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Decel & ITB throttle

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I am running a set of Borla EightStack look-alike Weber IDA's. The design of the throttle body is such that the injectors are above the throttle plates. System is running good, just looking to improve its driveability.

When you let off the throttle and the blades close, they tend to collect fuel. The AFR will go lean for a while. Then when you open the throttle again, the fuel that was collected now dumps into the airstream and the AFR will go way rich for a second before things settle down.

Any tips on how to smooth out the AFR chart line. I could put a decel factor based on TPS close rate but don't think that will catch the casual throttle closing. As a road car, my typical cruise TPS is between 3-8% so it is real easy to dip down to the full closed position without a dramatic closure rate.

Any ideas or suggestions


It may be possible to alter the mapping for more resolution at such small throttle openings.

Do you use a fuel shut-off, this can be a PITA if the rpm range, and opening, has the engine running around that point because it stutters as it cuts in and out. With that engine, and that car, it may be very noticeable - if it's enabled, you could try turning it off?


Appreciate your thoughts - I am ahead of you, I think, on the resolution thing. Running Alpha-N, my load (TPS) axis is 0, .5, 1.0, 1.5 - this is a screen clip of the lower load axis. I don't know if any finer resolution would be effective, but I am a newbie to much of this.


I do not run a fuel shut-off. I was curious if there may be some other type setting that could influence or predict this behaviour, other than 'rate of decel'

My thought would be to try and open the throttle a bit more to avoid the fuel puddling. You will likely need to reduce the timing a fair amount to keep the engine speed in check.

Just something to try.

Can't recall if you use an air bleed or throttle stop for the idle airflow, but a bit left field if the latter may be to file* a small notch the bottom of the throttle blades? Might work with air bleeds, too, as that's a big engine you have?

My thinking is you can then close the throttle blades more/fully with the airflow required for idle and some of the low load running being drawn through the notch along with the fuel, the turbulent, high velocity air being drawn through should help atomise the fuel, too. If you do try it - and it'd be at your own risk, after a good think and discussions with smarter folks than me - you're going to have to apply a lot of care to make sure all the butterflies have the same amount removed, to get even airflow. Kinda like drilling butterflies in the old days to get enough idle air with big engines and cam's, without having the butterflies opening into the transition metering holes.

*Or machine, or a flat - just something at the bottom of the butterfly blade to let the air and fuel through.

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