Thorp Air Command - T18.net

Does anyone have that model or similar model used on a 290? I don't want to have my rebuilt if it's just the wrong nozzle size.
MA3-spa
I think spruce shows a 10-3565 model but not my 10-3364 model.

The problem has been present since I bought it, the previous owner said it always did it as well. I pulled the carb down last year and everything looked good. All the passages were clear when I sprayed them with carb cleaner. Float height was as per service manual.

I'm getting lean bog past 2500 rpms and on take off with the last 1/4" throttle. In flight I tested with my jpi and at 2500 rpm cruising at 4500 ft Full forward mixture only gives me 120* rich of peak for my leanest cylinder. My rear 2 cylinders are hot in climb and both run 50-70* hotter than my fronts. I fixed the inbalance between 3/4 with new baffling. I think it's either a carb air distribution issue or just being too lean on climb. In cruise the egt/cht rise sharply if I push it to 2600 rpms and it won't even make it to 2800 rpms which is redline.

Round trips over an hour usually pull 7gph on full rich mixture at 3500-4500 ft altitude.

950$ for new carb swap, nozzles are about 150$. I do'nt want to have an overhaul to find out that my part number is too small and I have to buy another carb anyways. The same time I don't want to spend money on a new nozzle if the carb is just worn out. Everything looked in decent shape visually when I tore it down last year but I'm not an aviation carb guy.

I think a continental 300 carb also works and i heard is has a better egt spread than the ones for the 290.

If anyone has a carb I can swap to see if my problem is better I would appreciate it or if someone has the next larger nozzle for my carb. I can get it at Ky dam and mail it back or I would pay shipping to have it mailed to and from me.

Thanks!

Jeff

Maybe I'm missing something, but I think you are too rich. I wouldn't lean until break in is over, but it sounds like you might have just had the break in. Have you tried to lean while it is bogging? I don't have any experience behind an O290, but shouldn't the burn be much lower?
Cubes

I think Binder may be right. My O-290G can suck down 9+ GPH at full throttle, leaned for best power at low/moderate altitude, although I run a MA4SPA from an O-320. I think fuel flow is a matter of cubic inches & RPM more than carb size for a fixed pitch O-290.
The Golden Rule is 1 gph fuel burned per 10 Horsepower-hour, so 100 HP requires 10 gallon per hour flow rate.

_________________
Scott Emery
EAA Chapter 326
T-18 N18TE

I just noticed your comment about running a gravity feed fuel system in the other thread about tank vents.
There are many posts in the Thorp archives about gravity fuel system requirements, which include the size & routing of the fuel line. You should research that, and also make sure the little filter in the carb inlet is clean.
Anything that slows the fuel flow to the float bowl can cause lean operation at full throttle & high RPM.

_________________
Scott Emery
EAA Chapter 326
T-18 N18TE

This issue of bog on full throttle was present all this past year and the previous owner said it had always done it the 4 years he owned it. When I pulled apart the carb and removed all the nozzles last year everything inside looked good. No worn shafts, needle and seat had zero wear, float height by the overhaul manual, all passages easily passed carb cleaner when sprayed. No worn or deteriorating seals (although that doesn't mean they can't still leak).

I don't remember seeing an aerated (peppered) main nozzle when I had it apart. I didn't pay much attention but I know that caused some issues with atomization of fuel and was a fix that marvel said needed to be performed when the carb was changed to a 1 piece venturi which my carb has. Maybe I just have poor distribution on the rear cylinders which is causing higher temps and them to lean out sooner than the fronts.

So I don't think it is a break in related issue. On that note I didn't even burn .5 qt of oil in the 4 hours of flight this past weekend. Running it 2500 rpms which is over 75% for my altitude. I had it set to 6qts on the line and after 4 hours it is sitting just enough below 6qts to notice a space between the level and the dipstick. My temps have settled down quite a bit as well. I have 8 hours on the engine so I think it's close to being broken in. On the hot days I thought leaning it improved when on the ground but when I'm in straight and level flight using my JPI monitor I hit peak EGT quite fast when I start to lean at 2500 rpm. When I try to lean to 125* rich of peak I can only get 120* rich of peak on my leanest cylinder and I'm only at 2500 rpms not the 2600 continuous or the 2800 5 minute rating this engine has. The high engine temps on climb also suggest I'm too lean for cooling.

