When I built my first two IC engines, Jerry Howell's V-2 and V-4, I also built the carburetors for them. Jerry's carb is fairly complex, having high and low speed needles as well as a functional Bernoulli butterfly valve. The three copies I made (the V-2 required two) worked fine, but it seemed like I was continually adjusting the needles to not only start the engines but also to keep them running. With my limited experience I wasn't sure if this was due to the carb design, the engine designs, my machining, or whether constant fiddling is just the nature of the beast.
When I built my third engine, the Hodgson nine cylinder, I noticed the Howell carb design with its .25" bore had been licensed and included in the plan set I had purchased. After talking with Lee about the performance of such a tiny carb with the H-9, it wasn't clear that he actually had much personal experience with it. His carb of choice is a larger and very difficult-to-find unit from a vintage Cushman scooter. In fact, he mentioned that he maintains membership in a Cushman club in order to get access to them.
My research into what other builders were using for their H-9s was also disappointing. Those who were satisfied with their carbs were using models no longer available. Being intimidated by the complexity of the H9's induction system as well as the large number of machined parts that need to play nicely together, I decided to go with a commercial carb in order to reduce the number of unknowns that would be dependent upon my machining abilities.
After a few false starts I ended up with a Super Tigre #12163145 - a dual needle barrel carb with a .35" bore. Although my final gasoline settings seemed extremely lean, the adjustments were not overly sensitive; and the carb seemed to work well on my H-9. Once dialed in, the transition from idle to full speed was smooth, and I never had to mess with the needles again.
The induction system on my T-18 is even more intimidating. With no real expertise I designed my own diffuser and plenum based more upon good looks than any flow theory. The front row intake tubes are nearly twice as long as the rear row intakes which, themselves, are pretty long. In addition, the thermal characteristics of my T-18 heads and cylinders which affect the intake temperatures just have to be much different from those on the H-9. I felt Jerry's carb was too small for this engine; and so, again, I decided to go with a commercial unit. My tried and proven Super Tigre, though, hasn't been available in this country since the Japanese tsunami of 2011.
Choosing an RC carb for a multi-cylinder model engine from a distributor's website is a bewildering experience. The selection is typically large but the availability is in constant flux since the carbs are designed for particular RC engines that seem to come and go with the frequency of women's fashions. And, RC engine size isn't a sufficient spec to use when shopping for a carb for a multi-cylinder model engine. The requirements of the two engines can be very different. For example, I'm more interested in a carb that can provide my 18 c.i. display engine with a quality idle and smooth transition to mid-range rpm compared with an RC enthusiast who wants his .60 c.i. acrobatic single cylinder engine to make reliable high rpm power.
During my search, I came across Perry Carbs now owned by Gary Conley who is a member of the model engine building community. I was attracted to his website because it includes an inventory listing by Venturi size which, to me, is a more useful spec than a recommendation for a particular single cylinder engine. I spoke to Gary about my requirements which included a maximum 3500 rpm. He assured me one of his carbs was capable of performing even better than the Super Tigre.
I purchased his model 1401 which has a .312" Venturi. Perry carbs are also barrel carbs with high and low speed adjustments. A needle valve is used to control the fuel flow at high speed. But, an adjustable disk with a cat's eye opening uncovers a slot in the spray bar to regulate the amount of fuel at idle as well as in the mid-range region. Because I plan to use gasoline instead of typical RC methanol-based fuel, Gary supplied a special disk with an opening better calibrated for use with gasoline. For some reason, no documentation arrived with the carb, but two of the photos show the information I was able to locate online.
The design has evidently evolved a bit since the version shown in the exploded diagram. The throttle arm is now plastic, and the high speed needle no longer requires a tool to make adjustments. After installing the carb on my engine and hooking up the throttle linkage, the sloppy feel resulting from flex in the plastic throttle control arm was a bit of an annoyance. So, while looking for something to do while progress on the radial is currently weather stalled, I machined a brass replacement that feels much better. While the carb was still off the engine I decided to disassemble it to better understand its internals, and it was probably a good thing that I did. The area between the o-rings on the aluminum disk was covered with a sticky gunk - maybe dust encrusted o-ring grease - and there was a piece of aluminum swarf stuck in the narrow fuel slot. I don't believe any of this could have from my end earlier in the week since I'm running a fuel filter just in front of the carb.
The instructions mention that a only few thousandths movement of the idle control disk can make significant changes in the idle performance. I noticed the two o-rings tended to stick and slip when I attempted to rotate the disk a small amount (the documentation warns about this). So, it feels like this method for making small low-speed adjustments may be more difficult compared with a more conventional low speed needle. I'll reserve final judgement, though, until I actually go through the process. My T-18 carb adapter is designed around the standard carb dimensions of the Perry as well as a couple carbs salvaged many years ago from my younger son's RC junk box. If the Perry doesn't work out, I may have some other plug-n-play possibilities.
Perry's recommended starting point for the high speed adjustment is one turn open from fully OFF. After checking, I found that mine had been set 2-1/4 turns from fully OFF while I was first-starting my engine a few days ago. As I suspected at the time, the carb was probably set ridiculously rich; and I probably should have waited until I was familiar with it before attempting to start the engine. Anyway, it seems no harm was done; and now, with a bad case of cabin fever, I'm ready for the weather to warm back up. -Terry
