Robbie9146

Hi guys, I'm a first-time poster so be gentle!...

I've got a '99 SV650 and it's backfiring on a constant/closing throttle and it feels as though one cylinder is cutting out at a constant speed (I think it's the rear cutting out, I'll explain why in a minute). On acceleration the bike runs fine, except at the beginning of the throttle opening when the second cylinder takes a second to jump in. Here's where I'm up to so far...

- The bike has had 2 new sets of spark plugs. The reason why I think the rear cylinder is cutting out is because the first new rear plug became black very quickly, the front plug looked normal. I think the rear cylinder is cutting in/out and the build up of unburned fuel is causing the spark plug to look as though it's been run rich.

- The carbs have been cleaned twice, once by myself and once by a mate who runs his own workshop. He has also balanced the carbs for me and re-routed a breather pipe to give it more "still air" so it can operate properly. He now reckons the carbs need cleaning in an ultra-sonic cleaner which he has just ordered to clean the inaccessible passages, but that's going to take 3 weeks to arrive so I want to use that time to eliminate as many other possibilities as I can.

- The first time I took the carbs off I set them as per the Haynes manual settings. I might not be Jerry Burgess but I'm reasonably competent with a Haynes manual in front of me...

- The battery has been charged both by a battery charger and running the bike so I don't believe the battery is at fault although I haven't checked the specific voltage. Despite it being a new Varta battery only a few months ago the bike has been stood for a couple of months since then and it needed a fair bit of charging to bring it back. All seems to be OK now though and it seems to be holding a charge.

- I've checked both of the HT leads, they seem in reasonably good condition although I found that the rear HT lead was not fully connected to the spark plug. Fixing this has not improved the bike's running.

- I've checked the rear choke plunger and that seems OK. I've also managed to make the choke lever stiffer while trying to improve it's action... Go figure.

- The bike has this week gone through half a tank of fresh petrol which hasn't made any difference to the running, ruling out water in the fuel.

- Yesterday I checked the rubber boots between the airbox and carbs for any air that could be leaking past. They're fitted properly (now - no idea why they weren't before) and this hasn't helped the problem.

As a bit of background, my brother (accidentally) set fire to the bike about 2 years ago (it's a long story). The fire caused cosmetic damage to the right-hand rear seat cowl area and under tray. He has since replaced all the parts that were affected, but I have started to think that the fire could have damaged an electrical component such as the ignition unit or something similar behind the seat cowl that could be responsible, although as far as I can remember the bike was running badly before the fire - it's all becoming a bit hazy now though. The bike is also on it's third reg/rec. which is something that was suggested on a similar thread.

As a result of this and the misfiring/backfiring problem the bike has hardly turned a wheel for the last 2 years. I've recently got the bike back on the road and started using it despite the running problems just to get out on the road (the first few GPs of the season are usually enough to get me craving my bike after the winter) but I'm really scratching my head on this one and it's beginning to test my resolve... I've tried all the things that are cheap/free to fix and I wanted a second opinion before I start throwing money I haven't really got at the more expensive possibilities.

Has anyone had any similar experiences, or any other ideas on what may be causing the problem? Any suggestions would be greatly appreciated and could be useful.

PS. Sorry for the long thread but I just wanted to give as much background into the problem as I could remember!