O-320-B2B (160 hp) - applying full throttle results in huge EGT drop

Dear Forum members

We have just had our O-320-B2B engine overhauled. Due to several replacement parts not being available in the short term (covid...), we exchanged the engine for another one that was already refurbished and ready to go (same model, same specs). While we are still running the engine in at TSO = 8 hours, I found a few peculiarities that I would like to discuss and ask your advice about it.

We operate the plane for glacier landings, typically around 8'000 - 10'000 ft. However, the behavior also occurs at lower elevations. First thing to note is that for a cruise power setting at 8'000 ft (2400 to 2450 rpm), I need to pull back the mixture quite a bit more than with the old engine - roughly 1.5 fingers. We have an Alcor EGT with Cessna/Vernier mixture, so it can well be I just need to adjust the EGT indication a bit lower to see a good reading on the EGT (good use of the scale available). For best power, let's say peak -100°F, I need to apply a lot of mixture. If I pull a bit more, the engine quickly runs rough. It seems quite non-linear with mixture setting. That's not of concern yet.

However, when I apply from this power setting full throttle (without even enriching the mixture), the EGT drops by 150 - 200°F and I have the gut feeling that the engine is not producing significant surplus of thrust. Also, going to climb full throttle (and optimized mixture setting for cruise, as before) will only yield 2400 rpm with 70 mph indicated, with a less than thrilling climb performance. Since we do glacier landings, we typically have 1700 - 2000 rpm during the final approach, and then apply full throttle on the uphill part and of course for take-off. If I pull the mixture all the way out to optimize for full throttle operations, it runs very rough once I reduce to 1700 rpm. Thus, this is not very practical.

To me it looks like the part I should look at is the carburetor (Marvel Schebler) which was also refurbished. The carburetor float is made of composite I guess.

What are your recommendations? What should I do or look at? Static pull tests at various power settings? What should I test in flight to get a better idea of that is going on? Regarding idle mixture setting, it looks like it's not really far off the optimum. It's really the behavior when transiting from cruise throttle to full throttle.

Thanks a lot for your help,
Maurice

Do you have fuel flow numbers by chance? What was done to the carburetor and by whom? Does it ever appear to make full power at any mixture setting or always seems off? Have you pulled a plug and looked at it, real white or soot black?

GM

My first thought is that your "economy" jet is working well.

I apologize if my technical term is not correct, but your carb is designed to add a bunch of extra fuel at full throttle for cooling. Often full throttle is used at slower speeds, and you are asking for high power, both lead to overheat. At full throttle you get an extra amount of fuel to compensate for this. (Why partial power takeoffs should be avoided under most circumstances).

Depending on what altitude and ambient temperature you live, the carb overhaul shop may have installed larger jets than you had previously, causing a richer mixture and lower power. Checking the plugs is a great start to find out.

Also, depending if the engine shop did a run in on the engine, it might take a little while for your engine to break in and loosen up...

rv8drvr said:

Do you have fuel flow numbers by chance? What was done to the carburetor and by whom? Does it ever appear to make full power at any mixture setting or always seems off? Have you pulled a plug and looked at it, real white or soot black?

GM

Click to expand...

Hi GM
Thank you for helping out.
We do not have any fuel flow indicator. Carburetor was factory overhauled for engine revision after TBO. As stated, due to non-availabilty of replacement parts of Lyoming for our original engine, we took another engine in exchange.

I will try to mix for full throttle on my next flight.
Plugs not inspected yet, will the next thing to do.

Thanks!

Thanks AKtango58

It was a cold day and OAT was around -18°C on 9000 ft. I do not know where the carburetor was refurbished, but it was reassembled by the engine shop (which has very good reputation).

Short run-in (1h) was done in shop, then by airplane mechanic in flight, then we did continue to run in. but the engine only has 8 hours so far.

Will inspect the plugs and report.

thanks!
maurice

Check to see that the float bowl screws are tight. Grab the air filter and give it a shake, it should not move. Also look at the sides of the float bowl for fuel stains have someone shake a wing while you look at it. You density altitude at 18C and 9000 ft would certainly make it run very rich over 2550 RPM. What kind of RPM drop do you see with carb heat full rich at 2400 RPM? DENNY

What is you airfield attitude?
DENNY

Also inspect your carb heat/air filter box and cable controls. Any carb heat that leaks by the flapper when cold air is selected will reduce performance and create a richer air/fuel mixture. The flapper has to move but ideally with low leakage around the sides. Make sure it's fully closed when cold air is desired.

