O-320 Running too lean

[Rybo]

It's been about 4 months since I purchased my RV6A with O-320b2c and fixed pitch sensenich. Now that I've fixed my non-functioning EGT gauge, I've come to realize my EGTs are very high on takeoff and fuel flow is too low, about 10gph at sea level. I can maybe get it to to close to 11 once I get the rpm and speed up once airborne. Clearly I'm running way too lean. I can really only get it very rich of peak above 7500'.

Fuel pressure is good, gascolator screen is clear, throttle and mixture cables have proper travel. I have the 10-5217 carb which it seems may be the wrong choice for my engine.

Is there anything else to check prior to switching out to a 10-3678-32 which I've read might cure this issue?

Is drlling out the main jet likely to achieve the same end result?

Thanks,
This forum is a godsend for a new owner like me.

 

[D-Dubya]

Well, I'm more experienced with fuel injection systems than carburetors (at least on aircraft), but I do know that you don't want to drill out the metering jet. Those are precision drilled orifices that we can't easily replicate with basic tools. You may be able to purchase a larger metering jet and install it. Find a manual for that type carburetor and see if and how it can be done.

As I recall, there should be an enrichment valve at WOT that should come into play as well. If it's not working properly, that could also be causing you problems. Maybe someone with better knowledge than me on this will chime in.

Also, Paul Dye made a video out there somewhere that talks about carburetors and fuel injection systems. I think it was one of the Kitplanes FWF videos.

 

[Avanza]

Your engine O-320b2c is 160 hp and standard carb is 10-5135.

The carb 10-5217 is for the O-320-D2G, O-320-D2H, O-320-D2J, O-320-E2H,
O-320-H2AD, O-320-A2C, O-320-D1AD, O-320-
E1J, O-320-E2D, O-320-E2G, O-320-E3H, O-320-
D1A, O-320-E3D, O-320-E2A, O-320-D3G engines.
Some of them 150 hp and some are 160 hp.
So your carb may work for your engine.
I would look for a air leak on the induction side.

Good luck

 

[Kyle Boatright]

I had an over-lean condition with a new replacement carb that was the identical part number as the one it replaced. Sent it back to the factory twice. First time they adjusted it to the very top of the fuel flow spec for that carb. Second time, they drilled the carb jet two sizes oversize. Now it runs too rich, but that’s something I can manage from the cockpit.

As explained to me, there are 3 different carbs for O-320’s, each of which have modestly different fuel flow characteristics. The nice folks at the Marvel Schleber factory can probably talk you through your options.

https://msacarbs.com/contact-us/

 

[Taltruda]

Your engine O-320b2c is 160 hp and standard carb is 10-5135.

The carb 10-5217 is for the O-320-D2G, O-320-D2H, O-320-D2J, O-320-E2H,
O-320-H2AD, O-320-A2C, O-320-D1AD, O-320-
E1J, O-320-E2D, O-320-E2G, O-320-E3H, O-320-
D1A, O-320-E3D, O-320-E2A, O-320-D3G engines.
Some of them 150 hp and some are 160 hp.
So your carb may work for your engine.
I would look for a air leak on the induction side.

Good luck

That carb is too lean for the efficient RV airboxes… switching to the 10-3678-32 will fix your problem.. you’ll get 14gph on takeoff.

 

[YellerDaisy]

Fuel pressure is good, gascolator screen is clear, throttle and mixture cables have proper travel.

There is also a screen inside the carb (at the fuel inlet)... an easy thing to check if you haven't already.

 

[lr172]

Well, I'm more experienced with fuel injection systems than carburetors (at least on aircraft), but I do know that you don't want to drill out the metering jet. Those are precision drilled orifices that we can't easily replicate with basic tools. You may be able to purchase a larger metering jet and install it. Find a manual for that type carburetor and see if and how it can be done.

As I recall, there should be an enrichment valve at WOT that should come into play as well. If it's not working properly, that could also be causing you problems. Maybe someone with better knowledge than me on this will chime in.

Also, Paul Dye made a video out there somewhere that talks about carburetors and fuel injection systems. I think it was one of the Kitplanes FWF videos.

actually the mfg just drills them with the same kind of drill bit we use. They sometimes put an angle at the entrance and/or exit with a countersink, but not universal and that angle makes very little difference in the flow rate of the jet. Yes they use precision drill bits, but no more precise than the numbered bits that we use. Hot rodders have been drilling out jets for years. The holley jets have specific angles at the entrance and exit, but highly doubt the marvels use that. I would just drill oiut the jet. I had a 3678 and still need to go up one or two drill sizes. Then like Kyle, it was too rich, but just use the red knob to control.

