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[Geraintosaurus]

The advice was very helpful Adrian, thanks - I've sealed up the exhaust pipe and am waiting on the carb gasket



HOWEVER



Today I took it out for a ride after sealing the exhaust valve and a lot of white smoke came from the valve - this was what it was doing when I got it though so I kinda just assumed this was normal. The afterfire problem did seem to have been fixed (woopee!) but then after a few hundred metres I lost power and couldn't start it again. The tank was running low so I decided to rule out low fuel as a reason first. Filled it up and it did indeed start (the carb backfired on one of the tries but only once so not sure what to make of that) but then it began to cut out again and I had to keep the throttle open to stop it. After pulling over and kickstarting, it would only go if the throttle was wide open and even then only very slowly, with the occasional loud bang out the exhaust as if it was only occasionally firing and perhaps very rich?



My thoughts are: all I did was seal the exhaust, and unless I'm mistaken all that could do is increase the pressure in the chamber. Either this is affecting piston rotation and thus throwing off ignition timing, but then wouldn't that have happened straight away rather than after a few metres, OR the increased pressure is keeping a bit more air in the chamber after the exhaust stroke, making it too lean to fire straight away and thus over rich the next stroke due to the leftover fuel, causing the alternating nothing-then-big-bang from the exhaust. Does this sound right?



I will reseal the carb as this might sort the mixture, and redo the exhaust, and then try playing around with timing. If anyone has any better ideas, let me know!



Cheers,



Geraint

[Count Johnny]

Hi Geraintosaurus



Whereabouts in the country are you?



Your name would indicate that you are in Wales and, if you are, it might be worth your giving your bike to Paul Henshaw, of Performance Classics, and asking him to fettle your bike so you can get some trouble free miles in and learn a little more slowly.



But - if you decide not to do so - I wouldn't worry about timing, or anything else, until you've fixed that airleak on the carburation side.

[Count Johnny]

PS

If you've run it very low on fuel, you may have dragged some shite into your carb which might explain you most recent woes.



But, as I said, I'd recommend a visit to Performance Classics or - if you really don't want to do that - you can add dismantling and thoroughly cleaning your carb, to the task of curing the manifold air leak.

[Geraintosaurus]

Hi Count Johnny,



Thanks for the advice - unfortunately not in Wales but am thinking of taking it to a shop in Watford. Question is how to get it there. I recently cleaned the carb but yeah probably worth doing it again just in case - it was a very sudden thing so it sounds like it could be a blockage.



If that is the problem, looks like I might have to re-coat the tank but whew is that an expensive job!

[Geraintosaurus]

Sealing up the exhaust valve solved my backfire problem handsomely; I will have to redo it with the beer can idea soon though as the seal is already leaking.

The newer problem turned out to be a main jet that somehow shook itself loose! I think I should have guessed it was a carb issue, given how suddenly it occurred. Lesson learned.

Thanks again everyone for the advice!