Stromberg Carbs

Friends,

The subject here is the most popular light aircraft carb of all time,the Stromberg NAS-3 and 3A. These were fitted to roughly 70,000 aircraft over a production run that lasted several decades. It looks like a very simple carb, and in some ways it is. But in reality, it is a highly engineered design that was produced by the world’s leading manufacturer of aircraft fuel systems.  At the time of production, this was Stromberg’s most basic product. They also made ultra sophisticated pressure carbs that were on the most powerful multi-row radials. This carb comes with an experience pedigree that no experimental carb can come close to matching.

Above, a Stromberg NAS-3 mounted on Dave the Bear’s Wagabond. We finished this aircraft in our Edgewater hangar in 2004, when Dave worked as part of  “The Hangar Gang.”  We used this combination with a 2,700cc Corvair and a Sensenich 64×35 prop to conduct a lot of jetting tests. We were not new to the Stromberg, as our Pietenpol, and our second test mule, Gary’s Skycoupe, both used the same carb. Besides these Corvair powered planes, the carb was also on several certified planes we had at the same time like Grace’s Taylorcraft and Gus’ 120.

When it comes to gravity feed carbs, I like Strombergs  because they have literally millions of hours feeding air and fuel into flight engines. I know them and trust them, and if I had any little issue with it, I’d have a mountain of expertise to draw on, not just other people flying one, but professionals with decades of documentation. I don’t have to think about it, it isn’t a variable. The one limitation on the carb is that it is not suited to use on a plane that requires a fuel pump. Thus, it isn’t good for a 601XL or a 650, but it is a good match for any high wing plane or one with a header tank. (The sole exception to this is the 750 because the factory now recommends that builders use a back-up fuel pump because of the plane’s high angle of attack capability.) I consider the Stromberg a much better carb for the Corvair than any of the other gravity feed carbs like a Monett Aerocarb, Posa, etc. The typical price for an old but functioning NAS-3 is $250. If you shop around, you can find them for half of this.

Above, an overhaulled NAS-3 that went on the Pietenpol of Dave Minsink.

The carb comes in two venturi sizes, but they are interchangeable, so if you have a core from a 65hp with a small 1.25″ venturi, you can replace the venturi with a larger 1.375″ one that came on the 85 and 90hp carbs, and you can rejet the carb to the bigger model. If you wish to have your core rebuilt, the best place we know of is D&G Fuel Systems in Niles, Mich. It is owned by a good guy named Russ Romey. I have worked with Russ over many years, and dozens of Corvair powered planes are flying with carbs that he rebuilt. If you don’t have a core, you can buy a Stromberg outright from Russ.

When Dave the Bear’s Wagabond was finished,  Gus did the original test flights. One of  the things we carefully worked out was the jetting. Here is something not to try at home: We did successive tests to slightly pull the mixture on full power climbouts, looking for a slight increase in rpm, which would indicate the carb running slightly rich to protect the engine from detonation. This was done carefully, and still Gus aborted a take off after reaching 150′ because the engine was hinting at detonation at the combination of timing and mixture  we were trying. Dave kept rejetting the carb until we got what we needed.  From the EGT reading, I think  that the correct A/F ratio at this setting was about 11:1.  

We spoke with Russ from D&G during this testing  and when we were done, he  began to jet the Strombergs he sells for Corvairs this same way. This is what we came to call a “Super Stromberg” to differentiate it from others jetted differently. The Corvair will run with one taken directly off a 65 or 85 Continental, but if you run it hard to get the engine’s full potential, it will be too lean. If you’re close to flying, and you know that you’re eventually going to have the carb redone internally, let me encourage you to do it before you start flying. Dave’s plane ran a whole lot better when we had it finished, and today people can just get their Stromberg to run this way right off the bat. The jetting works on any Corvair from 2,700-3,000cc.

Some models of the carb have a mixture control, others do not. The ones we used in our Pietenpol and the Skycoupe had no mixture. If I were planning on flying over 10,000′, I would have one, otherwise I would use either model.  Most of the light planes that originally came with NAS-3s only had a mixture control as an option, not standard equipment. Although this may sound a little crude for an aircraft carb, the mixture control isn’t a requirement for flying. People logged a lot of happy hours flying J-3 Cubs without a mixture control. 

One of the most common questions asked about Strombergs is their susceptibility to carb ice.  Although it is technically more prone to carb ice, I treat every carb as if it were prone to carb ice and I use carb heat on every plane and utilize it habitually any time the power is reduced below cruise setting. I consider the ice issue a small point compared to the other issues experienced by unregulated flat slide carbs. Sticking slides on planes that are on short final is far more of a risk . The Stromberg’s butterfly throttle plate always opens smoothly, and the carb is well known for holding an adjustment throughout a very wide range of temperatures and conditions. I have flown the NAS-3A on Grace’s Taylorcraft in temperatures ranging from 15F to 105F, and it does this without adjustment. Other than having the filter screen cleaned, the logs show that the carb on her plane has not been worked on since 1974. This is reliability.

Another point to consider is that the carb is very tolerant in changes to fuel density. A great number of certified planes that use these carbs have auto fuel STCs. This change is just a paperwork change that goes with the plane; there is no adjustment to the carb to start running auto fuel. Likewise, the jetting will not change when you alternate between auto fuel and 100LL in an experimental aircraft fed by a Stromberg.  This is not so with some experimental aircraft carbs. These two fuels have different densities, and some carb designs really need to be rejetted when changing back and forth between fuels. As much as I like working on planes, this is a lot of extra work compared to having a carb like a Stromberg.

The Marvel MA3-SPA is the only carb commonly used on a Corvair that has an accelerator pump. All the others, including the Stromberg, need a primer for starting. When the Stromberg has one, it will start in almost any weather you would choose to fly in. Grace’s Taylorcraft  engine is a C-85, but it is equipped with the exact same Stromberg carb we are speaking about. With no preheat, and the engine cold soaked at 20F, I can give it 3 shots with the manual primer (same type as in the Aircraft Spruce catalog #05-19920) , open the throttle slightly and crank the starter for no more than 2 seconds. The engine will light right off and run at a  high idle. ( I let it warm up for 5 minutes with the carb heat on. Not  only does this vaporize gas better, it does also slightly enrichen the  mixture by lowering the density of the air entering the carb.)  It was not  required to work the primer once the engine started, I just leave it locked. I  turn the carb heat off for a few seconds every minute to evaluate how well it  was working. Within one minute, turning it off gives a 75 rpm rise, indicating it is flowing hot air.  Even though they are different engines, this data should give people thinking of using the Stromberg a good indication that they will start easily when cold. On an actual Corvair, the higher compression and the stronger spark would make the engine light off even faster.

The Stromberg is an affordable, proven carb for gravity feed Corvair powered planes. I consider it an excellent carb for a Pietenpol, Kitfox, Highlander, Wagabond, and many other airframes. If you have a question about a specific airframe, drop me a note, I will be glad to share what I have learned.

Thank you.

William