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I have a 500 - I've changed to digital ignition, cleaned and balanced the carbs - but no matter what 2 highly respected mechanics do ,the bike rides perfectly for 15-20 miles then one cylinder starts to cut out then cut in - the plug on the cylinder that is the problem is black but when changed the fault just recurs.
Can anyone help - what are we missing?

Is it the same cylinder? If so, do you have a faulty coil?

You could try swapping the coils around and see if the fault moves.

Which ignition system do you have?

Mike

PS - Kinross is a nice part of the world.

Hi Mike
Thanks for the reply - I fitted a brand new digital system to cure the problem all it did was extend fault to occur from approx 1.5 to 15 miles running...
Yes it is the same cylinder - the rear one. For the first 10+ miles it runs like a dream then the cylinder starts cutting in and out getting progressively worse. 2 really good mechanics have looked at it and cannot understand what is going on

Hi,

Can you tell us which ignition system you have installed please.

Does it recover if you let it cool down?

Thanks
Mark

Hi I'll get the name of the system it's the one recommended by the guys who advertise in the magazine and yes leave it to cool down and everything seems fine....
Really appreciate the help

Plug black wet or black sooty?
If the whole ignition has been changed and your resident rocket scientists are flummoxed another regular culprit is choke plunger seal degradation. Or knackered rubber seals on the choke plungers. Does the fault swap cylinders if you swap carbs? Or swap coils?
I'm guessing you mean the Sachse ignition system? A stronger spark would likely cope with over-fuelling for longer than a traditional 40 year old system.
Also the float height if a long way out could make it over rich. On another bike that fooled me for a while, the main jet was loose in its thread so excess fuel was leaking past that.
Another clue-when left to cool down a wetted plug can dry out!

I'm checking but I do believe when the carbs were swapped the system seemed to correct itself however the ignition was changed at the same time. The plug is I believe black and sooty not wet (but I need to check I am correct on this. I think it is a carb problem but they've been cleaned any worn parts replaced etc and the problem persists.

and the choke plunger seals? If they leak your bike will never be right.

You can quickly swap the choke mechanisms and see what happens, and have a look at the end seals whilst at it. Will it start without choke?
John

It won't start without the choke - once again thank you for the help
Rod

Bummer!
I was also going to suggest swapping carbs - in fact, I believe you can swap the entire top ends piece by piece if you want as they're the same.
(Correct me if I'm wrong, someone.)

Have you checked the inlet manifolds aren't split?
DD

When I first got my 3.5 it would cut out on the front after exactly 7 miles then recover when cooled and do exactly the same thing again. It was uncanny how precise the distance was for it to recur. It turned out to be the stator coil breaking down when hot, or at least getting it rewound solved the problem.
thanks - Duncan

To me as well that sounds like some lectronic part cannot deal with heat.

One of the mechanics did swap the carbs this changed the cylinder that was cutting out - rebuilt that carb and the problem seemed to go away - actually the distance before the problem increased the road test wasn;t long enough...
Really appreciate everyone's help. it's going in to a really good mechanic on Monday - its a beautiful bike just hope I can find the problem and fix it - will try everything suggested until we find an answer