Well,..yeh...I guess. Depends on how the missing feels and how bad it is - 1 bad carb vs 3 good cylinders..I'm not sure how much would be noticeable. I would step thru the electrics before going to the fuel system. It needs doing anyway. But always approach solving anything methodically - not dynamically trying to zero in a "likely" cause. You've done the plugs/leads, now check the points - are they badly pitted, out of gap,...clean/check and/or replace...then..
- ignition timing. Follow the manual to check timing is set true.
- coils. Related to spark plugs,,,but - now check HT leads back to colis are sound. I had 1 that was nearly broken thru where it entered the coil. It is an easy job to replace the HT leads in the colis, but does require pemanent disfigurement and scarring - cutting them open. Well covered on this site.
- back to plugs - did it do it
before u replaced the plugs? If not, you may have a naff plug. Get a spare and swap out 1 at a time.
Given it farts about only when you are running at speed and not always - I'd have to think fuel contaminant or irregular supply.
- So now to FUEL. There are better experts then me on this subject - but here's my advice.
- is fuel in tank clean and pretty much free of sediment? no? get busy. Don't go futher with any crud in there!
- check fuel lines from petcock to carb rack - must be downhill WITHOUT kinks.
- check each rubber coupling from carb to head for leaks, cracks etc. Spray with WD40 or something with motor running to see if revs increase to indicate a leak. You often hear a farting "fffhhhttt" sound from a leak here. Check the retaining bands at both ends are zipped up.
- drop off the float bowls and pull out mains jets, emulsion tubes, float valves, idle jets. CLEAN, blow, check replace. Can be done on bike with all bowls off,...replace the inside ones 1st to leave room for outers. Getting retaining clips back on can be annoying. Check u have suitable jets for your engine. Stock may or may not suit your config if u have different header/pipes. e.g. i have 40 idle jets and 120 mains - printed on them.Good grey/brown plugs should mean this is likely to be AOK though.
- try to check float bowl level - advice on this site for using the overflow outlet with a tube attachd to measure contents.
Else,..and best,...pull off the carb rack and pull it all open and clean, blow air and check float levels at 26mm.
After this - I'd hope for someone like TwoTired to come and help..
Good luck
Mark