TO fuel flow with an FP prop is much lower than one would expect, due to the low RPM. Most FP props are climbing at around 2500 RPM. 2500/WOT on a 320 B/D is around 90% power and Lyc says 12.5 GPH is required at 90%.

 

[edstepec]

I had the exact same issues on my RV6A. The O320 came off a Cessna 172 that cruises at 100 knots. Put that in a FAB and 160 knot aircraft, very lean conditions. Upgraded carb to 10-3678-32 and solved all my problems.

 

[BH1166]

I have 750 hrs running same carb 6A. I too have egt numbers I thought high on TO. I have all these hours in Savvy Aviation’s database. I talked to MASCARBS , nice guy. I also looked honestly at take off rpm/OAT/Altitude. Then I looked at the Lycoming charts on fuel flow per data mentioned. Perfect no, dang close to specs, yes. I got Savvy to look at things in the database of my flights, only once flagged as fuel flow lower than desired. I run lean of peak, my egt at peak high1400’s, one low 1500’s, lean of peak mid 1400’s. Hold on, it gets better. I always cruise 2600+, yep, engine going to explode. One more point of difference…… 93 octane no lead/Alcohol. I fly 100+ a year, I borescope annually. Savvy also gave credence to FAB pushing a LOT of air into carb, potential exacerbating things. MSCARBS did not offer anything but you can take the carb off ,send to them to do magic( likely change jets). You could spend $2000 and buy new carb flows higher, sell old one. Here is the thread where I was looking to fix a perceived by me issue. Take the fuel savings and go fly…..IMHO.
https://vansairforce.net/community/showthread.php?t=216796

 

[BH1166]

Most recent flights reports…….received today

 

[Taltruda]

I have 750 hrs running same carb 6A. I too have egt numbers I thought high on TO. I have all these hours in Savvy Aviation’s database. I talked to MASCARBS , nice guy. I also looked honestly at take off rpm/OAT/Altitude. Then I looked at the Lycoming charts on fuel flow per data mentioned. Perfect no, dang close to specs, yes. I got Savvy to look at things in the database of my flights, only once flagged as fuel flow lower than desired. I run lean of peak, my egt at peak high1400’s, one low 1500’s, lean of peak mid 1400’s. Hold on, it gets better. I always cruise 2600+, yep, engine going to explode. One more point of difference…… 93 octane no lead/Alcohol. I fly 100+ a year, I borescope annually. Savvy also gave credence to FAB pushing a LOT of air into carb, potential exacerbating things. MSCARBS did not offer anything but you can take the carb off ,send to them to do magic( likely change jets). You could spend $2000 and buy new carb flows higher, sell old one. Here is the thread where I was looking to fix a perceived by me issue. Take the fuel savings and go fly…..IMHO.
https://vansairforce.net/community/showthread.php?t=216796

I find it hard to believe you are flying lean of peak.. with a carb. Sure maybe one cylinder might go LOP, but to be truly lean of peak, you have to get ALL cylinders past the peak on the lean side. By the time you get the last cylinder past the peak, the first couple are so far lean, that they stop firing.

 

[petehowell]

You can make LOP work!

Here is my O-320/-9A on a trip to South Bend this weekend. Level at ~7500, Same carb as OP(with a drilled jet), cruising LOP. I had about 1/4" of carb heat on and the throttle was just off WOT. Can get the mixture distribution evened out by playing with those levers. I fly LOP almost all the time.

Over 32MPG here with a nice push - on mogas, too!

I find it hard to believe you are flying lean of peak.. with a carb. Sure maybe one cylinder might go LOP, but to be truly lean of peak, you have to get ALL cylinders past the peak on the lean side. By the time you get the last cylinder past the peak, the first couple are so far lean, that they stop firing.

 

[patterson]

Ream the jet

I had the same problem on my factory 0-320 D1A. It came stock with a 10-5217 carburetor. When I had the engine IRAN'd at Lycon, I paid for porting and polishing the cylinders.This yielded an increase of 16 hp (per dyno). The engine then had 1400-1450 EGT readings on climb out due to more power, same amount of fuel. I Reamed the Jet from .096 thousandths to about .098. Fuel flow slightly increased, temps on EGTs came down about 100 degrees. I never tried running LOP. Fuel flow at altitude 10k was about 7.2 gph at 75 degrees ROP

Use a drill press and a reamer. ymmv

 

[BH1166]

I find it hard to believe you are flying lean of peak.. with a carb. Sure maybe one cylinder might go LOP, but to be truly lean of peak, you have to get ALL cylinders past the peak on the lean side. By the time you get the last cylinder past the peak, the first couple are so far lean, that they stop firing.

