Stumped

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Hello all,

I am a newbie to Nortons. I recently purchased a 75 MK III, a nice original bike, 16K miles. I have recently added a Boyer ignition, new coils, plug wires, plugs, new battery, removed, rebuilt the Amals.
My problem is that upon starting, (which it does easily), it runs good for maybe a minute or less, then the right cylinder begins to misfire. When I attach the timing light to that side it indicates intermittent sparking. I have triple checked the wiring, good grounds, ohmed the coils, changed the plugs. This really seems to me to be electrical...... Can anybody suggest what to try next, this really has me stumped.

Thanks, Terry
 

RoadScholar

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Make sure that your fuel delivery is up to snuff and that the tank is breathing properly.

My original fuel taps (1975 Mk III) had screens that were so full of crud that they could have stopped a bullet.

The North Andover OE replacements I purchased had holes that didn't align making fuel delivery inadequate for anythimg over 50 MPH. I purchased a set of Paoils from Rabers, problem solved.

With everything you have done I'd think the electronics would be the least suspect, you can always swap in to old points for a check, but I'd look for fuel delivery problems first.

RS
 
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Ohming the coils will not expose a high-voltage leak or breadown. The swap-the-wires suggestion is always a good diagnostic technique. However, why not simply replace the 35-year-old coils with a modern dual-coil? The old coils will always be problematic, and modern coils will support a slightly wider plug gap (higher voltage output). Electrical peace-of-mind is available for well under 100 bucks. Take a look at the Pazon dual-coil product, for example.
 

rvich

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If you take a spray bottle and mist the coils and plug wires while it is running in a darkened garage you can often see if the high tension leads or the coil are arcing out. It just takes a really light mist to see the effects. Might be worth trying.

Used to do this a lot back when my cars still had distributors!
 
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Oops,

Sorry Terry,

Had I read your post more carefully, I would have seen that you've already replaced the coils. Disregard my obvious recommendation. Do you have clear fuel lines to confirm that you are feeding both carbs? When you turn off the bike and the petcock after the problem occurs, have you unscrewed the bowl drains to insure that both bowls are full ? Still, I can't see a fuel problem causing a plug to misfire. I have had open plug caps in the past, so swapping the plug leads, as ludwig suggests, gets you started. If the problem follows the switch, you can eliminate fuel and plugs, and focus on the RHS coil, lead, and cap.
 

L.A.B.

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tban52 said:
new coils,

Have you tried refitting one or both of the old coils/caps/wires? Or did this problem occur before you fitted the new coils?
 
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