the valve guides while you're at it. Be prepared to find bad things that lead to cylinder replacement. In the event this doesn't solve the problem, I'd look at the possibility a magneto has gone intermittent. Mags have a service life of about 500 hours but almost nobody replaces them until they fail - and they can fail in some weird ways. In my airplane days, I replaced mags every 500 hours with a new ones - it was my butt in the left seat. |
#3
|
|||
|
|||
Pawnee problems
Our club has a Pawnee with an 0-540 that has been giving us fits for the
last year or so. We are having intermittent problems with power surges that can be significant and are sometimes even noticeable from the ground. The problem will occur one day on one tow, and then sometimes disappear for several weekends. One tow pilot seems to think that the problem seems to appear any time the airplane experiences noticeable negative Gs. In the past year or so we have replaced or rebuilt the following: Carburetor Ignition harness Fuel shut off valve Exhaust system Mags Spark plugs We have also cleaned the gascolator, checked the fuel tank for obstructions, and checked the vent system. The engine has about 1300 SMOH and early last season we did have a stuck valve in one of the cylinders. We switched from auto fuel to 100LL at the beginning of last season and my recollection is that the problem might have started or gotten worse shortly afterwards. I am thinking that the engine is probably due for a top overhaul, but the problem doesn't sound like sticky valves because some of the other tow pilots say that the engine runs smoothly when the larger power losses occur. I have never experienced one of the major losses myself, but I have had a few of the 100 rpm drops which cleared up after about 15 seconds or so. When it happened to me I did notice a very slight engine roughness which does sound like it could be a stuck valve. It also makes sense that if it is sticky valves that the problem would get worse running on 100LL. Any comments or suggestions on what to do next? Two mechanics have looked at it and have not been able to resolve the problem as yet, and we need our Pawnee back on the flight line. We are close to doing the top overhaul as a next step. Thanks, Bruce |
#4
|
|||
|
|||
Pawnee problems
On Apr 29, 1:07*pm, "bds" wrote:
"Tony" wrote wing tanks or header tank? *has there been any consistency in fuel level during these events? ive had engine problems in airplanes with header tanks at extreme nose up attitudes and low fuel levels. *your "negative G" towpilot could be on to something. is there an engine driven fuel pump on the pawnee, even with a header tank? *have you checked to make sure it is working? ive had a valve stick (on a cessna 150) and it was VERY noticeable, not "very slight" roughness. ********************* It has a header tank but the problem occurs regardless of fuel level. *If anything, it is almost always full or at least half full when the problem occurs. I think part of the reason that the roughness isn't as bad on the 0-540 is because you're losing 1 out of 6 instead of 1 out of 4 when a valve sticks. Bruce fair enough. does it happen only on tow or sometimes off tow too? one towpilot mentioned negative G's as a possible trigger. I wouldn't rule out fuel delivery issues. with a header tank, possibly coupled with a weak or not working engine driven fuel pump (assumption), you could have issues when a glider pilot starts to yank the tail up or if your towpilots like to dive away after tow release. |
#5
|
|||
|
|||
Pawnee problems
we had a similar problem years ago with a Pawnee 150.....would usually run
up fine on the ground but every couple of flights just after take-off when you were expecting it to get up to full RPM it would only get about 80% of the normal power ....unfortunately this was about the time you'd be lifting off with a glider on behind.....the mechanic replaced carburetor float, changed plugs, adjusted magneto's .all resulted in the same behavior...then finally we checked the exhaust and sure enough a baffle had broken off inside and sometimes would lay harmlessly away from the muffler inlet or exit and other times would just simply end up plugging the exhaust end of the muffler.....what goes in has to come out or less can come in and consequently when less comes in and out the motor doesn't work as intended...you might also notice a higher than normal engine temperature as an indication that this is the problem ...anyway, this is my bet! tim Please visit the Wings & Wheels website at www.wingsandwheels.com "bds" wrote in message ... Our club has a Pawnee with an 0-540 that has been giving us fits for the last year or