Yep, I understand, but per egt/fuelflow/gas pump, she is LOP. I run 6.9-7.4 gal per hour generally at my usual altitudes 4500-8500. Here is some data just gathered hunting for info like original post. 2nd line shows full rich, first to peak #1…… difference between #1 and last pretty tight. * Some call this the Ron Rivera test. AP on, level 3500’, fullrich, then slowly lean recording rise and drops. Data tells the story…… me thinks.

 

[dvalle]

Just seeing this and had same issue right after an OH carb from Marvel-Schebler, O-320 RV6A. Pulled my hair out thinking it was my fault, numerous cowl removals and test flights, it wasn't. Had to remove and send the new carb back to them. Fixed!

 

[Taltruda]

Yep, I understand, but per egt/fuelflow/gas pump, she is LOP. I run 6.9-7.4 gal per hour generally at my usual altitudes 4500-8500. Here is some data just gathered hunting for info like original post. 2nd line shows full rich, first to peak #1…… difference between #1 and last pretty tight. * Some call this the Ron Rivera test. AP on, level 3500’, fullrich, then slowly lean recording rise and drops. Data tells the story…… me thinks.

Wow, I didn’t think that was possible! So you cruise at 5.6gph and it isn’t rough? I gotta say .6 spread is better than I thought..

 

[BH1166]

Nope no roughness…. I usually see a 20-40 rpm drop when egt start down. Maybe I’ll take the Bose A30 off next time and listen el natural, but feel nothing for past750 hours over 6 years.

 

[n82rb]

Egt values mean nothing to determine lean vs rich. Lean vs rich is a math problem. Fuel flow tells rhe story. The only thing an egt temp tells you is how close to the exhaust flange probe is. The actual number means squat. With egt your watching spread between cyls. Fuel flow and cht will tell the story about mixture.

 

[YellerDaisy]

Egt values mean nothing to determine lean vs rich. Lean vs rich is a math problem. Fuel flow tells rhe story. The only thing an egt temp tells you is how close to the exhaust flange probe is. The actual number means squat. With egt your watching spread between cyls. Fuel flow and cht will tell the story about mixture.

I've heard Mike Busch state the same - I admit that I do not understand and would really like to.

I did a fair bit of work to my last engine (O-320) and could run similarly to Butch (though mine would be a bit rough at 25 LOP and more rough at 50 LOP). That involved sealing the air induction, a little tuning of the air around the FAB, Ross Farnum's intake 'manifolds', and so forth. As noted by Butch, I could then watch each cylinder EGT rise to peak and then fall to 25-50 degrees (as mixture was leaned). RPM would drop, power would drop (feeling, not measured), fuel flow would drop, CHT's would drop, oil temp would drop (slowly). When I can watch the EGT's peak and then drop, I can't wrap my head around why this is irrelevant and not actually LOP.

I understand that the temperature at the EGT probe is not the same as inside the cylinder (it has cooled) but given the above description, why is it not LOP?

Edit: I found a photo of the above. Apologies for the thread drift.
In the photo, you can see that all four cylinders are LOP - 34 to 45 degrees LOP. Fuel flow is 5.7gph at a DA of 12,555'.

 

[avrojockey]

I find it hard to believe you are flying lean of peak.. with a carb. Sure maybe one cylinder might go LOP, but to be truly lean of peak, you have to get ALL cylinders past the peak on the lean side. By the time you get the last cylinder past the peak, the first couple are so far lean, that they stop firing.

Usually WOT, you can get a carb to peak all cylinders as good fuel injection as the balance between cylinders is good. When you start modulating the throttle closed it drastically changes the balance.

I'm like Pete but simply WOT...my GAMI spread is about 0.1-0.2 GPH. Of course this is on newly overhaul carb and engine, so it works near flawless

 

[avrojockey]

Egt values mean nothing to determine lean vs rich. Lean vs rich is a math problem. Fuel flow tells rhe story. The only thing an egt temp tells you is how close to the exhaust flange probe is. The actual number means squat. With egt your watching spread between cyls. Fuel flow and cht will tell the story about mixture.

