In Search Of … The Economical Carburetor

Friends,

There are a lot of smaller research projects that we work on during a year. One of these that I had kicked around in discussions with Dan Weseman was the potential to use an off the shelf, one barrel automotive down draft carb. Although both Dan and I have made a lot of high-end stuff for Corvair conversions, we are both still interested in keeping the engine and it’s applications affordable. Ma-3 and Elison carbs run about $950 overhauled or new. The are great and serve an important purpose. Stombergs are $250 to $550, and are a very good choice for gravity feed planes. Aiming lower, we are moving into the possibilities of  Zenith 268 carbs and motorcycle units. Neither of these offer a significant savings over Strombergs.  I am not a huge fan of carbs without float bowls. Aerocarbs, Revfows and Posas have all flown on Corvairs, and they have their fans, but most builders would like a carb that has a float, or a design like the Elison or Rotec that stops flowing fuel when the engine is shut off.  I have owned and operated all of the above carbs, and I will gladly say that float carbs are much more accurate at metering fuel. 

Below, what we are looking at is a Carter 1 barrel from a 170cid Ford straight six. I bought this carb in a box from Discount auto parts for $149, and paid an additional $10 core charge. This is about as inexpensive as you will ever see for an overhauled carb. The concept here isn’t new. 15 years ago we tested one single barrel Corvair carb and found that it could produce more than 85HP on the right manifold.  This Ford carb is significantly better flow potential than the Rochester HV. If you go to our main website and look at the category of flying planes, Dale Jorgensen’s VP2 has been flying for more than 10 years on a single down draft taken from a Chrysler slant six.

Above is a shot of the manifold on the test stand. The test engine is a 3,000cc, 120 hp engine. It makes a good test here because we want to know if the carb is operating near its flow potential. I am guessing that most of the people who may be interested in a Ford carb would be building a 100hp engine, and if the cab demonstrated that it could produce a solid 90 or 95 horses, this would be plenty. This manifold was made from one of our regular cnc bent manifold tubes. The box under the carb is 1.5″ x 3″ tubing. There is a lot of initial reaction to guess that such a flow pattern would be restrictive, but in practice, it matters a whole lot less than most people suspect. We are building a manifold for a 3,200 rpm engine making .65 HP/cid, which is very different from making a manifold for a 160HP sport bike that needs instantaneous throttle response.

Above, the carb mounted. The small red lever is a lawn mower throttle arm rigged to run the choke, in a plane this would be done by a cable. Looking at the carb, think of these thoughts: It can take fuel pressure; the inlet fitting is 1/8″NPT, not a brass nipple for fish tank tubing; It has a choke; It has an accelerator pump; It has robust construction and has a low parts count. The only downside it that it doesn’t have an easy way to control the mixture in flight.  If you want to fly at 15,000′, this is an issue. If you want to use this on a Pietenpol, it probably isn’t. In the years we had our Piet, I can’t recall ever having the plane over 6,000′ MSL, and I would guess that it spent 95% of it life below 3,000′ AGL. It had a Stromberg, but the mixture was wired in the full rich position. I would lay a serious bet that 100hp Corvair in a Piet with no mixture control would have a much higher service ceiling than a an A-65 or 75 Piet with the most elaborate mixture control.

Above is the carb running on the engine. Initial results are very positive, It starts right up and runs well. It idled very smoothly at 650rpm, and I could probably get that a lot lower with some carb heat and a finer adjustment. I am going to install a different EGT set up and a 5 psi electric pump to do a little more testing. We will have more results and the system on display at Corvair College #23 next month.