Carburetor's, Fuel Flow, CHT's

[Skates97]

To hopefully help others I want to share my carb replacement experience, even though admitting to buying two overhauled carbs in the space of less than 6 months will probably result in my membership in the CB club being revoked.

I had an EDM 830 installed in 11/2017, about a year after I bought the plane. They were not able to put the carb temp sensor in because the carb was so ancient that it didn't have the spot to put the sensor. The shop said the carb would have to be sent out, drilled, and tapped in order to install the sensor so I just left the sensor on the shelf. In October 2019 I decided to replace the carb with an overhauled one for three reasons. First, 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 10-4161-1.

https://msacarbs.com/technical-data/engine-eligibility/

The carb that was in it was a 10-3878 which did not have an "M" stamped on it. There is a bulletin from Marvel-Schebler to modify the 10-3878 in Mooneys to increase fuel flow at WOT for better cooling. I will attach the bulletin here and upload to the "Downloads" section. When that is performed it is supposed to have an "M" stamped on the plate to identify that it has been done. Mine did not have the stamp and I the engine log states that it was a 10-3878 that was installed with the engine overhaul so I bought a 10-3878 and had my AP/IA swap it out.

After swapping it out I noticed that I did not have the same fuel flow as before and I had to back off on RPM's to keep my cylinder's from getting too hot where I did not have to do that before. 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 from that. Also, the only change to the system was the new/overhauled carb. 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 of these temps were running full RPM and WOT with 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 did have 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 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.

Also make note 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. 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 but he said no, that he would 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 10-4161 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. I think that the 10-3878 that was previously in my plane had the modification done to it but was not noted.

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 where with the higher fuel flow I was not backing off RPM's. I do not know what they would have climbed to because I was not willing to push the temps that high. It would appear 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 would 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.

 

A11-62.pdf

[ragedracer1977]

Great info!  Your fuel flow numbers vs temps are mirroring what I'm seeing.  

My instrumentation prior was a UBG16 and FP5L. I remember when I installed the FP5L I was originally seeing 16gph plus, but the last few flights prior to the EDM it was under 16.

I did have the mux8a.  It stores data.  I wonder if there's a way to power it up on the bench and pull the old data off? I never did, because I didn't have the right cable, but I'm thinking it could be interesting.  There should be at least 100 hours of flight time recorded, as much as 600 hours

Edited April 11, 2020 by ragedracer1977

[mark21m20c]

I will check my carb information . Thank you for the post !!!!

[Brian E.]

To add to Skates post I too changed from the 10-3878 to the 10-4161 during my last annual.  Not only did I have high CHTs, I also required 13 strokes to prime the engine when cold.  With the new carb I'm down to the customary 2 - 4 pumps for cold start and CHT's are ~25 degrees lower.  I'll check FF today but I do recall I'm right around 16 gph.

Update:  Just checked today, with everything full forward I get 16.5 gph before I pull the prop back to 2600.

 

Good luck.

[carusoam]

Great follow up Brian!

Best regards,

 -a-

[mark21m20c]

We have the 10-3878M carb on our 67 M20c . First flight after engine overhaul fuel flow was 16.5 on take off . CHT got up to 480 for 10-20 seconds then returned to normal range. Sea level and 70 degrees . Is this normal CHT temperature for engine break in?

[Hank]

I don't have fuel flow in my C, but all this talk of 15-16.5 gph for takeoff bothers me! This is what my Owner's Manual says:

tl;dr--why are you not showing 18 gph???

[orionflt]

6 minutes ago, Hank said:

I don't have fuel flow in my C, but all this talk of 15-16.5 gph for takeoff bothers me! This is what my Owner's Manual says:

tl;dr--why are you not showing 18 gph???

the 16.5 comes from lycoming's operators manual for the O-360

Brian

[carusoam]

For typical NA engine break in... cold, high power, max FF, full rich... altering rpm every 15 minutes or so...

Find the written recommendation by the engine manufacturer... or cylinder supplier... or mechanic doing the work...

The objective is to use high pressure in the cylinders, without over heating them... and oscillate rpms to avoid wearing a step in any one place... rpm actually has an effect on the travel of the piston and their associated rings...

Of course the proper oil to go with that...

PP thoughts only, not a mechanic...

Best regards,

-a-

[mark21m20c]

Thank you ..

I am following standard engine break in procedures . I was wondering what others are seeing as far as CHT on take off up till clean up ?  Is CHT of 480( 20 seconds then returned to normal range)on first flight after overhaul in the ball park of what others have seen on break in.

 

 My 1967 owners manual (interpolate FF from 2500 to sea level) 16.5 FF .Just hope to tap knowledge of others that have gone thru break in procedure of o360A1D.

Thank You

[carusoam]

c,

If you want to see interesting data from other M20Cs... find Raged’s threat that came before this one...

Break in temps typically have higher CHTs until the rings get seated...

If you have a food engine monitor you may see it happen... at least in the saved data...

The other thread is an excellent read for what else can happen if the carb possibly misbehaves... there is both good and un good examples...

Best regards,

-a-

[Hank]

10 minutes ago, mark21m20c said:

Thank you ..

I am following standard engine break in procedures . I was wondering what others are seeing as far as CHT on take off up till clean up ?  Is CHT of 480( 20 seconds then returned to normal range)on first flight after overhaul in the ball park of what others have seen on break in.

 My 1967 owners manual (interpolate FF from 2500 to sea level) 16.5 FF .Just hope to tap knowledge of others that have gone thru break in procedure of o360A1D.

Thank You

Best wishes for your new engine! A good break in will give years (decades?) of good operationnoperation. But that's a very brief time window you're asking about. From throttle forward, checking MP, pretty much my next glance at the panel is verifying Orange Gear Light for fully up, then a little later DG for my first turn. Liftoff to cleanup is literally a handful of seconds, like 5 or so.

[carusoam]

Also keep in mind the first couple of flights have a few things in mind... they are not long break-in flights... they are more like oil leak and loose connection checks in the extended traffic pattern...

Safety steps when things don’t go perfectly well...

Best regards,

-a-

[mark21m20c]

Thank you

OK how about from setting take off thrust to 1000 feet.

[Hank]

4 hours ago, mark21m20c said:

Thank you

OK how about from setting take off thrust to 1000 feet.

If I'm staying in the pattern, push everything forward, rotate, confirm positive rate, gear up, turn crosswind, pull throttle back at 600-700' agl, trim trim trim, turn downwind, adjust throttle for 1000' agl and <100 mph.