Forum

[Presto]

Calling all clever people! Can anyone explain how the so-called 'bi-starter' (cold start) system works on the fuel injectors used on the EFI. They are cable operated. The cable opens an internal passageway in the injector body. The plunger moving upwards opens an air-way in the injector body. But how does that give a richer mixture for starting?

[Michael]

I stripped and cleaned my throttle body recently (you are talking about the throttle body, not the injector here)... I am not entirely sure, Thack might know better, but I think what happens is that as you pull the bistarter lever, the little piston moves upwards allowing additional air to flow via a small dirlling into the throttle body, bypassing the butterfly valve. I *think* this 'tricks' the manifold air pressue sensor into thinking that the mixture is lean and so the injector injects a greater quantity of fuel thus actually enrichening the mixture... but that might all be guff!




The MAP sensor sits just in front and to the right of the bistarter piston, on top of the throttle body (it looks like a small, black cap)
The injector itself sits downwind of all this gubbins. BTW did everyone know that if your injector is guff or playing up you can get them cleaned professionally for around a tenner?! I found this out recently and saved a fortune:)

[ric]

Simply version... this additional air is detected by the MAP (Manifold Absolute Pressure) Sensor which informs the ECU more fuel is required.

[ric]

Oops, should have refreshed the page before posting.

[Presto]

My problem with that explanation is that I can see no air-passage in the throttle body that leads from the bi-start chamber to the MAP. The air drilling that leads to the bi-start chamber that is opened by the bi-starter plunger feeds through to the channel that opens directly into the venturi of the throttle body.

[ric]

It doesn't need a direct feed to the MAP Sensor Per se, the MAP detects change within the manifold. As soon as the Bi-starter lifts and the pressure alters, that change is detected and the mixture adjusted.

[Thack]

The bi-starter is very simple: it is nothing more than an air passage around the throttle plate. There is already one air passage around the throttle plate, which you adjust with that large screw to set the idle speed. The bi-starter has exactly the same effect as unscrewing that large screw another half a turn or so. When you open the bi-starter the MAP detects that more air is entering the manifold and the ECU then injects more fuel.



Note that this does NOT result in any enrichment. The additional fuel is matched to the additional air. It is nothing more than a crude fast idle device. The enrichment required for a cold start is determined separately using the temperature sensor alone.

[Presto]

Thanks Thack! (and ric too) that makes perfect sense, and seems to match what I can see as I look at the throttle body.

[Presto]

Second thoughts, if all the 'bi-start' does is allow extra air why not simply open the throttle slightly? Also would not the 'extra' fuel be cancelled-out by the 'extra' air and the mixture end up being 'normal', rather than richer?

[Michael]

True... but the perhaps using the throttle is too much extra mixture?