Curvy SV Misfiring and Backfiring

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!

Mine is doing similar, not looked at it too much, but I think its the carb on the rear cylinder sticking open, I might be wrong though. I have replaced plugs and HT leads so far! Not helpful I know, but I will let you know if I get anywhere with mine.

Did you check the HT leads with an ohmeter? Easy enough to do. I had a front lead that broke the wire, but the insulation was OK. Gave me an intermittent problem, though I don't remember it as exactly your symptoms.

[TamSV]

Had a similar issue with mine recently which turned out to be an air leak. The rubber boot on the front carb nipple had broken up. You could check that, as well as the vacuum hose between rear carb/fuel tank and the inlet rubbers (on the engine side of the carbs) for any damage. Make sure carbs are properly seated too.

The only other thing that occurs off the top of my head is have the carbs been balanced since they've been rebuilt? If they're way out of balance one could be running rich and the other lean maybe?

Originally Posted by Robbie9146

my brother (accidentally) set fire to the bike about 2 years ago

Now that story rings a bell. Did this involve an angle grinder and a chap named Mike by any chance?

By clean the carbs do you mean strip them bare, remove and clean the jets too?
If you run the bike at idle does the rear header pipe get as hot as the front one?

That's just a question but my guess from what you describe is that you've got a vacuum leak, its stopping the air getting sucked in giving you the rich rear cylinder. Only when you blat open the butterfly in the carb does the bike come to life as the compression in the cylinder sucks the air through with more umph.

Some good suggestions there, thanks guys. I've got the afternoon off so I'm going to start by checking the vacuum hoses. I don't have an ohmeter to check the HT leads unfortunately but if the problem still persists then I might have to invest in one.

When I cleaned the carbs I stripped them down to clean them and re-set them to what the manual recommended, but left the synchronization as it was when they came off the bike. When I took it to my mate's workshop he cleaned the rear carb again and balanced them properly for me, although he did say they were more or less spot on as they were.

As for the header pipes, I haven't checked how hot they get but the bike sounds like it's running fine at idle, nice and smooth. It's only when I'm trying to ride at a constant speed, through town for example, that the misfire/backfire starts and I end up giving old ladies a heart attack as I go past. Not to mention how jerky it can get as one cylinder cuts in and out.

@TamSV, yes you're absolutely right about the fire, Mike is my brother and was stripping paint off an engine block that he was re-building to put in his Corsa at the time. The block was on an engine stand in the garage next to my bike and the rest is history... No idea how you know that though given that this is my first post on here! Do you know any of the old Southport Riders guys? That's the only possible connection I can think of.

[TamSV]

You've got the right rear seat panel off my project bike. Mike bought it on ebay and sent me a message asking if I had some other bits and pieces he needed and explaining what he had done. I think he possibly bought an indicator and undertray off me too - the **** taking he got was free of charge.

I've sold a fair bit of stuff on e-bay but that one stuck in my mind.

Okay so this afternoon I've checked the vacuum hose from the carb to the fuel tap, and the corresponding breather cover on the front carb - both seem absolutely fine. I've checked that the fuel tap isn't leaking fuel back into the vacuum hose and into the inlet manifold and it is working as it should. I've also checked the rubber boots between the airbox and the carbs and the rubber inlets, all seems well and everything is seated correctly.

I've had the new plugs out, there's not much in it but the rear plug definitely looks like it's showing signs of abnormality. Also checked the temperature of the exhaust pipes and the rear isn't getting as hot as the front, so I'm 99% certain that the rear cylinder is causing the problem.

Based on that I took the cap off the rear carb based on a theory that a mate of mine has about one of the diaphrams possibly being split. The diaphram isn't damaged but it is slightly out of shape at the edge, so I took a photo and I've attached it to this post. My gut feeling is that it's not enough to cause a running problem but I thought it was worth mentioning just in case.

I have taken a video short of the bike running to illustrate the problem. I'm going to upload it to YouTube and post a link later when it's been accepted, so you can all see what I mean.

Does anybody have any other suggestions or should I start replacing electrical components now do you think? If so where would you start?

On the bright side I have been able to improve the operation of the choke lever! (You've got to take the small victories in life...)

YouTube have reliably informed me that when the video is live the link will be:

http://youtu.be/vHqNkJc83tw

Please have a look and tell me what you think could be the cause. It's less than a minute long but I think it gets my problem across. You'll have to excuse the sound quality but take my word for it, it sounds fine on idle it's just the backfiring that's an issue...

It's not quite live yet, I've only posted it now because I'll be out now until about 10pm, I just thought I'd leave it with you all while I'm out. Thanks again!

#1- I had a rear spark plug cap that kept popping off. It stayed close enough to the plug to start and idle OK, but on application of power, it missed and stuttered, just like yours is doing.
#2- I had a front plug wire (HT lead) that broke. Just the wire broke, not the insulation. As the wire bounced around, it would sometimes be a closed circuit, sometimes open. When it was open, again, identical to the operation of your engine.
#3- A bad plug, with high internal resistance or open internally would also cause what you have.

If you're not having the plug cap popping problem, I recommend you invest in new plug wires and plugs.

As a first step, you could swap the plugs to the alternate cylinders, run it, and check the plugs again. (or just check exhaust pipe temp differentials again. If the problem follows the plug, that's the problem. If it doesn't, it's likely the rear HT lead.