Interesting Zenith carb results

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Marc W

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Since I recently got my Thatcher CX4 flying I have been tinkering and tuning. The Zenith carb was dripping a lot of fuel after shutdown so I replaced the needle and seat. I had to take the airbox off to get the carb off and that exposed the bottom of the engine so I changed oil while I was at it. Plus I had some oil leaks to fix, etc. The list kept getting longer and the whole thing turned into a 3 week project.

Partly inspired by the March Kitplanes article on the Ballenger A/F monitor and partly because I always wanted to try an 02 sensor, I installed an NGK/NTK wide band A/F ratio monitor while I was at it. https://www.ngk.com/product.aspx?zpid=41326 My Zenith carb has the mixture adjustment on the bottom of the carb and I also made an apparatus to allow the mixture to be adjusted from the cockpit. I have made one flight with the 02 sensor and the cockpit adjustable mixture control.

NGK recommends recalibrating the sensor for every 3000' change in altitude. Calibration requires powering up the sensor and letting it hang in free air for a few seconds. My field elevation is 5200' MSL and the local mountains top out at a little over 11000'. Before I installed the sensor, I took it up the mountain in the car and calibrated it at 9000' since that should give good readings from 6000' to 12000'.

The first results are interesting. My engine is a Hummel 2180 with the Zenith 14992 carb. The ideal air fuel ratio is 14.7 to 1, where all the fuel and all the oxygen are combined. Maximum power comes at about 12.5 to 1. My engine idled the best at about 10.5 which is a little on the rich side but it was cold. Before I flew, I set the mixture on the ground at 2400 RPM to 12.5. Throttling back, the mixture went rich below 2000 RPM to about 9.5.

After take off, the mixture turned out to be slightly rich and I readjusted it to 12.5 during the climbout to 8000'. I played with it a bit at 8000' and found that the engine did not want to run much leaner than 12.5. Peak RPM occurred at 12.5 and RPM would drop around 50 RPM as the mixture got richer but it would run happily on the rich side. I then climbed to 12,500' and was amazed to see the mixture held at 12.5 all the way up. When I throttled back to descend the mixture richened below 2000 RPM to about 9.5 just as it had on the ground. On the ground, at idle with a hot engine, mixture went to a little over 11.

I didn't know what to make of the results. Everybody knows that mixtures get richer as you climb and you need to lean at altitude. It's in all the books! Mine didn't! I don't know whether to believe that gauge or not. I told another pilot who flys a Sonex with the Aerovee engine about it today and he said that he had read somewhere, that years ago, many people installed Zenith carbs on their airplanes because they hold a constant mixture through changes in altitude.

So, my preliminary results indicate that you don't need a mixture control on a Zenith carb! Set it and forget it! Wouldn't that be great! Further testing to follow!
 

plncraze

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As I understand air cooled engines they are really partially cooled by fuel. Aircraft carbs are designed to dump even more fuel than asked for during high power operation through the enrichment valve. If 14.7 is stoichiometric then it is not surprising it has to run richer. There are other things going on in the engine besides mixture and that means a richer mixture. Thanks for sharing! It's cool to see what these little engines will do.
 

Pops

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On my 1835 cc VW engine with a Zenith carb I change the mixture screw 1/2 turn between summer and winter, ( May and Oct ). Field elevation is 650'. Engine always ran good at any elevation without anymore adjustment. Have had it to around 8k when working strong lift. No cockpit mixture adjustment. Completely trouble free. For start put choke full on, flip 5 blades, make mag hot and choke off , start on first blade 95% of the time. When below 40 degs temp, give it 2 full strokes of prime , flip 5 blades, choke 1/2", mag hot and start on first blade and keep running with the choke as required until warm.
 
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poormansairforce

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Since no one else had a specific response I'll take a crack at it.

At altitude the density drops which means less fuel needed. But what no one points out is that the pressure drops as well. Pressure difference is what drives air through an engine and fuel through a carb so it depends on the carb set up.

