This article specifically only applies to those of us with an O-360. However, the rest of you with fuel injection likely know someone with a carburetor so you could keep reading to share your newfound knowledge with them. I hope that sharing my carb replacement experience will help others when they come to that point of ownership.
I had an EDM 830 installed in November 2017, about a year after I bought the plane. I bought a carb temp sensor, but they were not able to put it in because the carb was so ancient that it didn’t have the spot to put the sensor. The shop said the carb could be sent out, drilled, and tapped in order to install the sensor, but I decided to leave the sensor out and put it on the shelf. In October 2019 I replaced the carb with an overhauled one for three reasons. The last time mine was overhauled according to the log books was 1997 (22 years ago), it would allow me to have the carb temp sensor installed, and by buying an overhauled unit the plane would only be down for one day to have it swapped out.
There are two carbs that are listed for the O-360A1D that is in my plane, a 10-3878 and 10-4164-1. In addition to those two, there is a bulletin from Marvel-Schebler to modify the 10-3878 in Mooneys to increase fuel flow at WOT for better cooling, essentially making it the same as a 10-4164-1. The bulletin can be accessed and downloaded from my website here. When the procedure in the bulletin is performed, it is supposed to have an “M” stamped on the plate to identify that it has been done.
The listing of eligibility from Marvel-Schebler is available on their site here.
The carb that was in my plane was a 10-3878 which did not have an “M” stamped on it. I went through the engine log and it stated that it was a 10-3878 that was installed with the engine overhaul. Like any smart Mooney owner, what was in the plane was working, so rather than buy a 10-4164-1 I bought a 10-3878 and had my AP/IA swap it out.
After swapping it out I immediately noticed that I did not have the same fuel flow as before the swap. Climbing out I had to back off on RPM’s to keep my cylinder’s from getting too hot, not something I had to do previously and not something I like doing. WOT and full RPM to cruise altitude is SOP, who buys a Mooney to fly slow? The doghouse on my plane is in very good shape, and I had already replaced the baffling around the cowl opening and the felt around the starter/generator, so I knew that I was not having air flow issues. The only change to the system was the new/overhauled carb so logic says that is where the problem is located. I kept track of the data from my EDM 830 and got in contact with Marvel Schebler.
Prior to swapping out the carb the average high fuel flow at WOT was 16.2 with an average high CHT on the #3 of 408°. The highest I ever saw #3 get was 420° and that was departing Chandler, AZ in the afternoon late in June 2019. (It was hot enough I had to wear gloves to pre-flight because the skin of the plane was too hot to touch). All these temps were running full RPM and WOT, and the only measure taken to reduce CHT’s was climbing out at 120 mph. These numbers were taken from seven flights over June-Sept 2019, hotter months.
After swapping out the carb the average high fuel flow at WOT was 14.8 with an average high CHT on the #3 of 434°. The highest I saw was #3 at 453° departing Fullerton, CA in November of 2019, much cooler than AZ in June. On these flights I had to reduce RPM and climb out at 120mph+ to try and keep the CHT’s down. Had I left it at WOT and full RPM the CHT’s would likely have climbed much higher. These numbers were taken from fifteen flights over Oct-Feb 2019-2020, much cooler months than the first set of flights with the old carb.
Keep in mind that there are no adjustments that can be made to increase the fuel flow at WOT.
I went back and forth on emails with Marvel-Schebler checking different things that they recommended. By supplying the serial number of the carb that went in my plane to them, they confirmed that it bench tested at 16.7 gph at the factory and they expect to see between 16-17 gph on that model. After research I was able to find where people talked about a “Mooney Mod” for the carb but could not find anything specific other than mentions of it and that it was supposed to improve fuel flow. Further inquiries to Marvel-Schebler about this resulted in them providing me with bulletin A11-62 which is linked earlier in this article. It involves conversion kit 666-660 which in 1962 was $12.80 but now is $228 from Aircraft Spruce or $244 from Marvel-Schebler if you can get it.
https://www.aircraftspruce.com/catalog/eppages/carbconvkit.php
https://msacarbs.com/product/conversion-kit-666-660-f/
I took the bulletin to my AP/IA and asked if he would tackle it. He said no, he would have to send it out to a carb shop. After considering the price of the kit, plus the cost of the carb shop to perform it, plus the week+ down-time to pull the carb, send it out, and reinstall, I decided to purchase the overhauled 10-4161 so it would be a one-day swap.
After replacing with the 10-4164 the average high fuel flow at WOT was 16.3 with an average high CHT on #3 of 411. The highest I saw #3 get was 425. This was over six flights in March-April of 2020. As with the flights in the first test group, nothing was done to mitigate the CHT’s other than climbing out at 120 mph. These numbers are similar to what I was seeing with my old carb. The only conclusion I can come up with is that the 10-3878 that was previously in my plane must have had the modification done to it but was not noted in the log book and there was no “M” stamped on the plate.
The increase of 1.5 gph fuel resulted in a decrease in CHT’s of approximately 25° F, however this is a lower number than what it would actually be as I was backing off on RPM’s to keep the CHT’s down. With the higher fuel flow I was not backing off RPM’s. It appears that an increase of even 1 gph of FF at WOT and full RPM is probably worth a decrease of at least 20° in CHT’s.
If you are looking to replace the carb on your O-360 A1A or A1D I strongly recommend the 10-4161. It is the richer of the two carbs. If you are having issues with high CHT’s, I think it is worth looking at your FF at WOT and full RPM. If you are not seeing 16+ gph you are likely seeing CHT’s that are 25+° higher than what they should be.