so. We are having intermittent problems with power surges that can be significant and are sometimes even noticeable from the ground. The problem will occur one day on one tow, and then sometimes disappear for several weekends. One tow pilot seems to think that the problem seems to appear any time the airplane experiences noticeable negative Gs. In the past year or so we have replaced or rebuilt the following: Carburetor Ignition harness Fuel shut off valve Exhaust system Mags Spark plugs We have also cleaned the gascolator, checked the fuel tank for obstructions, and checked the vent system. The engine has about 1300 SMOH and early last season we did have a stuck valve in one of the cylinders. We switched from auto fuel to 100LL at the beginning of last season and my recollection is that the problem might have started or gotten worse shortly afterwards. I am thinking that the engine is probably due for a top overhaul, but the problem doesn't sound like sticky valves because some of the other tow pilots say that the engine runs smoothly when the larger power losses occur. I have never experienced one of the major losses myself, but I have had a few of the 100 rpm drops which cleared up after about 15 seconds or so. When it happened to me I did notice a very slight engine roughness which does sound like it could be a stuck valve. It also makes sense that if it is sticky valves that the problem would get worse running on 100LL. Any comments or suggestions on what to do next? Two mechanics have looked at it and have not been able to resolve the problem as yet, and we need our Pawnee back on the flight line. We are close to doing the top overhaul as a next step. Thanks, Bruce __________ Information from ESET NOD32 Antivirus, version of virus signature database 5072 (20100429) __________ The message was checked by ESET NOD32 Antivirus. http://www.eset.com __________ Information from ESET NOD32 Antivirus, version of virus signature database 5072 (20100429) __________ The message was checked by ESET NOD32 Antivirus. http://www.eset.com |
#6
|
|||
|
|||
Pawnee problems
What do you mean by a power surge? To me a surge is a momentary
increase in something. Are you experiencing a momentary power increase, or a momentary power reduction? The PA28-180 I used to be a partner in experienced a few 200 rpm drops at the most inconvenient times. It scared off 3 of the partners and it took several years for me to find the problem. I became more motivated after it happened to me during a high density altitude takeoff. It was caused by a P lead contact being shorter than spec. The P lead contact pushes against the contact spring mounted on the mag capacitor. The short contact didn't push the spring contact far enough away from the rear mag cover and allowed it to sometimes arc to the case. Dismantled the P lead contact and replaced the spacer tube with a longer one from a spare P lead (same length as the other mag) and the power loss problem has not been seen in the last 3 or more years. New mags, or even a complete engine rebuild, would not have fixed the problem unless the P leads were also replaced. This wouldn't normally be done since they are part of the aircraft wiring harness. Really bad mag design since if the P lead comes unscrewed the mag is shorted internally and behaves just as if the P lead is grounded. Andy (GY) |
#7
|
|||
|
|||
Pawnee problems
"Tony" wrote wing tanks or header tank? has there been any consistency in fuel level during these events? ive had engine problems in airplanes with header tanks at extreme nose up attitudes and low fuel levels. your "negative G" towpilot could be on to something. is there an engine driven fuel pump on the pawnee, even with a header tank? have you checked to make sure it is working? ive had a valve stick (on a cessna 150) and it was VERY noticeable, not "very slight" roughness. ********************* It has a header tank but the problem occurs regardless of fuel level. If anything, it is almost always full or at least half full when the problem occurs. I think part of the reason that the roughness isn't as bad on the 0-540 is because you're losing 1 out of 6 instead of 1 out of 4 when a valve sticks. Bruce |
#8
|
|||
|
|||
Pawnee problems
