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Misfire number 4 cyl.

MarkA

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Whitchurch, Shropshire UK
Hi, doesn’t matter what I have done so far, I can’t rid my bike of the misfire in number 4 pot.
Changed plugs, swapped HT leads, cleaned carbs out checked compression (128 lbs) and the rest are similar. Might it be the coil or the spark unit ......help.....lol!
 
Switch 1 and 4 plug wires leaving them attached to the same coil post to see if the issue stays at the plug, or follows the wire


 
I note the use of the words 'spark unit', is that bike a DOHC? If so the carbs can be 'clean' and present several problems anyway. For most people a single carb clean job is nowhere near enough.
 
Hi Amc49,
Yes it’s a DOHC. I have cleaned all carbs manually and fuel is getting through. I removed the plug on No4 pot and could smell fuel on the plug and a dark browning of the plug after a ride out later in the day. I doubt I’ve managed to completely clean the carbs out as I don’t have a ultra sonic cleaner. I checked all butterflies and adjusted them very slightly to bring them in line and checked the pilot jet adjusting screw and it was the same position as the other screws when checked. Will swap the coils over next. Thinking out loud..... should check the valve clearance ?
 
If the compression is that amount on #4 then not that but at some time it needs to be done.

The pilot jets clog to the nth degree and that done makes the carb go rich just off idle and it can easily wet the plug to not run as the ignition system is weak on its' best day. A leaking aircut will mess up off-idle running too.
 
You need to absolutely make sure the pilot is open using like a .013" wire size to check the opening, the easiest source of that is 'E' string off a guitar. Thinking the thinnest one.

You pretty much must break the rack apart to get to the aircuts except for the #1. That means removing the staked screws holding choke plates down. I grind the back ends off with a dremel and rock in it and then loctite the screws going back together. Elsewise you likely strip out the choke shafts removing the screws and then you need new parts. Not ALL chokeplates have to remove, only two. You end up with a pair pf carbs with a chokeplate and shaft still in them but can then work on all 4 carbs.

The carb tops MUST stay with their slides, they are matched parts.
 
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