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MAP Accel Mode and Clamp interaction? LEM V3/5

Nick Diggles

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Hi! I have an LEM V3 with a V5 upgrade, performing well for many years, and have recently re-tuned for E85.  Engine is an old Datsun L20b, turbo with a lumpyish cam, fabricated plenum and 65mm throttle body.  I have been using MAP Clamp of 52Kpa for years to help stabilise the idle. Row Steps are TPS+MAP.

I've always set Accel Mode to TPS without problem, but now on E85, low rpm mild throttle blips (especially when cold) show lean and die - the rate of TPS change is not enough to trigger Accel enrichment even though the MAP changes significantly. Tried richening the next Row which was not satisfactory. Increasing Accel Low helped trigger an enrichment event with less throttle movement but it supplied too much fuel.

I then changed Accel Mode to MAP and voila, the light blips now trigger Accel enrichment and problem solved, along with improved throttle transient 'feel'.  The problem now is that rolling into and out of throttle on cruise shows much richer than previously, and even very slow tip-in from the fuel cut is often harsh as the mixture is richer and 'ready to burn' rather than lean and 'fluffy' that allows a smoother transition to the cruise mixture.  This is a steady state condition that is not caused by an acceleration enrichment event.  Slow tip-in from the fuel cut shows sudden rich (often with jerking) , then normal cruise lean, then normal richening as the throttle is opened further.  Even without entering the fuel cut it will now show rich on cruise at the lightest throttle settings.

I don't understand how simply changing the Accel trigger caused this!  All I can think of is that now for some reason the MAP Clamp is limiting how lean the fuel delivery is at cruise, or under 52Kpa.

Any help or ideas are appreciated!  

Thankyou, Nick.

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