I'm not sure what you mean here...EGT has a lot to do with lean vs rich. All modern EIS and EFIS reference EGT when leaning, and the old dogs used to watch the color of exhaust stacks when leaning

 

[penguin]

My first RV had an E2D, at overhaul I installed 160hp cylinders and suffered the same high temperatures. Flying at 6000ft it was clear I was already very close to peak EGT.
I don't remember the exact size of the main jet but enlarging it by 2 number drill sizes brought me well rich of peak and brought the temperatures down somewhat - take it easy, one drill size at a time.
I didn't finally get the temperatures fully under control until the cooling air mass flow was increased by adding cowl louvres.
I found it was possible to operate LOP with worthwhile fuel savings.
Pete

 

[lr172]

Egt values mean nothing to determine lean vs rich. Lean vs rich is a math problem. Fuel flow tells rhe story. The only thing an egt temp tells you is how close to the exhaust flange probe is. The actual number means squat. With egt your watching spread between cyls. Fuel flow and cht will tell the story about mixture.

That is just plain wrong. EGT is directly correlated to mixture. True that absolute values are worthless, but the differential values are DIRECTLY related to mixture. EGT will consistently peak at 14.7:1 AFR (though the actual temp at peak can vary) and the differential values lean or rich of that point are very consistent once your factor out differences in ambient temps. To be fair, there are several things that will change the EGT values, such as timing advance, compression pressure, etc. However, they ALWAYS peak at 14.7 and that is proven science. CHT is NOT consistently related to mixture and countless things can move CHT around other than mixture. CHT is simply NOT a reliable tool for assessing mixture. If it was auto manufacturers would use head temp to adjust mixture instead of 02 sensors. While EGT is not as precise or fast as O2 sensors, it is the next best option.

 

[Rybo]

Well upon investigating further I found my current carb was leaking fuel at the accelerator pump gasket so I went for an overhauled 10-3678-32 from AS. After satisfactory ground testing, I took it for a test flight. Fuel flow at sea level takeoff is a good 1.0 to 1.5 gph higher and my EGTs at sea level are about 200 degrees rich of peak now instead of right at peak.

 

[n82rb]

That is just plain wrong. EGT is directly correlated to mixture. True that absolute values are worthless, but the differential values are DIRECTLY related to mixture. EGT will consistently peak at 14.7:1 AFR (though the actual temp at peak can vary) and the differential values lean or rich of that point are very consistent once your factor out differences in ambient temps. To be fair, there are several things that will change the EGT values, such as timing advance, compression pressure, etc. However, they ALWAYS peak at 14.7 and that is proven science. CHT is NOT consistently related to mixture and countless things can move CHT around other than mixture. CHT is simply NOT a reliable tool for assessing mixture. If it was auto manufacturers would use head temp to adjust mixture instead of 02 sensors. While EGT is not as precise or fast as O2 sensors, it is the next best option.

read what i wrote, EGT Value means nothing. the only thing that matters with egt is the peak and the spread between cyls. and spread only means something if the probes are installed right. if they are not, there will alwas be a spread between cyls.

 

[lr172]

read what i wrote, EGT Value means nothing. the only thing that matters with egt is the peak and the spread between cyls. and spread only means something if the probes are installed right. if they are not, there will alwas be a spread between cyls.

Well, you are welcome to your opinion and I am not going to argue with you.

 

[RV6_flyer]

Well upon investigating further I found my current carb was leaking fuel at the accelerator pump gasket so I went for an overhauled 10-3678-32 from AS. After satisfactory ground testing, I took it for a test flight. Fuel flow at sea level takeoff is a good 1.0 to 1.5 gph higher and my EGTs at sea level are about 200 degrees rich of peak now instead of right at peak.

Rybo:
Saw that there are new posts in this thread so I am revisiting it.

I also have the same B2B you are using but with a Constant Speed prop. I have always had the 10-3678-32 carb that you are now using. I have 3,583.8 hobbs hours and 26-years of successful operation.

With the LASAR electronic ignition, I can run LOP BUT the richest cylinder is only 7 degrees LOP when the leanest one starts missing. Yes some carb engines can run LOP but not as well as the injected engines.

With the constant speed prop, I get similar numbers like PeteHowell has in post #12. At 2,000 MSL yesterday, 22" MAP, 2,100 RPM, 5.5 GPH, 122 KIAS / 135KTAS. @ 22" and 2,100, I am always 6 GPH or less when leaned to peak EGT.