Shipchief: the fuel screen and gascolator screen was clear last year as well. Just checked and cleaned gascolator upon this rebuild again to make sure. There are 2 tests performed for fuel flow on this airframe in my log books. One performed in the last 10 years it shows 25gph fuel flow at the hose at the carb. That is without any ram air effect on the vent line. I would think that is double my required fuel flow for this engine so I should be able to manage it. Although now that I'm thinking about it at a high AOA I could see a reduced fuel flow. It doesn't explain level cruise though.

The bog also worsened slightly after rebuild. I'm getting noticeable more power as my take off rolls are quite a bit shorter than before and my climb is much faster. So if I did gain a few hp that would increase my fuel requirements as well.

I guess I could get an inline electric automotive fuel pump and plumb it inline and see if that changes anything. That would rule out a fuel starvation issue if the static drain of 25gph isn't enough and it's a pressure type thing. I would think if it was pressure based I would have a worse issue when my tank is low and it was the same when tested at less than 10 gallon in the tank as it was with 28 gallon.

I appreciate all the help. Keep the great info coming!

I have an 0-320, had the same leaning problems, could not touch the mixture below 5000 ft, it was lean on takeoff, lean on climb out and you didnt think about leaning it out until bout 6500ft and then you could not lean much at all. Pulled the main jet and opened it up a tad, problem solved. I have to lean on the ground which is normal, and I can lean it back as needed at altitude just as it is supposed to be I try for about 1350 on the CHT and all is well. I can get a fuel burn of bout 7 GPH at cruise at altitude. I always thought that maybe my Ram air could play a role in the mixture issue as my manifold pressure stands up real good at altitude, who knows. The bird has been flying for 10 years, opening of the main jet fixed the lean situation I had. Dan

Since I'm most likely still using the non-atomizing old nozzle maybe it would be my first test to open up the fuel jet. Is this nozzle a stepped nozzle or just a straight bore? I can look but I do know they recommend against drilling out a stepped bore nozzle because it can really mess with other throttle settings. If it's a straight bore then it would be easy to open it up and check. If it causes me issues I need a replacement nozzle anyways.

Oh wait, just noticed you are 0320 so you are probably using an ma4 instead of my ma3 so the main nozzle question wouldn't answer my stepped nozzle question.

Thanks!

Are you running a mechanical or digital tach? Mechanical tachs tend not to be very accurate.

If you are losing power because you are too lean then you should be able to climb to an altitude where you can get close to max rpm.

It is possible you may have too much prop for the engine. Static rpm and manifold pressure are good ways to check how hard the engine is working. Too much prop will lug the engine and bring your manifold pressure up just like taking too big of a bite with a constant speed prop. I believe conventional wisdom says if you are turning 2600 rpm then you should pulling 26" or less MP (i.e. don't over square it). High MP will also drive up temps.

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Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson

A pull from the Marvell Carb manual: http://www.insightavionics.com/pdf%20fi ... Manual.pdf
f. BACK-SUCTION TYPE ENRICHMENT SYSTEM, (Small MA-3, MA-4 Carburetors, Ref.
appropriate figure for a particular model). – The back-suction mixture control varies the pressure
in the float chamber between atmospheric pressure and a pressure slightly below atmospheric. This
pressure variation is accomplished by using a control valve located in the float chamber vent line.
The float chamber is vented to the low-pressure area near the venturi through a back suction
channel. This lowers the pressure in the float bowl. When then mixture control is in the rich
position, the vent valve is open and the pressure in the float bowl is raised to essentially the
atmospheric pressure, and a differential pressure exists across the main metering jet. This causes
fuel to flow out of the discharge nozzle. When the mixture control is moved to lean, it closes the
vent valve, and pressure in the float chamber is decreased to a pressure that is essentially the same
as that of the discharge nozzle. This decreased pressure differential decreases the flow of fuel.
So I'm wondering if your MA-3 has a leak in the back-suction control valve or a clog in the vent, because nobody really understands what it does, so it is usually ignored?
The manual also has a good troubleshooting guide, so It's worth reading the whole thing.

_________________
Scott Emery
EAA Chapter 326
T-18 N18TE

Mechanical tach. I think the shop has a digital one so I'll check my mechanical with the digital one to see how far off mine is.

That being said, the speed verses rpm at the altitudes tested should be about right for my plane. If anything my 140mph IAS is a little slow compared to the numbers I gathered from other 0290. That's not apples to apples of course.

I run a 68x68 Demuth wood prop. A few others with the 290 have been using the m76 cut to 68x73 with much higher airspeed. I woudln't think my 68x68 is too much prop for the 290 but i'm not an expert. I don't have a manifold pressure gauge to check pressure.

Since it's still in break in I haven't tried climbing really high to see how the leaning goes. I know I can get 120* rich of peak (at full mixture) at 4500ft. Once the wind dies down I'll climb it higher and see.

shipchief,

That is the manual I used when checking the issue a year ago. Since it occurs at the last 1/4" of throttle application I think there could be an issue with the economizer jet. It is the portion that is supposed to add additional fuel at full throttle for take off and climb. Since I can't get into that area of the throttle then maybe I'm not getting any fuel through that channel. I guess that could be aggravated by the leaner main nozzle as well.

A problem with the back suction circuit sounds like it would be opposite what I'm experiencing although the vent tube portion would be right on the money. If the vent is clogged I would be running lean since the pressure drop wouldn't occur to enrichen the mixture.

Looking at the economizer tube there are a few ports from the single tube. When I use solvent and air to clean that out it could all be going through 1 or 2 vents but not through all of them thus leaving an important one clogged. I'm going to use very thin safety wire though all the ports to make sure they are patent when I go through the carb this time. The same would go for the bowl vent.

You may want to think twice before putting anything into the nozzle that could scratch the inside of the jet / discharge nozzle. From my understanding, the inside finished is polished. If I were going to put anything into it, I might use very thin nylon fishing line as to not scratch anything. Going back (a long way) to when I was in A&P school, I remember that this discharge nozzle is directional, so you want it to send the fuel down the throat of the carb in the direction of the airflow. If the nozzle is not clocked correctly it could do funny things to the process including affecting the proper atomization of the fuel . You may want to call Precision Aeromotive in Washington, They don't work on the Marvel carburetors anymore but were the manufacture for many year and a wealth of knowledge. Tempest I think has purchased the rights to build the carburetors now, they may be willing to put it on there flow bench and check how it is flowing ? Most A&P's now will not go very far into the carburetors now because the need for a flow bench to ensure it is flowing correctly.

Phil
118BC

I am no expert either. It was just a couple of thoughts I had. First step is to get your carb right.

When I was exploring prop options, Catto had me climb to 8000 feet density altitude and collect numbers. I have only ventured that high a couple of times in my 25 years of flying and it surprised me I could get max rpm up there (I had never tried it before). My 160 hp O-320 is not working hard (I cruise at 22 inches and 2400 rpm, for the test I was able to get 2690 with 23.5 inches), even so, they recommended I go from the 68x74 I currently run to their 68x70 and predicted an increase in speed even with the 4 inch drop in pitch. The point I am trying to make is comparing one prop to another can be a great starting point but things can vary greatly between manufacturers and materials.

A manifold pressure gauge is helpful but not necessary for fixed pitch, especially if someone can give you good advice on static rpm numbers. Sorry about hijacking your thread.

_________________
Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson

I would focus on fixing the carb fuel flow problem first, and then deal with any other problems if they still exist.

The 10-3346 and the 10-3565 carb are identical except for the main nozzle size. The 3346 uses part number 47-719 nozzle and the 3565 uses part number 47-728 nozzle. My experience, along with others, is that the 47-728 (biggest nozzle available for the MA3-SPA carb) is too small to flow enough fuel when a 290 is installed on a fast homebuilt with more airflow potential than when installed on a Cub or Tri-pacer. The only option is to drill or ream the nozzle to a bigger size. It has been done many times before and works just fine. I would recommend getting a reamer to do a little more accurate job.

The 47-728 nozzle has an id of .0935" (#42 drill size) and needs to be enlarged to .0995" (#39 drill size). This worked good on my 290D and was maybe even a little small when changing cylinders to the later D2 cylinders with bigger face intake valves that flowed a little more air. If you want to be real picky, after reaming you can run a string through the nozzle with a little valve grinding compound on it to polish the sharp edges on both ends of the nozzle.

As a side note, some RV guys have to drill the nozzles on the 320 engine also, depending on installation.

_________________
Bernie Fried
N18XS

True Bernie. This time of year I always think about a larger jet for my O320.
Cubes