Nitro Engine Tunning 101

[Michowski]

Airtight is Right
In order for an engine to idle, transition and tune consistently it must be airtight- airtight meaning that no air can enter the engine except through the carburetor intake. Surprisingly, even brand new out-of-the-box high-end engines can have air leaks and until these leaks are found and eliminated, the engine won't run consistently and cannot be precisely tuned. The good news is, several years ago I figured out an easy way to check for and fix air leaks. You'll need the following:
• A small glass of soapy water
• A small brush
• About a foot of fuel tubing

Step 1. Be sure that the high-speed needle is open at least two turns out and the throttle is at fully open.
Step 2. Install one end of the fuel tubing on the carburetor nipple. You're going to be blowing into the other end of the tubing to pressurize the engine crankcase.
Step 3. Place your thumb over the carburetor intake to prevent air from escaping, then blow into the fuel tubing while brushing soapy water around the base of the carburetor, on both sides of the pinch bolt, around the fuel fittings, at the high and low speed needle valves, around the front bearing and even around the back plate.

If any bubbling occurs you've got an air leak. But don't be alarmed. Fixing the leak is easy and your engine will run remarkably better and more consistently when you've eliminated these leaks.


The Fix


Brushing soapy water around all possible air leak points while pressurizing the case will easily identify air leaks.
The most common air leaks seem to occur around the pinch bolt. Applying a dab of RTV (or silicon) over both exposed sides of the pinch bolt will fix this type of leak. RTV is also great for sealing any leaks around the base of the carburetor. Simply remove the carburetor and run a small amount of RTV around the base of the carb where the leak occurred. Reinstall the carburetor and allow the RTV to ooze out between the crankcase and the carb body. For leaks around the high-speed needle valve fittings, usually retightening the high-speed needle assembly will take care of these. Leaks directly at the high speed or low speed needle usually means there's a damaged O-ring that must be replaced. Many racers don't realize that the front bearing can leak air. That's why nearly every high-end engine uses a rubber sealed front ball bearing to not only keep out dirt, but also air. If your front bearing leaks air, it's time for a bearing replacement.

This simple leak check can easily be done even when the engine is installed in a car. After you've done this test on several engines you'll be surprised of two things: first, how often unwanted air leaks occur and second, how much better your engine runs, idles and tunes when it's airtight. To run right it must be airtight!