When I built my Minimax in the mid 90s I realized I could take the slide carb on the 340 and port the bowl vent to the carb throat to get altitude compensation and if I added some tubing and a needle valve to the vent system I could have mixture control. Never did because I didn't fly that high and, like Pops, I had a system of jet changes that worked.

When the internet happened I found out the some (all?)of Zenith carbs have been doing this for a long time. The main mixture control is done differently but the bowl vents to the throat. The vent pressure is controlled by an economizer system that reduces bowl pressure to lean the mixture during mid throttle by exposing the bowl to manifold vacuum which lowers the fuel flow rate. That system is turned off at full throttle so the mixture gets richer. The vent pressure drops as well as the altitude increases pushing less fuel into the throat.

I think many set ups are inadequate since the carb vent is using cowl pressure. A good tuner could make a zenith have the same fuel curve/efficiency as an EFI. Hope that helps you out.
 
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TFF

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First, it’s a tractor carb; it’s just going to work! One of the Marvel versions is on our 1966 Lincoln welder with Bosch mag; Continental flat head.
I would look at two things. One is exhaust and placement of sensor. If it’s too short it may skew the numbers. Also what is best guess on what the mixture change should be. I’m at 250 ft MSL. To 5000 is a big change. 5000 to 10000 is not as much from what I’m use to. More of a tweek.
 

Pops

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First, it’s a tractor carb; it’s just going to work! One of the Marvel versions is on our 1966 Lincoln welder with Bosch mag; Continental flat head.
I would look at two things. One is exhaust and placement of sensor. If it’s too short it may skew the numbers. Also what is best guess on what the mixture change should be. I’m at 250 ft MSL. To 5000 is a big change. 5000 to 10000 is not as much from what I’m use to. More of a tweek.[/QUOTE

My 1968 MF 135 farm tractor has a Zenith carb on the 4 cyl Continental flat head gas engine. The carb got dirty and I overhauled it about 20 years ago. Overhaul kit was less than $20. They just work. The Zenith carb from Great Plains on the 1835 cc , 60 HP, VW engine has been just as reliable. And I do like the choke.
 

Pops

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Quick Zenith question. What is the correct number Zenith carb for an 1835cc engine?


Zenith carb numbers are very confusing to say the least. If you buy the Zenith carb form Great Plains for the 1835 you will get the correct one for the size of the engine. We had a thread on this question before and I think someone posted the correct Zenith carb number for the different size engines. But at my age I can't be sure of much of anything :)
 

Marc W

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I'm looking at the CX5, how do you like the CX4?
I think the CX4 is a good design that is probably as simple to build as you will find in a metal airplane. Some of the detail design could be improved and there are some places where I would simplify the construction even more but overall it is a good airplane. One thing I really don't like is the low angle of attack the wing has on the the ground when built as a taildragger. I also do not care for the aileron design. I changed both of those. If you build the nosedragger the angle of attack doesn't matter and the existing aileron design does work so it isn't a big issue. My CX4 is a good performer. It will easily true out over 130 MPH and climb is adequate at the altitudes I fly. I think the CX5 is more of the same.

The Zenith carb is about as simple as a carb can get. Idle circuit and main circuit. Not all models have the economizer feature. Mine does not. I don't know how much the mixture normally changes with altitude. I just know I saw a solid 12.5 from about 6000' to 12,500'. I think that is remarkable performance! Just a little reminder for Pops.
If the Zenith had the cockpit adjustable mixture with fuel shutoff like a MS carb it would be great.
You don't need the mixture control, Pops!

The instructions for the NGK/NTK AFRM say to locate the sensor at least 12" from the exhaust port and to not put it downstream from an exhaust leak. It also should not be near the end of the pipe but they don't specify a minimum distance from the end. I have a 4 into 2 exhaust system. I put the sensor 12" from the port and 11" from the end of the pipe on the left side of the engine. There is a slip joint ahead of the sensor that I wasn't sure about since it is a potential leak. I am satisfied with the results since max RPM is right at 12.5. I will add that I am always suspicious of any instrument and always take their readings with a grain of salt.
 

