Black stains inside carb. Really old GL1000.

[Honest Andy]

So with southerly squalls whistling round the shed I decided to leave the helmet on the shelf and give the old GL some love with some well deserved maintenence...

This is what I found when I removed the carbs:

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There is blackness on the inside of the inlet manifold and coming all the way back up to the carb and marking the vacuum piston and carb body.
The blackness isn't heavily oiled and only some is removed with petrol or carb cleaner. More of a stain.

This has been happening for a while and I have noticed it getting slowly worse over the years on all cylinders but far worse on number 1, which always runs a little advanced.

So what do we reckon? Valve guides? Timing? Old age? Signs of impending doom or just a well used motor?

[SaferRides]

I've seen black gunge down the intake runners on the R1 after a float needle valve started leaking. But nowhere near as bad as in your photos.

[Honest Andy]

Yeah nah, not fuel stain from carb, these are carbony marks coming up from the cylinder. Ta.

[OddDuck]

Looks like backfiring through the inlet manifolds to me.

I'd suggest checking compression (leaky inlet valves / worn guides would allow this), if that's OK then have a look at resistance between the coils and the battery (best done with the ignition on but the bike not running and looking at dc voltage between battery terminal and coil terminal).

The resistance is something I've been through with the Ducati 900SS. The coil LT side is usually pretty low resistance, i.e. 4 to 5 ohms or so, and the connection to the battery needs to be much lower than this or else it's possible to get coils cross talking.

[Honest Andy]

Yeah I was thinking leaky valves and guides, it'a a high milage machine and pretty easy to turn over on the kickstarter...
Interesting point about the coils though, the coil for that cylinder failed not that long ago. I'll test them again :yes: