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- Dec 4, 2018
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Hello everyone,
First post after picking up our 1970 Cherokee 140.
We picked up the plane in MN and flew it back down here to South Carolina. During the flight the engine ran smooth, however at the first gas stop we did a run up before taking off and when switching to the left Mag, the engine would stumble, acting like a misfire, not a smooth drop, but rather like the spark was breaking up. We ran the engine at 2000 RPM, pulled back on the mixture for 1 minute and checked again, this time everything seemed fine, so we departed.
The next gas stop, once again during the run up, while testing the left mag, we would get an intermittent break up/misfire. We repeated the same procedure as before, and everything seemed fine and off we went.
Once the plane was back here, took it up for the first flight here, same thing again, left mag caused misfire, repeat procedure, everything looks good and off we went.
Last week though even after doing the same procedure, the engine would misfire on the left mag, also started to notice it would very occasionally do it on the right mag. It was a Friday, so I ordered up some spark plugs as the Champions that were in there had about 370 hours on them and 10 years.
The new plugs came in and the local mechanic walked me through how to change the plugs, which was super easy on the Cherokee. The plugs that came out were inspected and I was told that they look pretty good (picture attached, #1 plugs are on top and #4 towards the bottom of the picture). I installed the new set of Tempest plugs and took the plane down to do a runup and once again the left mag would stumble and misfire. The misfire though is not all the time, sometimes the engine will run for 5-10 seconds, then misfire. Other times it will misfire as soon as you switch the mags.
Talked with the local mechanic again and decided to rule out the switch and P-leads. So disconnected one P lead from the Left Mag, went down and did the static run up, once again the sporadic misfire was there. Repeated the same procedure with the P lead on the right Mag disconnected. The right mag would also sporadically misfire, but much less severe than the right Mag.
Today, we removed the Left Mag for inspection. It is a Slick Mag with about 150 hours on it. We opened up the Mag and checked the capacitor, the coil resistance and all the parts. It all checked out great, everything looked very good inside the Mag. We installed it and confirmed timing at 25 degrees and also that both Mag's were in time with each other. Went down to do run up, and the same thing happened, intermittent misfire mainly on left Mag, but sometimes on right.
So, I reached out to the previous owner and asked if he ever had a Mag check issue. He said he always did the Mag check at full throttle with Carb heat on. So, I went down and tried that and sure enough the Mag will check out with full throttle and Carb heat on.
The Cherokee does have a Powerflow exhaust installed on it and so I started doing some reading to see if there are any carburetor adjustments needed when installing the system on the Cherokee and can not find anything indicating any adjustments for mixture are needed with Powerflow system.
I did read on the Powerflow site about checking to see if my carb is too lean - https://www.powerflowsystems.com/forum/showthread.php?t=104
So, I went out again and checked a full throttle run up, slowly started to pull back the mixture and there is no rise in RPM before the RPM starts to drop. At this point I am starting to think that there might be a fueling issue causing the misfire and looking for feedback. Another note, on the trip back of 11.5 hours we burned about 8 gph the whole trip without leaning the mixture, not sure if that helps or not, but figured I would put that out there.
Attached is also a picture of the new Tempest #1 spark plug after doing the last runup test. For my day job, I help design automotive parts and if I pulled this spark plug on a normal car engine I would say it looks lean, but still new to the much more low tech airplane engines and leaded fuel to know for sure.
Anyway, any information or things to check would be appreciated. I did also check all of the intake runners to make sure the rubber connections are tight along with the hose clamps.
Thanks again,
Dave
First post after picking up our 1970 Cherokee 140.
We picked up the plane in MN and flew it back down here to South Carolina. During the flight the engine ran smooth, however at the first gas stop we did a run up before taking off and when switching to the left Mag, the engine would stumble, acting like a misfire, not a smooth drop, but rather like the spark was breaking up. We ran the engine at 2000 RPM, pulled back on the mixture for 1 minute and checked again, this time everything seemed fine, so we departed.
The next gas stop, once again during the run up, while testing the left mag, we would get an intermittent break up/misfire. We repeated the same procedure as before, and everything seemed fine and off we went.
Once the plane was back here, took it up for the first flight here, same thing again, left mag caused misfire, repeat procedure, everything looks good and off we went.
Last week though even after doing the same procedure, the engine would misfire on the left mag, also started to notice it would very occasionally do it on the right mag. It was a Friday, so I ordered up some spark plugs as the Champions that were in there had about 370 hours on them and 10 years.
The new plugs came in and the local mechanic walked me through how to change the plugs, which was super easy on the Cherokee. The plugs that came out were inspected and I was told that they look pretty good (picture attached, #1 plugs are on top and #4 towards the bottom of the picture). I installed the new set of Tempest plugs and took the plane down to do a runup and once again the left mag would stumble and misfire. The misfire though is not all the time, sometimes the engine will run for 5-10 seconds, then misfire. Other times it will misfire as soon as you switch the mags.
Talked with the local mechanic again and decided to rule out the switch and P-leads. So disconnected one P lead from the Left Mag, went down and did the static run up, once again the sporadic misfire was there. Repeated the same procedure with the P lead on the right Mag disconnected. The right mag would also sporadically misfire, but much less severe than the right Mag.
Today, we removed the Left Mag for inspection. It is a Slick Mag with about 150 hours on it. We opened up the Mag and checked the capacitor, the coil resistance and all the parts. It all checked out great, everything looked very good inside the Mag. We installed it and confirmed timing at 25 degrees and also that both Mag's were in time with each other. Went down to do run up, and the same thing happened, intermittent misfire mainly on left Mag, but sometimes on right.
So, I reached out to the previous owner and asked if he ever had a Mag check issue. He said he always did the Mag check at full throttle with Carb heat on. So, I went down and tried that and sure enough the Mag will check out with full throttle and Carb heat on.
The Cherokee does have a Powerflow exhaust installed on it and so I started doing some reading to see if there are any carburetor adjustments needed when installing the system on the Cherokee and can not find anything indicating any adjustments for mixture are needed with Powerflow system.
I did read on the Powerflow site about checking to see if my carb is too lean - https://www.powerflowsystems.com/forum/showthread.php?t=104
So, I went out again and checked a full throttle run up, slowly started to pull back the mixture and there is no rise in RPM before the RPM starts to drop. At this point I am starting to think that there might be a fueling issue causing the misfire and looking for feedback. Another note, on the trip back of 11.5 hours we burned about 8 gph the whole trip without leaning the mixture, not sure if that helps or not, but figured I would put that out there.
Attached is also a picture of the new Tempest #1 spark plug after doing the last runup test. For my day job, I help design automotive parts and if I pulled this spark plug on a normal car engine I would say it looks lean, but still new to the much more low tech airplane engines and leaded fuel to know for sure.
Anyway, any information or things to check would be appreciated. I did also check all of the intake runners to make sure the rubber connections are tight along with the hose clamps.
Thanks again,
Dave