"bildan" wrote Going back to 100LL probably contributed to the problem. Lead deposits build up on the valve stems and inside guides causing them to stick. Unleaded fuels are one reason cars last so long these days. First check your engine for intake leaks (and for exhaust leaks while you're at it) The rubber hose sections on the intake runners are notorious for developing leaks. One way is to pressurize the intake system with the blow side of a very clean shop vacuum (new is good) and spray the outside of the engine with a soap solution looking for bubbles. If the intake system is tight, your suspicions on sticking valves may be correct. You'll need to read up on the infamous "Lycoming rope trick" which reads in part: "Insert about 8 feet of 3/8 inch nylon rope through the spark plug hole; then turn the crankshaft until the piston moves the rope snugly against the exhaust valve." See: http://www.lycoming.com/support/publ...fs/SI1425a.pdf This procedure allows the valves to be removed from their guides and cleaned without removing the cylinders from the engine. You should also clean the valve guides while you're at it. Be prepared to find bad things that lead to cylinder replacement. In the event this doesn't solve the problem, I'd look at the possibility a magneto has gone intermittent. Mags have a service life of about 500 hours but almost nobody replaces them until they fail - and they can fail in some weird ways. In my airplane days, I replaced mags every 500 hours with a new ones - it was my butt in the left seat. ****************** We did check the intake tubes and couplers and tighten up the clamps but we haven't done anything like the test with the vacuum cleaner - that sounds like a good idea. I recall reading something about the Lycoming rope trick last year when we had the stuck valve on the one cylinder. I am thinking that if we had one stick, it is reasonable to assume that we likely have problems with others. Bruce |
#9
|
|||
|
|||
Pawnee problems
"Tim Mara" wrote in message ... we had a similar problem years ago with a Pawnee 150.....would usually run up fine on the ground but every couple of flights just after take-off when you were expecting it to get up to full RPM it would only get about 80% of the normal power ....unfortunately this was about the time you'd be lifting off with a glider on behind.....the mechanic replaced carburetor float, changed plugs, adjusted magneto's .all resulted in the same behavior...then finally we checked the exhaust and sure enough a baffle had broken off inside and sometimes would lay harmlessly away from the muffler inlet or exit and other times would just simply end up plugging the exhaust end of the muffler.....what goes in has to come out or less can come in and consequently when less comes in and out the motor doesn't work as intended...you might also notice a higher than normal engine temperature as an indication that this is the problem ...anyway, this is my bet! tim Please visit the Wings & Wheels website at www.wingsandwheels.com We did find problems with the baffling inside the muffler over the winter and thought it was our "aha" moment. The muffler was rebuilt but the problem remains. Bruce |
#10
|
|||
|
|||
Pawnee problems
"Andy" wrote in message ... What do you mean by a power surge? To me a surge is a momentary increase in something. Are you experiencing a momentary power increase, or a momentary power reduction? The PA28-180 I used to be a partner in experienced a few 200 rpm drops at the most inconvenient times. It scared off 3 of the partners and it took several years for me to find the problem. I became more motivated after it happened to me during a high density altitude takeoff. It was caused by a P lead contact being shorter than spec. The P lead contact pushes against the contact spring mounted on the mag capacitor. The short contact didn't push the spring contact far enough away from the rear mag cover and allowed it to sometimes arc to the case. Dismantled the P lead contact and replaced the spacer tube with a longer one from a spare P lead (same length as the other mag) and the power loss problem has not been seen in the last 3 or more years. New mags, or even a complete engine rebuild, would not have fixed the problem unless the P leads were also replaced. This wouldn't normally be done since they are part of the aircraft wiring harness. Really bad mag design since if the P lead comes unscrewed the mag is shorted internally and behaves just as if the P lead is grounded. Andy (GY) Sorry - it is a sudden loss, not a sudden increase. Poor wording. Thanks for the tip - we will take a look at this this weekend. Bruce |
|
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
To Pawnee or not to Pawnee...that is the question... | Travis Beach | Soaring | 56 | March 24th 14 06:21 AM |
Looking for Pawnee | [email protected] | Soaring | 1 | July 13th 08 12:35 AM |
To Pawnee or not to Pawnee...that is the question... | Travis Beach | Soaring | 4 | October 17th 07 01:31 PM |
PA-25 PAWNEE | Rafgsa Centre | Soaring | 2 | March 3rd 05 03:13 AM |
Pawnee | Damien Dyer | Soaring | 2 | November 2nd 04 05:35 PM |