Question: What is wrong when an engine runs well on the ground or when I hold it firmly before flight but the engine bogs down or stops shortly after becoming airborne?

Answer: The best way to sort this out is to use a test stand. Set up the test stand fuel tank and fuel lines properly. Get the engine running and satisfy yourself that the engine runs well on the test stand.

Now put the engine back in the model. Start the engine and set the mixture with the model on the ground. With the engine at high RPM, pick the model up and holding it nose high, gently hold it between your finger tips. Emphasis on gently! Don't drop it but try to barely hold it. Be careful. The idea is let the model be lightly held nose high but be safe while you are at it. Wait about 30 seconds with the nose high and the model held loosely.

If the engine bogs down after a period of time when it is held lightly in a nose high attitude, the mixture is too lean. Try a richer mix (generally by rotating 1/8 of a turn out on the needle valve) and do the test again. Normally you will get to a point when the mixture is about right but if you find that you cannot get a reliable setting check the following:

- the glow plug (change to be sure)

- the fuel (10% max for VMAX engines) with at least 18% blended oil (castor & synthetic). Do NOT use more than 10%. Your engine has been designed to work best with up to10% nitro.

- the plumbing (tubes tight etc)

- muffler pressure (gasket in, bolts snug, tubing securely attached)

- the prop (back off a bit on pitch and/or diameter)

If you still encounter problems when held lightly in a nose high attitude, pay attention to your finger tips where they contact the model... is the airframe vibrating? Do you feel a bit of a tingle in the tips of your fingers?

Vibration is the hidden boogy man in some of these cases. Here is what happens... on the ground or when tightly held the model does not vibrate as much. When loosely held in a nose high attitude or in flight... high frequency vibration is able to make the fuel foam or ingest air bubbles. This leans the mixture and bogs the engine down (overheats). To reduce vibration you MUST MUST MUST balance the prop AND the spinner. Then make sure the fuel tank is mounted with the rubber gasket in place and the tank cradled by foam or the dowels on the power module. If you are using a non-wood prop and after balancing the problem persists, change to a balanced WOOD prop and try again. Remember non wood props flex and change pitch a bit with RPM and this induces some vibration. Wood props hold their pitch better and vibrate less once properly balanced.

In conclusion: If the engine runs on a test stand it is something about the model or engine prop/spinner that is the cause of your problems. If the engine runs in the model when level on the ground, you are likely going lean in the air. Examine for the causes of the engine going lean in the air... what is reducing fuel flow or increasing air flow into the carburetor? Remember that vibration is much more of a problem than many people realize... balance the prop AND the spinner. Try a balanced wood prop.