A10 won't start :(

[Greybeard]

Some progress then   

[muskrat]

G'day owain.
Sounds like the pilot circuit is blocked or too small jet. Is the pilot jet screwed into the body above the float chamber or pressed in opposite the air screw? Early concentrics had the screw in but later had a non adjustable jet pressed in.  Hesitation at cruise could be needle jet and needle or slide (they wear).
Cheers

[owain]

Cheers everyone, the bike is back in the garage for this evening. I quickly removed the spark plugs though and they are already black and they were brand new. They were not wet or oily, just coated in black soot. So a problem with the carbs jets/ needle position seems like a logical place to look.

Oh and the concentric is the old type of concentric with a screw in pilot jet.

It seems a little odd though that the bike has been running fine with this carburettor and jets for the last few months and all of a sudden it's running so rich that spark plugs foul within a few minutes?   

The other suspicion I have is that the valve sleeves are allowing oil to get past the valves and into the combustion chamber. Perhaps the oil leak from the rocker box could be an indicator of a cylinder head issue?

The carb is definitely the cheapest and quickest area to investigate first. Failing that, it's a little annoying if the valve sleeves do need replacing as a machinist checked out the cylinder last summer and told me everything was in good order 

[KiwiGF]

Ah well I may have spoken too soon actually. It seemed to run ok yesterday. It wasn't sounding very good today though. When I accelerate it sounds good. When I'm cruising it sounds like it's 'hiccuping'/ running a bit poorly. I thought that maybe this could be due to the timing being slightly off. If I didn't give it any throttle. It dies on idle. I had to twist the idle screw in the carb really far in to keep it alive at a fast idle.

I'm thinking of taking the carb apart checking all the jets e.g. blowing them out with compressed air. Then giving it a go again. If it is still running a bit off when cruising then I'll reset the timing again to see if that improves the situation 

In my experience compressed air is not always enough to clear the idle circuit on the concentric, to clean the idle circuit properly you need to get an understanding on “how” the air and petrol gets to the adjuster screw, and thereafter to into the manifold. I’ll have another look for that diagram of the idle circuit.

[muskrat]

Yes I had one that was so blocked I had to remove the brass welsh plug to clean it out. Check the passage in the bowl that feeds the pilot jet, often overlooked.
Black sooty plugs is the other way so the air passage may be blocked. I had an XT that was giving similar problems. Found the aftermarket filter was blocking that passage.
The needle and needle jets wear, especially noticeable at cruising throttle opening and can upset pilot settings.
Cheers

[morris]

The other suspicion I have is that the valve sleeves are allowing oil to get past the valves and into the combustion chamber. Perhaps the oil leak from the rocker box could be an indicator of a cylinder head issue?

Oil in the combustion chamber will leave a trail of blue smoke behind you but won't make your engine run rough.

[muskrat]

One exception to that ^^^ morris. If the inlet guide is bad enough and on small throttle openings crankcase fumes/air can be drawn in upsetting the mixture.
Cheers

[Guy Wilson]

Just too add...
I had an almost identical experience yesterday on my 53A10, coughing, spluttering, running on one cylinder under load, black plugs etc. There was a little bit of oil in the left hand pot, no real smoke though, but  I assumed it an exhaust valve not bedding correctly or a valve seat.
I pulled the head off and it all looks pretty fine. Valves sealing perfectly..no sooting on the rocker side..
If only I'd read this yesterday.... thank you for sharing all this. It has an impact around the world and keeps bikes running,
Guy