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Grey Wire, TPS, AP, ACV, Surging, My Findings 2006 WR450

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OK, so you may have seen some other posts, my UK bike has 94dB Scorpion pipe, 45 pilot, stock adjustable needle, 165 main etc, no bogging, loads of power.

GREY WIRE: I had no idea if this had been touched, or even if my bike had one! But yes, it was there, connected, and earthed. (see pic at end)

Hooked it out and up the road. Hmm, not a lot different, the bike was very powerful and still is! No change at all at lower revs, but I think a back-to-back test would show a slightly stronger upper mid and top.

The point to make is that the bike is no harder to ride with the wire disconnected and in my opinion it is pointless putting in a switch.

Maybe engines in higher state of tune (YZ cams etc) might show more difference?

THROTTLE POSITION SENSOR (TPS): Well this is an easy one, disconnect it and see for yourself!

On mine, at higher revs the bike still pulled very strongly (which surprised me) but had a very noticeable effect at lower revs/throttle, with a not very desirable soggy, vague feel to the throttle action. Easier to ride, possibly, but not a nice feel.

More importantly, the abrupt power delivery off idle that makes this bike less than ideal for rocky/slow stuff was not affected by disconnecting the TPS, or indeed, made worse by the grey wire.

For this reason, again, in my opinion it is pointless putting in a switch.

SURGING: Easy, its caused by lean jetting, end of. How do I know, well start your bike, push in the choke immediately and see if it will run. Like crap? Jetting is too lean (as mine is), and the bike will surge terribly at low throttle.

You can mask it a bit by making the pilot richer, and/or raising the needle, but ultimately it needs a needle with a thinner straight part. Disconnecting the TPS will help but its only masking a different problem!

ABRUPT POWER AT LOW REVS: Well, this is my next experiment, as in my opinion(!) its caused by the AP delivering too much too soon. By adjusting it so it kicks in say at 1/8 throttle rather than immediately, it should be possible to get a more manageable power delivery when going very slow.

Of course too much delay will cause a bog so we will have to try and see…


Grey wire:


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An update to this, after experimenting with the Accelerator Pump and the Air-Cutoff Valve

Disable the Air-Cutoff Valve, something I have done on other bikes to good effect, normally to smooth the on-off throttle transition at low speed, most noticeable on my XR650.

To do this, remove the spring and insert a 5mm piece of flexible tube between the ACV cover and the diaphragm – it keeps the valve in the “normal” open position.

Interesting, on this bike it made no difference, apart from introduce a lot of popping and gurgling on the over-run, which of course it is put on in the first place to prevent!

So that was returned to normal. With richer jetting, particularly the pilot, you don’t need the ACV, but its put on there for:

a/ lower emissions, allowing a leaner pilot jet, and

b/ a pilot jet rich enough to prevent over-run popping does not always give very smooth low down pick up or idle.

I am going to try a larger pilot anyway, and will experiment with this again.

Accelerator Pump: - first I checked it, and it was set to spec, starting at approx 3mm slide opening. I turned the screw in ½ turn so there was an obvious small amount of twist-grip movement before the AP kicked in.

A test ride revealed very little difference, no bog, but it felt very slightly softer for the first few degrees of throttle. The engine still picks up very strongly and a little abruptly when going slow in first, so no improvement there.

Adjusting my fuel screw had the most effect on smoothing low speed running so in conclusion, leave the AP as standard, and try a richer pilot…….


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