I just replaced the carb sockets on my plane. What prompted that was that I reviewed all my engine readings from 2013 and found that the EGT's had gradually increased throughout the summer. Upon inspection, I saw that the boots were deteriorated. I decided to do a carburetor rebuild at the same time since both the 25 hour inspection and the 150 hour needle/jet replacements were past due.
Up until the rebuild, everything appeared to be running fine. I've been running the engine every few weeks during the winter. The last time I ran the engine was the beginning of April. I did not notice anything unusual at that time.
However, during the rebuild I found several things not right (according to the online videos on this site and the EAA site):
1- One of the carburetor caps was installed backwards
2- The jet needle clip was on position 1 of the jet needle, where the specs say (for that specific jet/needle) should have been on position 3
3- Neither needle jet had an o-ring
4- Both needle clip assemblies were installed AFTER the white plastic retainer (should have been first)
I bought the carburetor replacement kit and a new needle jet and jet needle, and followed the videos for disassembly and rebuild. That all went well.
Then I did the carburetor balancing, according to the video on this site (which I thought was awesome). The first part is the mechanical balancing, which, as you know, involves adjusting both carbs so that the slides move simultaneously. Then you do an 8 minute runup at 3000 RPM and then adjust the idle speed, if needed.
This is where my problem is. When the engine is at 3000 RPM, everything seems fine. But when I throttle back to check the idle speed, the speed "lopes" (I'm told by a real mechanic that that is the correct term) or "seeks", which really means that the RPM gradually oscillates. It is not a fast oscillation at all. But the speed does oscillate over a 600 RPM range (1700-2300).
I was able to observe the engine as it idled, and every time the "loping" was at the low end of the RPM range, there was a lot of gas venting out of the carburetor vent tubes - both of them!
My understanding is that the vent tubes are there to allow excess gas to vent out if the float bowl overfills.
With the engine stopped, I checked and both of the float needles moved freely. I checked the float valve arms in one of the carbs, and when I gently pushed the arms up, the gas flow into the carb stopped. To me, that means it is working correctly.
I have to believe that, since all was well before the rebuild (even though I found those issues listed above with the carbs), it has to be something that I did or did not do correctly. Keep in mind that I am NOT a mechanic, and my learning curve is very high.
So why is the RPM "loping" at idle and why is gas being spewed out at the low end of the loping RPM range?
Please help and thanks in advance for all replies!
Mike
Up until the rebuild, everything appeared to be running fine. I've been running the engine every few weeks during the winter. The last time I ran the engine was the beginning of April. I did not notice anything unusual at that time.
However, during the rebuild I found several things not right (according to the online videos on this site and the EAA site):
1- One of the carburetor caps was installed backwards
2- The jet needle clip was on position 1 of the jet needle, where the specs say (for that specific jet/needle) should have been on position 3
3- Neither needle jet had an o-ring
4- Both needle clip assemblies were installed AFTER the white plastic retainer (should have been first)
I bought the carburetor replacement kit and a new needle jet and jet needle, and followed the videos for disassembly and rebuild. That all went well.
Then I did the carburetor balancing, according to the video on this site (which I thought was awesome). The first part is the mechanical balancing, which, as you know, involves adjusting both carbs so that the slides move simultaneously. Then you do an 8 minute runup at 3000 RPM and then adjust the idle speed, if needed.
This is where my problem is. When the engine is at 3000 RPM, everything seems fine. But when I throttle back to check the idle speed, the speed "lopes" (I'm told by a real mechanic that that is the correct term) or "seeks", which really means that the RPM gradually oscillates. It is not a fast oscillation at all. But the speed does oscillate over a 600 RPM range (1700-2300).
I was able to observe the engine as it idled, and every time the "loping" was at the low end of the RPM range, there was a lot of gas venting out of the carburetor vent tubes - both of them!
My understanding is that the vent tubes are there to allow excess gas to vent out if the float bowl overfills.
With the engine stopped, I checked and both of the float needles moved freely. I checked the float valve arms in one of the carbs, and when I gently pushed the arms up, the gas flow into the carb stopped. To me, that means it is working correctly.
I have to believe that, since all was well before the rebuild (even though I found those issues listed above with the carbs), it has to be something that I did or did not do correctly. Keep in mind that I am NOT a mechanic, and my learning curve is very high.
So why is the RPM "loping" at idle and why is gas being spewed out at the low end of the loping RPM range?
Please help and thanks in advance for all replies!
Mike