Dana

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The Zenith on the Mosler in my Fisher had a cockpit adjustable mixture, which was an option offered by Mosler. It was just a u-joint and shaft turning the mixture screw with a knob in the cockpit. Like Pops, I only adjusted it twice a year, once in the spring and once in the fall.
 

Marc W

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I have put a few more hours on the A/F ratio monitor now. I do not use the mixture control. It was very useful to set the mixture initially but there doesn't seem to be much point in constantly adjusting it. I have the mixture set to 12.5 for take off and climb. As I stated in my original post, it will hold 12.5 all the way to 13,500' at full power. It goes richer about a point to 11.5 at cruise settings. I figure that is close enough and I doubt it makes much difference in fuel consumption. I am trying a different prop now but it hasn't effected the mixture at all.
 

Pops

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I think the Zenith carb is a better carb for the VW than the Stromberg carb is for the small cont's. I also like the choke.
 

TFF

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The octane need goes down the higher you go, so there actually may be some leaning available on that. It would be interesting to see what EGT it runs. Rich is great for acceleration and cooling, but once stabilized, you maybe giving up power and fuel burn using O2 instead of EGT It would be cool to have both for the information. What is the EGT of a VW leaned, 1200F?
 

Marc W

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I don't know what EGT's the VW's run. I will ask the man I borrowed the prop from. I think he has EGT gauges on his Aerovee.

I can lean it more but I haven't fully tested that yet. It does seem to run fine between 13:1 and 14:1. The plan right now is to run it as is and see how the plugs look at next oil change. Leaning it further would be valuable for cross country flights where you want to maximize range. I am still in Phase 1 so it doesn't matter much right now. It would be nice if the mixture went leaner at cruise. Maybe the Zenith with the Economizer would work there. As it is, fuel consumption seems to be running 2.5 to 3 GPH so I have nothing to complain about there.
 

Pops

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My 1835 cc, 60 HP, VW engine burns 2.9 / 3.0 GPH at 2650/2700 rpm. About 33 hp at cruise of 80 mph. WOT on climbout is about 3150/3200 rpm with the 60' x 26" prop and a static rpm of 2950/2975 rpm.
 

Marc W

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Good information there. Thank you for that! Even though I have a bigger engine, at my altitude, I probably don't have much more HP than you.
 

Vince Castellano

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I'd like to share a discovery with you. I have a Mosler CB40 with a bottom carb, it has a heater box, piped to the exhaust box and it's manual. I started restoring a 1884 Corvette C4 with crossfire twin throttlebodys. The Aircleaner System was designed for interstate iceing of the TB's. So It automaticaly controls air temp. The Air cleaner housing has 2 outside air inlet doors, that are opperated by a vacumn bellows actuator on each, and thats controlles by a Tempature vacumn switch! when it gets Icy at the TB's The switch opens vac to the outside air doors, closing them off, and the only air source is piped to a box around the exhaust to the bottom of the air cleaner housing. constantly maintaining air tempature! The system is simple and I thought of making one for my Mosler!
 

glenbradley

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As owner of the prototype Thatcher CX5 I have wondered why its engine -Revmaster 2300- does so well at altitude even with a non- leaning Zenith carb. My Youtube video shows it at 10k feet running normally and still climbing 400 fpm. The big wing plus Zenith carb have worked well for almost 900 hours already. If I remember, I turn main mixture jet half a turn for winter/summer but it hardly knows the difference. Our new CX7 which I will test fly soon has same engine and is side by side. I love that engine carb combination. Set it in five min and forget it..... just like a tractor. Smiling.l
Dr Glen Bradley
Thatcher Aircraft Inc.
 
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