The fuel system is composed of the fuel, fuel tank, fuel lines, and the exhaust pipe. The integrity of the fuel system is critical to the operation of your nitro RC engine.
How it works
The fuel system stores the fuel in the fuel tank and delivers it to the carburetor where it is mixed with air for combustion in the engine. A vacuum pressure is generated by the engine that pulls fuel from the tank into the carburetor, but this vacuum doesn’t provide enough pressure to supply a consistent amount of fuel to the engine. Additional pressure, called “back pressure”, is supplied from the exhaust pipe to the fuel tank. Without this back pressure, you’ll constantly be chasing the tune on your engine.
Common Issues
The fuel system works properly when all of the components are air tight. Even the slightest leak can cause a drop in pressure and lead to tuning and performance issues. Leaks commonly occur at the following points:
- Fuel lines – A crack or obstruction in the fuel line, especially around the fuel tank, carburetor, or exhaust pipe ports.
- Fuel tank – Fuel tanks are prone to cracking over time or the filter in the tank can become clogged with dirt. If you leave fuel in the tank after running, it can gum up and cause blockages.
- Exhaust coupler – The silicone coupler that connects the exhaust header to the exhaust pipe can create an air leak if damaged.
- Exhaust header – The metal exhaust header that is attached to the engine is sealed by either a gasket or o-ring and an air leak can be created if the seal is damaged.
Any damaged components should be replaced or cleaned.
Fuel Storage
Always close the cap on your fuel container after filling your fuel bottle and never store fuel long term in the fuel bottle. Nitro fuel will pull water from the air if exposed and degrade quickly. An air tight fuel container stored at reasonable temperatures will last a very long time.
If the fuel container is metal, like with VP fuel, store it off the ground to prevent rusting.
Other Air Leaks
Other common areas where leaks can occur, but are not contained within the fuel system, are the backplate of the engine and the carburetor. Both of these components have o-rings that should be inspected and replaced as needed if tuning issues are occurring.
The Front Bearing Leak
The front engine bearing is another area that many well meaning people exclaim will solve air leak symptoms, but I’m not convinced this actually solves anything. Here’s how I believe this common misconception began:
When an engine is showing symptoms of an air leak and all the fuel system components are ruled out, the logical conclusion is that there is an air leak in the engine. A commonly recommended method for testing an air leak on an engine is to place it under water and blow air into it through the fuel line going into the carburetor. Then you look for any bubbles coming out of the engine, which tells you where the air leak is. If bubbles are seen coming from the front bearing, the logical conclusion is there there is an air leak in the front bearing.
The problem with this conclusion is that engine bearings are not designed to be air-tight on their own. It is only when the engine is running that a film of oil covers the front bearing and creates an air tight seal.
In general, I wouldn’t recommend performing the water bath pressure test on your engine. Instead, take the engine apart and inspect the backplate o-ring for tears and the front bearing for play on the crankshaft.
Sealing an engine
While we’re on the topic of air leak solutions, let’s talk about sealing an engine with silicone or RTV. This is another often recommended solution to an engine that is showing air leak symptoms. Some even recommend sealing a brand new engine proactively. The recommendation is to seal the backplate and carburetor with RTV or a similar silicone sealant to close any air gaps that might be present due to poor manufacturing.
Personally, I only recommend this solution as an absolute last resort to save an engine that just won’t run properly, despite having all of its fuel system components, carburetor seals, and engine seals replaced. The risk of sealing an engine is that the sealant can deteriorate and end up inside the engine where it can cause issues.
Here’s thebug 404’s thoughts on the front engine bearing leak myth: