Brown stain on intake valve

[MattJ]

The engine is from a KTM500EXC and has barely been used. It's going in a single seat race car that we're building at uni and the engine has been run in and then only started a few times after that. It's possible that the injector has been positioned wrongly when it has been started up a few times, but would that cause that? It doesn't rub off, it seems to be stained on there.

https://s14.postimg.org/5hak7hc9d/DSC_0923.jpg


Any ideas?

[mentalboy]

looks like good old fashioned rust pitting from where I'm sat.
Has it been left open for a while, possibly in a damp environment?
How have you tried to clean it?
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[weasley]

It looks like lacquering, probably from fuel (but potentially from oil, if there is a crankcase breather fed into the intake air).

For fuel it would mean there is, or has been, opportunity for liquid fuel to sit on the back of the valve when it is shut (and hot). This could be down to poor injector location, spray pattern, injection timing, poor injector needle seating, poor airflow, over-fuelling or any other factor that would allow fuel to settle or pool on the valve tulip (rather than atomise and stay in the airflow). Or it could just be crap fuel.

If there is a crankcase breather into the intake air, oil mist can carry over but this really doesn't look like classic inlet valve fouling from oil, which tends to be washed away by the fuel spray anyway.

Lacquers like that can be pretty stubborn to shift, although they are often more a cosmetic issue than anything (although they can act as a base for further deposition).
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[MattJ]

Cheers for the replies. Weasley's idea seems to be the most likely. It's probably only been running for around 2 hours in total and it's been kept in the lab which isn't damp at all. It's definitely not rust because it doesn't rub off at all, also, both valves are in the same environment so if it was rust then they would both be the same.

Just been speaking to a lecturer who say's it's very likely varnishing/lacquering due to there not being enough fuel on the stained valve and therefore not being cleaned. The throttle body doesn't have any way to locate it on the intake manifold therefore it would be very easy to have it injecting more fuel down the valve on the right in the picture which cleans it but not the one on the left. The worry is that it has happened so fast. If the engine had been run for thousands of hours in total then it wouldn't be much of a worry but 2 hours running time is. It doesn't seem like anything terrible at the moment but I think we just need to keep our eye on it and make sure the throttle body is positioned correctly.

Cheers

[Ariel Badger]

That inlet Port needs polishing like a mirror and the guides need to be shaved to an oval profile to give less restriction to the gas flow. I can no see the valve seats but I will bet that they have far to much contact area and will benefit from a re-profile to 1mm of contact. What fuel are you going to use? Methanol will allow you to up the CR if the crank is up to the job.
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[gavcarter]

You revved it so hard one of the valves shit them selves.....

[stevo as b4]

[Ariel Badger]

I am not saying it is anything other than an excellent inlet tract but it could still take a little bit of work to gain that extra HP.
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[sickpup]

[gavcarter]

[stevo as b4]

Oh I agree it could be improved a fraction more with work to get the final few % of flow, say for a race/competition motor etc. But for a road bike without loads of engine work, I would not bother.

It's definitely not the type of head you could show a grindstone to and make huge flow gains, and for many DIY Noob's like me, there's more potential to bugger it up than improve things!

I was thinking that EFI would negate the need to have a rougher finish on the inlet port, as your spraying the fuel directly onto the back of the valve, and for a DI petrol engine, you could have a mirror finish inlet with no issues at all?

[bladerunner]

Agreed the modern thought is mirror polishing causes MORE drag on the incoming charge .....think of the dimples on a golf ball,they reduce the amount of air drag and improve distance covered vs a smooth ball of otherwise identical dimensions and weight
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