Could this be some kind of breather issue? Some piping in the fuel tank? (I don't know much about carbed 900s). I've had similar issues on FI bikes that have been solved by cleaning up the fuel filler cap. But I am no mechanic, sorry if I'm talking rubbish.
Don't know about the SS but, on Monsters with the same engine and carbs, there are some little plastic pods that the carbs vent into. I believe the function is to protect the carbs venting from airflow, causing pressurisation issues, not sure how critical this is or whether the SS has a different setup but maybe it's something related to this, especially if it only happens on the move?
The breather issue is relatively easy to diagnose and fix. If the tank can’t breathe, because the breather holes in the cap are blocked, the effect will be that fuel being pumped out of the tank to the carbs will create a vacuum in the tank, starving the supply of fuel. If this is what is happening, try and open the cap. It should be difficult to do so as the vacuum will be ‘sucking’ the cap closed. If this is the case, simply clear the breather holes in the cap, the collar below the cap and at the breather/overflow nozzle at the top/front of the tank. There were tales of some tanks imploding due to this vacuum, but they may have been urban myths.
Hmm, well OK. Firstly, thank you for all your replies while I've been doing other stuff over the past week. I have had another crack at it just now and am fairly confident that it isn't a sticky float. To test I checked the drain on both carbs to see whether there was fuel in the float bowls and there was. Then I ran it down the road. It started up fine as normal, running on both. A couple of hundred meters down the road and the misfire is still there under load. Took it back home and checked the drain on the RH (front/horizontal) carb bowl and there was still fuel in it, so I don't think that's the cause. I doubt it's tank vacuum as that would affect both carbs/cylinders, rather than just the one, but it's a good thought. @Dukedesmo - good shout on the carb breathers - you are right the SS does have the plastic pods and, guess what, one of the breather hoses had popped out and was lying on top of the engine cases. Sadly though, it was the one for the vertical cylinder, not the horizontal, so its not that either. I've put it back in now. I'm now at the point of trying all things again. I will replace the HT leads I think to eliminate them as a cause (BTW - is there any difference in HT lead quality? You can pay £2.99 inc postage or £15 pus postage for a 1m length of HT wiring on eBay, seemingly both the same spec: 7 mm O/D, 21 core, 0.3mm, 1.5 mm2 cross section. Don't want to buy absolute dross, but also don't want to get ripped off by confusing price with quality, if it's all the same stuff!). For the time being I have popped the CA coil back on the the horizontal cylinder and taken it for a run. It's still lumpy and misfiring, but seemingly not as bad as it was previously. The front pipe was good and hot when I brought it back. Have also been on a charm offensive with the neighbours, so now they sympathise with my issue, rather than just thinking I'm a noisy hooligan disrupting their peace. At least I hope they do. They've stopped scowling at me anyway. Putting the standard cans back on instead of the carbon Microns while I worked through it all was probably a smart move... Cheers
If it's a harsh cutting out i would point somewhere (again) at the ignition system, if its softer or gradual then i would be looking at the fuel side.
Yeah, I think I agree. The fact that it is OK on start up and tickover but struggles under load suggests fuel. The carbs have been cleaned but I guess there could still be some rogue gunk in there. Going to have to get them out again and have a proper look.
