Curvy SV Misfiring and Backfiring

My fiance recently bought a Curvy that wasn't running well, presenting the same issues you seem to be having and was also sat for a while.

I stripped the carbs (twice) balanced them and reset mixture screws, replace spark plugs and tried the battery off my SV. No joy.

As a last resort i drained the tank and found water in the fuel, so i had to remove the fuel tap and completey drain it, then leave to air overnight to get the last of the water out.

re-fitted it all and refilled it and it runs a dream.

Might be worth considering it, esp. if the bike was left outside?

It really does sound like a weak spark.
If the plugs and wires are OK, I'd check the coils next, rear cylinder first.

May be worth checking that the choke cable hasn't rusted inside, common problem on old SV's. The plungers will look ok but the cable prevents it returning properly.

Originally Posted by RogerNeville

May be worth checking that the choke cable hasn't rusted inside, common problem on old SV's. The plungers will look ok but the cable prevents it returning properly.

Good idea, but the way it started in the video, I assume it was warmed up, and it sure didn't sound like it was running on the choke, i.e. too rich at idle.

Thanks for the suggestions. I think I mentioned that the rear cap hadn't been fitted properly, but whether or not this was due to it being forced off or whether it's just something that I overlooked when I replaced the plugs the first time I don't know. Something I did notice inside the plug cap when I was working through some of the suggestions I got was that inside the rear plug cap there are 2 small slots either side of the centre of the cap where the plug goes. One of these slots seemed to have a small build-up of something that I can only describe as rusty-looking. (I know what you're thinking - probably rust then!) I'll try to get a photo to show you but I don't know if the camera will be able to pick it up with it being so far inside the plug cap.

The bike is kept in a garage so I really don't think there's going to be any water in the tank, although I had thought about taking the tank off and draining it just to clean out any sludge while I was in a spannering mood. Unfortunately I didn't have a container to drain the petrol into at the time so it got put on the ever-growing to-do list, along with the fork seals and the head race bearings and cleaning up the fork lowers...

The bike had been run for a couple of minutes before that video was taken and was warm, but as you pointed out it starts fine and idles fine and doesn't sound like it's running on the choke. There is a small amount of popping from the exhaust when you stand right next to it but as far as I can remember that's always been there and several people I've asked about it have just put it down to the baffle in the exhaust. The plungers do look okay but the cable could probably do with replacing. The cable itself is a bit stiff and one of the ends of the cable splitter seems to have been damaged and isn't holding one of the cables as well as it ought to.

I've got all day tomorrow to have another look, but I go away on holiday on Monday and I still need to get packed and ready, so if nothing turns up tomorrow it's going to have to wait until I get back. I might have an hour to spare tonight but I'll just have to see how things pan out.

Thanks again for all your suggestions and I'll let you know if anything works. I'll also have a go at getting a picture of inside the rear plug cap so you can see if you think it might be something that could cause a weak spark.

It may be that the plug cap is popping off, but, if you stuck it back on for your test run video, it was on then and shouldn't have been causing any problem. It's only when it's near, but not touching the plug that you'll get a weak spark.
Did you try swapping plugs front to rear to see if the problem followed the plug?

[yorkie_chris]

Sounds like an air leak to me. Or at least a bit lean.

Toot in the jets/water in bowls, split or loose rubbers, split or missing vacuum covers, split vacuum lines etc.

Just to give you a quick update I spent most of Friday night and Saturday working through various possibilities with the HT leads and coils. The rear cap was loose in the HT lead to the extent that it didn't have to be unscrewed to be removed so I trimmed it down by 10mm to get to fresh lead and that improved it a little. The suspect substance inside the cap actually turned out to be glass-like and not metallic as I first thought. That has now been removed but I'm looking at replacing the caps and leads (front and rear) when I get back from my travels next week. If that doesn't cure it I'll look at new coils and getting the carbs thoroughly cleaned in my mate's new ultrasonic cleaner.

On a slightly more depressing note, a friend of mine knows someone who races a CBR600 at club level and also rides a curvy SV650S so he knows quite a bit about mechanics in general and SVs in particular. He thinks that the head gasket on the exhaust side of the cylinder could have perished and would therefore be letting air in causing the backfire. Does anyone think this is at all likely? I want to eliminate as many possibilities as I can before I even think about stripping the top end of the engine down.

[yorkie_chris]

Not at all likely and the way you describe that isn't even possible.

[embee]

I'd guess he was meaning the exhaust header pipe sealing ring (where it seals against the cylinder head/port) rather than the head gasket. Air leaks into the exhaust joints can sometimes lead to pops and bangs on overrun.