6.5 Getting a good idle 6 TUNING THE MS3PRO
6.5.0.7 Correct Fuel Engines don’t like to idle lean; a lean air/fuel ratio is one of the biggest causes of a surging
idle. On an engine with sequential injection, this is simple enough - get the engine idling at 14.7:1 for gasoline, and
you’re good to go. Batch fire engines, however, have issues at idle with intake pulse reversion sending some of the
fuel into an adjacent cylinder. So, if your setup only allows batch fire, you’ll need to add a little more fuel so all the
cylinders can stay at 14.7:1 or richer. Batch fire engines typically idle best in the mid 13’s, so if you’re leaner than
that, put in some more fuel and see if that stabilizes the idle.
You’ll need to make sure the air/fuel ratio is maintained during cold start. If your idle hunts when the engine is
cold, but stabilizes when it warms up, try more warm up enrichment.
6.5.0.8 Correct Timing Timing is a very useful tool for controlling your idle speed. Ideally, you’ll want the timing
to be set to slightly below the timing that would give it the most power - on most piston engines, this typically works
out to somewhere in the 10 to 18 degree range. Then you can add a row around 300 RPM below your target idle
speed if you find the idle to be unstable. The timing in this row can be bumped up by around 2 to 4 degrees over
the idle timing. The 3d view below shows a spark table set up this way.
Setting up your timing like this has a stabilizing effect on the idle. When the idle speed drops, the timing
advances, increasing power and moving the idle speed back up. The result is an automatic feedback loop that
helps maintain a constant idle speed. Not every engine really needs this. However, you need to look out for timing
curves that do the opposite. If you’ve specified less timing below the target idle speed than you have at the idle
speed, this often makes for an unstable idle as the ECU pulls timing while the idle speed falls. This can make a
slight disturbance in idle RPM amplify itself into an idle speed that cycles up and down rapidly.
Too much timing can be trouble. If the timing at your regular idle speed already puts you at the most power
AMP EFI MS3Pro manual version 1.202, firmware 1.5.0, 4/21/2017 Page 122
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