KTM 125 duke stalling

[A100man]

Cant help think a Lambada sensor might have been a poor choice.

https://c.tenor.com/Aft1eoNHdhcAAAAd/lambada-dance.gif
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Now: A100, GT250A, XJ598, FZ750

Then: Fizz, RS200, KL250, XJ550, Laverda Alpina, XJ600, FZS600

[gavcol]

The Lambada sensor effects the air to fuel ratio. It does this each second by measuring the exhaust fumes. It is activated once it's internal heater reaches operating temperature.
My thought process was that it works fine for the first 20-30 seconds (heat up period).
Following that it reads the exhaust fumes composition each second and gradually adjusts the fuel flow through the injector. After 30 seconds the bike gradually degrades and more throttle has to be applied to keep the revs. As if the engine is being choked or under fueled
On paper and in my head it sounded like a possible cause.

[Nobby the Bastard]

[gavcol]

Oh wow, apologies for being a bit slow on the uptake there.

[xX-Alex-Xx]

[Robby]

Fuel pressure check first, and get the battery on a charger. 125s use tiny batteries and all of this diagnosis will have run it down a bit.

Then (but not simultaneously) I would try unplugging sensors and seeing what happens. If you're lucky the ECU will revert to a base/safe map and still run but give you a warning light. That's a way of diagnosing a sensor giving the wrong (but valid-looking) reading.

Also worth knowing - pretty much every sensor is either a hall effect sensor counting blips per minute, or some kind of variable resistor giving the ECU a reading of between 0 and 5 volts. They are not complex instruments. The ECU is basically a small computer containing a large spreadsheet telling it what to do with all the values it's receiving. The only really important inputs are throttle position, intake air mass and engine speed.

[gavcol]

Alex, Robby, Robby thanks for the really good info. That makes sense about the lambda sensor and the open/closed loop.
I have charged the battery a few times through these diagnostics but have put it back on charge again. I've also had my jump pack hooked up to it as well for extra charge.

Tried removing the throttle position sensor and the TMAP sensor while running, both gave engine management light and the bike behaved the same as before. I've also managed a Cylinder compression test and it gets up to 175, my ratio is 12.8:1 https://calculator.academy/compression-ratio-to-psi-calculator/#f1p1|f2p0 says this is 188psi. So I think that it's compressing well, thoughts?
I've also checked the voltage of the fuel pump whilst running and it holds steady at 13.5 V, all they way up to it stalling.
I'm now going to order a fuel pressure tester.

[Robby]

The compression is good.

I'm not sure that checking the voltage at the pump is the same as giving it an independent feed of electricity. The purpose of the diagnostic test is to prevent any signal from the ECU telling the fuel pump to shut off, so that diagnosis can focus on the injector and ignition side of things.

If it is gradually dying over a few seconds, it's fuel. More specifically, it's a gradual change in the amount of fuel. If it's suddenly dying, it's spark or total fuel cutoff.

If fuel pressure is OK, then my next best guess is that after about a minute it's transitioning from the warm-up map (like the choke on a carb) to normal temperature running. This means reducing the amount of fuel going in (leaning out the mixture).

A fuel injector is just a valve that opens for as long as it's told. The amount of fuel that passes through it is dictated by fuel flow rate (or pressure) multiplied by time.

If pressure is low, or the injector is clogged or damaged, enough fuel could still get through it during warm-up to run the engine, but it would not have enough fuel to run afterwards. This is similar to a clogged jet on a carb.

So if the fuel pressure is ok, next place to look is the injector.

[gavcol]

My fuel pressure tester arrived today. Sits at a steady 36psi through out which is within book values of 36-42psi.
Had the battery on charge overnight and that now has fluid seeping out the top. New battery arriving tomorrow

[jaffa90]

Are you talking 36psi before and after starting the engine?

[gavcol]

Yep, turn the ignition on. Pressure goes up to 36psi. Start it up and it sits steady at 36 untill the engine does and then it gradually reduces a few psi before holding steady.
To me thats working fine. Pump and regulator valve keeping a steady pressure

[gavcol]

RESOLVED

Short version it was the valve seating. Couldn't get the shims correct so bought a new cylinder head. Shim clearance was set perfectly on those and bike now runs great.
The valves closed enough when it was cold but as they heated up they couldn't close correctly, 1 on the inlet side and one on the outlet side.
In the old head I had sprayed some carb cleaner in to both sides in an effort to clean it out. This probably cleared out a load of carbon that had been sitting in the valve seat acting as a gasket. With that gone the valves retracted further.

On the upside I know have loads of spares

[carbon90]