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Modified rapture!

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On Friday my very extended top-end rebuild finally came together — and the bike started second kick!

However, loud popping and banging in the left-hand exhaust quickly set in, and by the time I had ascertained that oil was returning to the tank it was evident that the bike was now running only on the r/h cylinder.

Modified rapture! (to quote Nanki-Poo in The Mikado).

At this point I was called in for tea, and on Saturday was only able to spare a little time for further investigation.

Obviously there is a wide range of components involved that might by chance have chosen just that moment to fail, but to try and speed up diagnosis I'd appreciate suggestions of things that might be particularly susceptible after a lengthy lay-up (just over two-and-a-half years).

Saturday's work confirmed that there is no spark on the l/h cylinder, but a serviceable one on the r/h (the engine will fire but not run).

So far I think I can rule out

  1. the contact breaker unit: the points seem in good condition and the gaps were both exactly 0.015". Arguably the fibre wicks were a little dry, but evidently this didn't impede the initial start-up
  2. the coil; I have on hand a pair of brand-new coils from AN, and substituting one of these for the existing one brought no change.

For what it's worth the bike is a '72, but it is a rolling project and has clearly been the subject of various interventions in an adventurous previous life.

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Well here's a how-de-do! - but, for act 1, Young man [do not] despair for I've got a little list...

Try swapping the plug leads? 

Swapping/replacing the plugs

Good luck, I know there is beauty in the bellow of the blast

Cheers, Mike

 

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1/ Wiring fault or broken wire?

2/ Points sticking open?

3/ Condenser u/s?

4/ Dodgy plug or plug cap?

5/ Are the points assembled correctly, ie: wire correct side of insulation washers?

If RHS is firing, does swapping the RHS  coil wire over to left hand coil now result in left side firing? If so, then coil is ok, and fault probably lies in one of 1,2,3  or 5 above.

With the points closed, If you 'flick' the points open, is it still no spark? If so, that eliminates 2/.

Can you swap condensers over and see if no spark goes with it? If so, then it could be 3/.

Try running wires directly from battery to points to bypass as many switches as possible to help check for 1/.

Hope this helps,

Regards, George. 

 

 

 

 

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My first check would be the carbs...A year is more than long enough to allow the bush in the blind drilling to block itself.

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..How does a blockage in the carb stop a spark occurring?

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Thanks to Mike and George and Richard for suggestions; this afternoon I swapped the HT leads over and the spark changed sides!

Luckily I had new HT leads from AN already in stock — fitted them, and all good so far.

On the way to this I also checked the capacitors and measured coil resistances, which turn out to be bang in the middle of the range specified in the workshop manual.

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Put the old ones in the bin! If you leave them on the bench they might end up in the 'they'll come in useful one day' pile and in a few years you will be revisiting this situation!

Seriously though, glad you sorted it.

George 

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That put a smile on my face, been there done it. I hoard all kinds of "treasure".

 


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