On this page I have compiled what I have found out about how to keep the Cox .049's happy. Many people, including myself, when first getting into small engines such as these can find it very frustrating to get them and keep them running. Cox engines have built up a notorious reputation, and if you have been in the modeling hobby any length of time, I am sure you will have heard how 'fussy' or 'unreliable' these engines can be. Nothing can be further from the truth IF you take proper care of them and know how they work.
Lets start with the basics. I will describe the functioning of the common Cox reed valve type engine such as the modern Surestart, or the older Black Widow, Golden Bee, etc etc. When the piston travels upwards and passes the exhaust ports, it begins to compress the air/fuel mixture in the upper portion of the cylinder. The glowplug should be hot, and when the right pressure is reached, the air/fuel mixture ignites and the piston is pushed back down. On its way down, the piston builds the pressure in the crank case up, closing the reed valve. The piston then uncovers the exhaust ports and combustion products are discharged. Transfer ports cut into the inner walls of the cylinder are uncovered, and the pressurized fuel/air mixture in the crankcase travels up these ports and into the top of the cylinder again, removing more exhaust gasses in the process. On the pistons way back up from the momentum of the propeller, a low pressure region is created in the crankcase, and the reed valve opens, allowing fuel/air mixture to pass into the crank case ready for another cycle. This cycle is repeated hundreds of times per second, even for the lowest of performance 049's.
So why the long explanation? Mainly because if you are going to be running one of these engines, or any engine for that matter, I believe it is important to know how they work. This can be of great help in diagnosing problems that may arise during the running of the engine, as you will see here.
So what can you get from the above explanation? Well because the engine is relying on crankcase compression to get fuel/air mixture into the cylinder, it is not going to run worth a darn if the crank case is not sealed. The gasket on the back of the crankcase and the o-ring on the venturi come on these engine for a reason! It is important to make sure these are kept in good condition and that the bolts that hold the backplate or tank on are tight so this seal remains airtight. On the left is an image from the rear view of the crankcase. This is where the airtight seal is important. When installing the backplate or tankmount, remember to put the gasket between it and the crankcase.
Because the engine needs to compress the fuel/air mixture in the top of the cylinder, a sloppy worn out piston will not do a very good job. Always run your engines with a fuel that has a high oil percentage to minimize the wear induced by running. If you are not going for all out speed, run a tad on the rich side so extra oil will help protect your engine. If you have a crack up, disassemble the engine to make sure there is not grit that could score the piston or cylinder and cause a decrease in compression. Make sure you glow head is screwed in tightly with at least one copper gasket under it to ensure a good tight seal. A feature particular to Cox engines is the means of fastening the connecting rod to the piston. This is achieved with a ball and socket type joint. Over time, this joint can loosen, resulting in loss of compression and hence performance, and eventually if allowed to worsen, a permanently damaged piston. To keep this joint in slop free working order, you need to buy a piston reset tool, which allows you to tighten up this joint before it becomes a problem. Davis Diesel Development sells a reset tool, and the come up on Ebay frequently.
The reed valve can also be a source of trouble. The old copper-beryllium ones are very very easy to bend, and if they get bent when you are installing them or removing them, you may as well toss them out, they will only provide you with problems. More one reed valves in the next paragraph.
Perhaps the most important aspect required for a good running engine is CLEANLINESS. I cannot emphasize that enough. Probably most of the problem an engine has in running are the result of dirt where it shouldn't be. Dirt or gummed up castor oil in the fuel system will not allow your engine to suck fuel properly. Take apart the gas tank, and soak the parts in methanol overnight. Then, put on your safety glasses and blow the needle valve assembly out with compresses air. Be sure the reed valve is removed before using compressed air or you will damage it. For good crankcase compression, the reed valve has to seal well on its seat. This seat MUST be kept clean and free from gummed up castor oil for you engine to run properly. The reed valve seat can clearly be seen in the picture to the left. A leaky reed valve will bring you nothing but grief.
Now that you have learned the basics in the function and the care of your engine, you can get around to starting it!
First, what equipment do you need? Well you need a well charged battery and a clip (above left) to attach to the head in order to heat the glow plug. You will need clean fuel, and some means to fill the tank, I use a syringe with a blunt needle to fill the tank (above middle). In this picture you can see the fueling syringe, and the oiling syringe. I always put a few drops of after run oil (Marvel Mystery Oil works great) into the engine after a flying session to protect and lubricate it. If your engine has a spring starter, hand propping is very easy, but if you don't, you can either hand prop by flicking the prop over with your finger or a chicken stick, our you can use a 1/2a electric starter. I would tend not to use the electric starter on these small engines because if you flood it, you can run the risk of breaking something. Plus if your engine is maintained well, it should start up no problem by hand. Also indispensable is the Cox wrench. You can disassemble the whole engine with this, and, important for field, work-remove the glow head. That is really all you need to start your engine. So here is what you need to do to get the engine running:
Cox 049's will run on a very wide variety of propeller. I have heard of people running props as large as 8x5 and as small as 4.25x4. For general hacking around a 6x3 will run well, and also a 5x3. I prefer the 5x3's for most work. Cox makes both these sizes in durable nylon that will survive all but the most violent contacts with the ground. If you are going for flat out speed a 4.25x4 prop will serve you well. You probably won't be able to find this size, so it is common practice to cut down a larger prop.
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Following these guidelines, you will have no trouble starting and running Cox 049's. Mine frequently start 1st or 2nd flick, although the initial run of a flying session may be a little harder to start when the engine is cold. If everybody knew what you now know, perhaps small engines wouldn't have the reputation they do. So next time someone complains about how fussy their 049 is, you can show them what to do!