quote:

ORIGINAL: hpi apollo

i would use something other than blue thunder, fuels with high/higher amounts of castor oil are a bit more forgiving at tuning, vapor lock can be caused by an engine that is running lean, inadeqaute airflow, or both, you can use blue thunder if you want, i would get some castor oil and add it to the fuel, not a big deal, and they run much cooler, when i made my own fuel for the first time, i used 10% straight castor oil and my engine had to be leaned over a full turn to run right, then it was flying and running cool

quote:

ORIGINAL: RCtruckRacer

Mine has less info, but may be good for someone new to the hobby to keep in their wallet, pit box etc. A quick reference tuning guide if you will.

Why do we ‘tune’ our engines? How is it done? This guide assumes that you are familiar with all of the parts of your R/C's engine. If you are unsure what a Low Speed Needle (LSN), High Speed Needle (HSN), idle screw etc are, please refer to the manual (RTFM).
Tuning a nitro/methane 2 stroke engine literally means adjusting the air/fuel mixture inside the chamber. This is done by means of adjusting the amount of fuel that is allowed to flow into the carburetor via needles which can be opened and closed. Most of us know that there are a wide variety of factors that affect that air/fuel mixture in the chamber other than just adjusting the needles. Because of this an engine may have to be tuned several times in the same day for optimum performance. Not maintaining an optimum air/fuel ratio can result in poor performance, or even engine failure!

There is a point, in every engine, at which air/fuel mixture will allow the engine to obtain its maximum HP and RPM’s. Tuning your engine to that point can be dangerous, as the mixture is somewhat lacking in the lubricants necessary to give maximum protection to the engine. Therefore, most people will want to run slightly rich (more fuel in the mixture) of that point. This is referred to as the ‘Sweet Spot.’ Some racers will run at pure max potential, but they also don’t usually care if the engine only lasts a gallon… Running at those settings can be very dangerous because being too lean is just a very slight adjustment away.

And running lean is the real engine killer, not temperature. With proper lubrication, the internal components of an R/C engine can easily withstand temperatures of 300F, maybe even more. The thing is, these engines are designed to operate at temperatures around 210 – 270 F. While there is the possibility you will encounter an engine and conditions that dictate the temperature at proper tune is over 300, it will be a rare occurrence. What that boils down to is that there is a good possibility that there is a problem if your engine reaches too high of a temperature. That is why it’s a good idea to monitor the engine temperature, but not use it as a guide for adjusting the air/fuel mixture in the engine.

So, how do you get to that ‘Sweet Spot?’
1) Start out with rich settings. If you are unsure how to go about this, an easy way is to start with stock needle settings. This includes the idle screw - make sure it is set at factory settings, usually a 1 or 2 mm opening for the idle stop. The factories always set engines rich for break ins.
2) next, warm up the engine. Try not to hammer the throttle too much while its still bone cold, but slowly work your way up to a couple WOT runs.
3) Now that the engine is warmed up (1/2 tank is good), do a couple WOT runs of about 50-100ft in front of you and pay attention to the pitch of your engine. The higher the pitch, the more RPM's its doing.
4) Now lean out the HSN by 1 hour increments (30 degrees, 1/12 of a full circle etc) and continue to do some WOT runs in front of you. The engine's max RPM's should continue to go up each and every time you lean out the HSN by those 1 hour changes. If it doesn't, you have reached the top end and need to richen by an hour. Make sure that you do not continue to lean out the HSN after max RPM's have been reached. You will damage the engine.

Congratulations, you just found the sweet spot. Now, if you notice that the car/truck bogs a bit at take off, lean out the LSN (again 1 hr increments) until it will take off at the touch of the trigger.
And you're done. At this point I like to temp the engine to get a baseline reading for the day.

This is a simple and fool proof method of tuning that everyone can do. Regardless of your experience.