What air filter are you using? Some designs can be more restrictive and cause a richer mixture and lower power. If suspected temporarily remove it and test for performance.

Gary

Edit: Read especially the troubleshooting section in the first link:
http://www.insightavionics.com/pdf files/MA-4 Carb Manual.pdf

https://wiki.ad7zj.net/wiki/images/8/85/MSA_Aricraft_Carburetor_Service_Manual_Models_MA3A_MA3PA_MA3SPA_MA4SPA.pdf

I had this exact thing happen to my 0320-B2B. When I pulled carb heat, the RPMs Increased. When I landed I saw streaks extending down the carb float bowl and it was loose to the touch. Had BJ's rebuild the carb, and I guess this was not an unusual finding. Kinda spooked me a bit!

I don’t think you are too far off the Mark. My CarbonCub has a different carb but acts the same way at WOT. I will see at least 150 temp drop on egt when fire walled. This is George’s economizer at work
sounds like your new jetting is different
if you can find someone who really knows carbs and jets and how to set a carb up it’s money well spent
jim

Hi everyone

Thanks a lot for your valuable input. I will check air filter / filter box / cable controls / float bowl and rpm drop at 2400 rpm with carb heat on and full rich mixture.
Airfield elevation is 2150 ft, but glaciers are typically 8'000 - 11'000 ft. I don't want to go too high during the initial hours of the engine (I try to remain > 75% power)
We use the round air filter with this sticky/glue-ey filter material. It is still the same model as before the engine change.
Thanks also for the carburetor manual. very helpful. I will go along the troubleshooting list and see if I find something.

Maurice

Any chance in trying your old carb?

I had a Marvel Schebler fall apart on approach to Pecos. Had no idea what went wrong until I got the carb off (78 years old at the time; 98 degrees on that ramp). Screws just came loose.
I use Strombergs - most of the screws are safetied.

On your problem - the accelerator pump puts a slug of fuel in there any time you advance the throttle. That has to drop the EGT, no?

Is it possible your main Jets too small or partially blocked on carb and your engine is going lean of peak at full throttle. I have my Avstar jetted to 14 gph at full throttle for additional cooling.
The fact that you are not making power as you think could be the problem

Thanks to all who contributed with their hints. After optimizing the idle mixture and inspecting carb heat box, air box etc. the problem was not solved. when applying full throttle at higher elevations (8000 ft above) it still felt as if power was missing and rate of climb was below expectations.

the old carb was sent to overhaul in the meantime gone. according to the technical documents I have the old one was a Marvel Schebler 10-3678-32, which is the same model as the new one.

after some discussions the engine overhaul shop recommended to exchange the carb with another model, the Marvel Schebler 10-4910-1, as he said he had problems with the 3678-32 in one other occasion already. I just read the -32 has en economizer jet, however I found zero documentation about the 10-4910-1. I have not tried it yet, it will probably be installed later this week or next week.

What is your opinion, anyone having experiences or knowledge about the differences between these two carburetor models? Thanks again for your help, really appreciated.

Maurice

If you have the economizer jet that dumps lots of extra fuel in at WOT, your mixture will be very rich causing the EGT and power to be reduced; this is what it was designed to do for low altitudes and hot days to keep engine temps down.

You are flying in cold, high altitude- you are flooding the engine at WOT, pull the mixture out a bit. It might take a bit more than you leaned the previous engine- possible the economizer jet is a little more open than the one in the previous carburetor.

The more I read the more it sounds like subtle differences in the carb setup, and it requires different mixture settings than you used in the past. Each engine and Carborator has it's own personality, and sometimes we need to relearn how to adjust it for best operations.

A last thought: If plugs are looking normal, and temperatures are in parameters in cruise, and the only issue is power loss from excess fuel in the WOT situation, it really sounds like the economizer jet is a tad more rich than you are used to. Lean to desired and fly.