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Popping, poor running

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MarkCR750:
Just when I need the bike for the distinguished gentlemans charity ride it develops a problem!, it started popping a lot on rundown, tickover uneven, No3 downpipe is getting warm not hot, I've just swapped HT leads 2 and 3 over, no difference, new plugs, no difference, raising tickover to 4k No3 pipe does get hotter accompanied by backfires, I'm hoping it's something simple like a blocked primary but not at all sure it is, here's the old plugs, as you can see the No3 porcelain is white, any thoughts greatly appreciated

hairygit:
I'd remove the tappet caps on the offending cylinder and check the valve clearances and the valves are not sticking or bent, easier than removing the carbs! If all is well there, turn the fuel off, then remove the drain plug from each carb in turn, and see if the offending cylinder has the same quantity of fuel as the others in it. If they are roughly equal, it's carbs off and check for crud in them, as you have eliminated points/condensors/coils as an issue by swapping leads and it making no difference.

hairygit:
Also, try a new plug in the dodgy cylinder!

MrDavo:
Something else you could check - one of my plug caps was going AWOL occasionally, giving me a misfire that came and went, with occasional spells of running on 3 pots. I found it by checking the resistance on each cap with a multimeter, after unscrewing them from their lead.

K2-K6:
They were talking about the distinguished gentleman's ride on the wireless yesterday old chap ;) I just caught a bit of it and thought it was for bicycles,  sounds more interesting to see if it's real proper bikes.

The plug from no 3 just seems to be saying lean compared to the others. As hairy says have a check of the valve clearance just to make sure your not missing anything.

Swop 2 and 3 plug leads to see if it changes the running on those two to cross check the caps and leads.

Without being in front of it,  looks like primary circuit on number 3 could be restricted. Maybe worth trying to see with a pipe into the vac check hole on that cylinder and dangling into a small pot of petrol to see if it'll tickover by drawing from an auxiliary supply and show if the problem is caused by that area.

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