The Snow tandem double acting gas model engine designed by Doug Kelley is a classic among model engines. It is a bit different and as a result attracts attention at model engineering shows. It is a long build requiring attention to squareness and concentricity. It can be very disappointing when it doesn’t run like you expected, so I thought I would post this thread about my experience improving my Snow.
Many of the Snow engines built to the drawings have exhibited a common running issue. I have seen this on at least four other engines, including mine. The engine will start and run for 30-60 seconds and then it slowly loses RPM, then stops. No adjustments seem to help. The engine is restarted and it loses speed and stops again. This repeats few times until it won’t re-start. Let it sit for 15 minutes and it will start and the whole cycle repeats itself. At other times the engine will slow quickly and stop. If your Snow engine has a similar problem I believe there is a solution.
The scale of the Snow model engine obviously presents a multitude of problems related to its scale, but I will focus mainly on the fuel system. The Snow fuel system uses a fuel/oil mix to provide lubrication to the cylinder and piston rod bushings. Neither require much lubrication, but should have constant lubrication which they get with a fuel/oil mixture. Conventional two cycle oils should be avoided. They were designed for high temperature, high speed, hard working engines and the Snow model engine isn’t anything of that sort. The Snow model is slow speed and produces rather low exhaust temperatures so a much lighter oil is all that is needed. Marvel Mystery oil (MMO) seems to do the job nicely. I have used 50:1 and 60:1 fuel/oil ratios of MMO with good results and still have a very wet engine inside. I believe the fuel/oil ratio could be increased to 70:1 or higher, but have not tried it yet.
The carburetor size is important. It must be small. The slow speed of the Snow (400-500 RPM) provides ample time for the needed fuel air mix volume to flow through a very small carburetor opening. I have found that an RC barrel type carb with a throat diameter of .120” (3 mm) is just about optimum for the Snow. A self compensating type is preferred, but due to the constant speed operation of the Snow engine this feature isn’t essential.
Real Snow engines had a fuel mixer adjacent to each valve cage with a common throttle control rod linked to a governor. These mixers used a gaseous fuel so mixing was good and control of mixture was not as important as on a liquid fuel system. The Snow model engine intake manifold is rather unusual. The individual intake runner lengths are all equi-distant to their respective valve cage. They are almost 6.5” long from carburetor to valve cage. When the engine is started the carburetor and adjacent portions of the manifold become quite cold due to the vaporization of fuel in the carburetor. There is little mass to the manifold so it will become cold very quickly well beyond the carburetor junction in both directions. There is little, if any, immediate heat feed back from the engine mass. For a long time I had thought this was the root of frequent running issues with my Snow. I tried using a heat gun on the carb after the engine was started and the results were very encouraging. The engine would run as long as I kept the carb area quite warm. One Snow builder built a jacketed intake manifold that utilized engine exhaust gas heat to warm the intake manifold. It didn't compete with a hot air gun. I suspected that moisture, fuel, and oil were condensing inside the manifold and that condensed fuel was then re-vaporizing inside the manifold upsetting the fuel air ratio after only a short period of running. In my mind, this theory correlated with the engine running for a while and then slowly stopping. But then I began to wonder if the problem was an accumulating slug of condensate itself. And if this was so, how would condensate flow uphill to purge itself from the rising intake runners of the Snow. My wife's old percolator coffee pot came to mind.
I felt a new intake manifold design might help if condensate, or condensate accumulation really was the real problem. The manifold needed should allow any condensate to be carried along into the engine instead of accumulating inside the manifold and restricting intake air flow. It also needed to be capable of providing heat to the carburetor area to offset the significant cooling effect of the vaporizing fuel. This manifold design was a simple flat plane log type manifold installed at the elevation of the intake valve cages. The intake runner from the manifold would be as short as possible. The carburetor would be center mounted as before. No gaskets would be used at the valve cage flanges to get more heat transfer into the manifold. I used a section of K&S .188” x .375” rectangular brass tubing for the main manifold. The cross section area is a bit larger than the original Snow tubular manifold. My idea was to first build a dry log manifold using one rectangular tube for the air intake flow only, and to try it on the engine. Then build one with two tubes soldered back to back with hot return coolant routed through the second tube. This would provide heat along the full length of the manifold, and around the lower part of the carburetor.
The running issues of my Snow had confined it to the shelf for over three years and I was very anxious to get it running properly. The first single tube dry manifold was installed shortly before the East Ohio Model model engineering show in Zanesville last October. With the new dry manifold, the engine started easily and it ran,…..and it ran…and continued running for over 30 minutes before running out of fuel the first time on the new manifold. I was cautiously optimistic! Not only did it run like never before, but the minimum smooth engine operating RPM had dropped from 500-550 RPM to 400 RPM. I put about 3 more trouble free hours on the engine before the show. It ran without with problems at Zanesville for at least 3 hours. A few months later it also ran well at Cabin Fever for over 6 hours without any unexpected stopping. It will now run at 300-350 RPM if you watch it, but still seems happiest at 400 RPM. The intake manifold was still getting quite cold near the carburetor junction, but the cold area did not extend too far. The new short intake runners were back feeding heat to the manifold very nicely. I considered the new log manifold a success. My Snow will run continuously for about 40-45 minutes with this manifold before reaching a return coolant temperature of 165 degrees F, but the hotter it gets the better it seems to run.
I built the second jacketed manifold after Cabin Fever to see if things could get even better. I was surprised to find that performance wise, there was no noticeable operational difference between the dry or wet manifold. Both run almost the same. That was a little disappointing at first. While I was just as happy with either manifold from a performance standpoint, I noted that the jacketed manifold had three side benefits over the dry manifold. First, it solves all of the coolant flow and frequent coolant spillage problems of the Snow since it becomes a closed circulating system. Secondly, the heat absorbed by the carburetor vaporization contributes to engine cooling and extends the engine continuous operation period indefinitely at 150 degrees F. I like to run my engines. And third, the fuel consumption seems noticeably reduced. The only drawback to the wet manifold that I can see is that the distinctive Snow coolant overflow tubes and drain funnels must be removed.
In my opinion, condensation in the original Snow intake manifold can become a percolating restriction to intake air flow disrupting and/or stopping engine operation. The flat plane log intake manifold will allow any excess manifold condensate to be purged as it accumulates allowing for a free flowing intake system and uninterrupted engine operation. It seems to work for me and I now consider my Snow engine one of, if not the most dependable running engine I have built.
A few photos of my three intake manifolds are included. I am sure my log manifolds can be improved upon, but they work. I would be very interested in hearing about any other Snow modifications that have improved running performance of the Snow engine.
Jeff
