Misfire at idle only! - SV650.org

gadget · 07-11-20, 08:28 PM

Hey all, my 99 curvy has just developed a misfire when idling only, as soon as the revs pass 2000 it fires spot on. It's never ridden when wet so I've discounted water in the electrics problems, the new plugs have done approx 1000 miles, brand new battery, reg/rec is spot on... charging at 14.4v, starts first push of the starter and idles well for the first few seconds but when warming up it starts to misfire and sounding like it's steadily firing on one pot, the other cylinder cuts in and out radomly, Rev it passed 2000 and all is well. It did this a week ago but when I took it for a good 20 mile run it idled perfectly when I stopped so I assumed it just needed a good 'blow out'. Now it's playing up again. When passed 2000rpm it pulls like a train with no hesitation at all, a while back I opened the tank cap and quite a bit of water poured into the tank because the flexible drain tube was kinked under the tank as it had rained the night before and the bike wasn't under cover as usual! I used some of that 'Sta-bil' additive as it claimed it could safely pass the water through the carbs to be burned with the fuel, not so sure it's done its job tbh.
I'm thinking maybe water in the float bowls / tank still!? Or maybe the float bowls/ tank are clear and it could be the choaks sticking!?
Any advice what to check first? Or anything else that may not be so obvious?
Tbh ... I don't get to ride it very often, maybe 3-4 times per month due to work and every day stuff going on,
Maybe it's contributed to these issues?

redtrummy · 07-11-20, 09:09 PM

stick a good dose of Redex in the tank and run it for a month or so then start thinking! Could be pilot jet problems

Dave20046 · 07-11-20, 09:41 PM

Choke sticking is very common on the curvy. That or possibly a small airleak?

I'd check the chokes and springs aren't gummed up and are moving properly, grease them and oil the cable.

Biker Biggles · 08-11-20, 09:36 AM

Agree with the above. If there is water getting into the tank you need to sort that.

embee · 08-11-20, 04:59 PM

Sticking choke plunger would be my guess from the described symptoms.

If you have one plunger not closing fully it will make the mixture on that cylinder far too rich at idle when warm, leading to misfiring. With the throttle further open the effect of the choke enrichment is reduced to the point where the cylinder will fire (as long as the plug isn't completely carbon fouled).
I have had this happen on my curvy when it was nearly new, having cleaned and lubed the plungers and cables where they join at the carb body it has never happened again. There was white crusty deposit, possibly from flux from the cable soldering?

gadget · 08-11-20, 06:50 PM

Cheers all for the suggestions and advice, I kind of suspected the choke plunges tbh as I have already sorted the water drain tube problem by adjusting the tube so it stays straight when the tank is down. So ... next job is to inspect the chokes and clean n lube them along with the cables. Then for good measure take it for a nice long run (in accordance with 'lockdown' regulations) of course! and see if that does the trick, I probably will also drop the float bowls to empty any water that may still be lurking in there and then clean the idle jets before I re-balance the carbs.
Oh joy !!

gadget · 12-11-20, 03:24 PM

Quick update on misfire, as it was a nice dry day I thought I would service the choke plungers, needn't have bothered tbh as they were in perfect condition but at least I can eliminate them, I took them out anyway and thoroughly cleaned them, oiled the cables and the choke plungers with graphite oil... smooth as silk.
Started it up from cold and it fires on both spot on ... , but less than 30 seconds running and it starts to fire on one cylinder again! As soon as I take it up to 3000rpm it cuts back in and fires on both cylinders.
So now I'm thinking either a blocked idle jet (hence only up to 3000rpm the problem exists) or a dodgy spark plug/coil or lastly maybe still some water in the float bowls! Any ideas where would be best to start to diagnose the problem? Having said that I still haven't taken it for a good long blast, maybe it would benefit from a tank of fresh fuel and a bit of redex!?

Seeker · 12-11-20, 03:41 PM

If you have (or can borrow) a strobe light that picks up its trigger from being clipped around the HT lead, it should help diagnose whether it's electrical or not. You would look for a gap in the strobe pulses and you could compare cylinders - you would not be looking to time the engine.

gadget · 12-11-20, 05:49 PM

Quote:

Originally Posted by Seeker

If you have (or can borrow) a strobe light that picks up its trigger from being clipped around the HT lead, it should help diagnose whether it's electrical or not. You would look for a gap in the strobe pulses and you could compare cylinders - you would not be looking to time the engine.

That would be a good shout but I don't have one and don't know anyone who has unfortunately so it's back to ... process of elimination. Btw ... can a plug misfire at a given rpm then burst into life when it reaches a certain rpm? I'm clearly not an electrical guru so have to ask these seemingly silly questions.
Having said that I'm still learning towards a fuelling issue, really not looking forward to removing the carbs for a jet clean.

