SOHC.co.uk Forums > CB500/550
Ongoing PD46 carburettor woes (possibly)
keithtraffic:
I did think about that but I can't make the No4 lead reach to the No 1 cylinder - is there a quick way round this?
K2-K6:
You'd have to dismount the coils and physically flip them over to facilitate reaching the plugs.
Maybe of benefit though, run it in dark conditions and carefully view the 1 lead closely to see if you have any leakage. You can usually see a spark but also hear one too, like a loose tappet.
If there's obvious spark you can move the lead with a stick to see if the cylinder chimes in. Where they pass rocker cover is normal suspect.
keithtraffic:
I will give that a go tomorrow - although I am not sure that would explain the situation where I can spray carb cleaner into the vacuum tube screw hole and the cylinder picks up and runs pretty cleanly (for a second or two).
K2-K6:
Yes was thinking the same as you, that it fires with spray cleaner.
But to explore what's happening, cleaner is much more volatile than fuel mix and will fire with veeeery weak spark. So is it cutting in with cleaner for the above reason (comprised spark), or because it had no fuelling from carburettor.
Guess you could go either way to prove / disprove, one of which involves carb removal :)
I had one recently that took an embarrassing time to get it. On a V8 had similar symptoms of dead cylinder on cold start, chimed in after a while, same again if left for 1/2 hour, exhaust temperature confirmed the cylinder # ended up finding a break of HT lead just inside the rubber boot entering plug cap. You could see a carbon track where the spark was travelling too (not ngk caps) once fixed it's absolutely perfect.
Just another quick reality check, have you swapped the plug caps over between 1 and 4 ?
keithtraffic:
Well its all a bit bizarre!
I played around with trying to get the coils off and turning them round but that seemed to be bit difficult in practice and as the no-4 cylinder lead almost reached the no-1 spark plug in any case, I carefully put the coils back in place and fashioned a small 'bare wire' extension, just long enough so it would reach the spark plug from the plug cap, taking care to make sure it could touch any of the cylinder metal work.
But before trying this out I though I would put everything back properly, just to re-check the baseline - and lo and behold the engine fire up and after about a minute was ticking over fairly smoothly - a touch fast at 1600 revs but with no indication of the backfire - and cylinder 1 was definitely working as it got as hot as the others very quickly.
Nevertheless did then swap round the plug leads but this made no difference that I could perceive - so put them back properly again and the engine fired up easily on the starter button with no throttle - and then just sat comfortably at 1600 revs.
So - either disturbing the coils and plug leads has done something or perhaps what ever blockage there might have been in the No1 carb has moved - but I really have no idea which it is!
Assuming the weather here tomorrow is OK I will the bike out again and see how it goes - but any thoughts on the above?
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