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#9402
Eric Davis
Participant

Ray,

I have experimented with using EGT to tune bottom-ends by by logging patterns in EGT temperatures under consistent weather, oil, gas, engine setup, and other conditions. .  My preference is to tune the bottom-end of 2-stroke engines based on length of stable idle, throttle response, corner acceleration(s), and overall temperature.

Focus on selecting a good combination of idle jet (or inner/outer idle jet for Dellorto), air screw adjustment, and idle speed that allows the engine to idle around 2k for 5-10 seconds. Then, you should notice the engine starting to load up on fuel, and the idle speed should start to slow down.  When you blip the throttle to clear the engine, the engine should stumble, then clear.  If you notice a pattern where the engine idles for a long period of time (e.g. 20 – 30 seconds) and/or bogs on quick throttle blips, then your bottom-end is too lean (General Rule).

The easiest way to finalize your bottom-end adjustment is to ask the local top racers what their jetting and air/idle screw adjustments are, and ONLY change the air idle screw setting to identify the best setting for you.  For example, if the local racers are using a 60 outer idle on a Dellorto, with the Air Screw at 1.5 turns, start there.  Then, adjust your air idle screw inwards 1 full turn, blip/clean the throttle, then notice the difference.  Do the same process at 2.5 turns out.  You’ll definitely notice some changes, and be able to make a more educated decision for your environment, weather, engine, etc.

You can get a good understanding of the bottom-end, its’ principles, and tuning approaches by reading A. Graham Bell’s book “Two-Stroke Performance Tuning”.  It’s one of the best books on the market, and thoroughly explains quite a few key concepts.

Also, if Riley Will is lurking around here, he’s probably forgotten more than I know. :)