Seeker · 12-11-20, 05:59 PM

The only plug failure I've ever had (other than 2 stroke plug fouling) was on a curvy SV. It didn't like low revs but worked better at high revs (but not perfectly). NGK have had such a problem with counterfeit plugs they released a tech bulletin
https://www.ngk-sparkplugs.jp/englis...ake/index.html

07-11-20, 08:28 PM	#1
gadget Member Join Date: Aug 2019 Posts: 397	Misfire at idle only! Hey all, my 99 curvy has just developed a misfire when idling only, as soon as the revs pass 2000 it fires spot on. It's never ridden when wet so I've discounted water in the electrics problems, the new plugs have done approx 1000 miles, brand new battery, reg/rec is spot on... charging at 14.4v, starts first push of the starter and idles well for the first few seconds but when warming up it starts to misfire and sounding like it's steadily firing on one pot, the other cylinder cuts in and out radomly, Rev it passed 2000 and all is well. It did this a week ago but when I took it for a good 20 mile run it idled perfectly when I stopped so I assumed it just needed a good 'blow out'. Now it's playing up again. When passed 2000rpm it pulls like a train with no hesitation at all, a while back I opened the tank cap and quite a bit of water poured into the tank because the flexible drain tube was kinked under the tank as it had rained the night before and the bike wasn't under cover as usual! I used some of that 'Sta-bil' additive as it claimed it could safely pass the water through the carbs to be burned with the fuel, not so sure it's done its job tbh. I'm thinking maybe water in the float bowls / tank still!? Or maybe the float bowls/ tank are clear and it could be the choaks sticking!? Any advice what to check first? Or anything else that may not be so obvious? Tbh ... I don't get to ride it very often, maybe 3-4 times per month due to work and every day stuff going on, Maybe it's contributed to these issues?

07-11-20, 09:09 PM	#2
redtrummy Member Join Date: Jan 2019 Location: N W Leicestershire Posts: 248	Re: Misfire at idle only! stick a good dose of Redex in the tank and run it for a month or so then start thinking! Could be pilot jet problems Last edited by redtrummy; 07-11-20 at 09:10 PM.

07-11-20, 09:41 PM	#3
Dave20046 Member Mega Poster Join Date: Jul 2008 Location: Sheffield Posts: 10,274	Re: Misfire at idle only! Choke sticking is very common on the curvy. That or possibly a small airleak? I'd check the chokes and springs aren't gummed up and are moving properly, grease them and oil the cable. __________________

08-11-20, 09:36 AM	#4
Biker Biggles Member Mega Poster Join Date: Jan 2005 Location: Barnet Herts Posts: 5,068	Re: Misfire at idle only! Agree with the above. If there is water getting into the tank you need to sort that. __________________ On a clear day we stand there and look further than the ordinary eye can see.

08-11-20, 04:59 PM	#5
embee Member Mega Poster Join Date: Jul 2003 Location: Warwickshire Posts: 2,801	Re: Misfire at idle only! Sticking choke plunger would be my guess from the described symptoms. If you have one plunger not closing fully it will make the mixture on that cylinder far too rich at idle when warm, leading to misfiring. With the throttle further open the effect of the choke enrichment is reduced to the point where the cylinder will fire (as long as the plug isn't completely carbon fouled). I have had this happen on my curvy when it was nearly new, having cleaned and lubed the plungers and cables where they join at the carb body it has never happened again. There was white crusty deposit, possibly from flux from the cable soldering? __________________ "Artificial Intelligence is no match for natural stupidity"

08-11-20, 06:50 PM	#6
gadget Member Join Date: Aug 2019 Posts: 397	Re: Misfire at idle only! Cheers all for the suggestions and advice, I kind of suspected the choke plunges tbh as I have already sorted the water drain tube problem by adjusting the tube so it stays straight when the tank is down. So ... next job is to inspect the chokes and clean n lube them along with the cables. Then for good measure take it for a nice long run (in accordance with 'lockdown' regulations) of course! and see if that does the trick, I probably will also drop the float bowls to empty any water that may still be lurking in there and then clean the idle jets before I re-balance the carbs. Oh joy !!

12-11-20, 03:24 PM	#7
gadget Member Join Date: Aug 2019 Posts: 397	Re: Misfire at idle only! Quick update on misfire, as it was a nice dry day I thought I would service the choke plungers, needn't have bothered tbh as they were in perfect condition but at least I can eliminate them, I took them out anyway and thoroughly cleaned them, oiled the cables and the choke plungers with graphite oil... smooth as silk. Started it up from cold and it fires on both spot on ... , but less than 30 seconds running and it starts to fire on one cylinder again! As soon as I take it up to 3000rpm it cuts back in and fires on both cylinders. So now I'm thinking either a blocked idle jet (hence only up to 3000rpm the problem exists) or a dodgy spark plug/coil or lastly maybe still some water in the float bowls! Any ideas where would be best to start to diagnose the problem? Having said that I still haven't taken it for a good long blast, maybe it would benefit from a tank of fresh fuel and a bit of redex!?

12-11-20, 03:41 PM	#8
Seeker Member Mega Poster Join Date: Jun 2016 Location: NE Lincs Posts: 1,069	Re: Misfire at idle only! If you have (or can borrow) a strobe light that picks up its trigger from being clipped around the HT lead, it should help diagnose whether it's electrical or not. You would look for a gap in the strobe pulses and you could compare cylinders - you would not be looking to time the engine. __________________ 2016 SV650AL7 2023 GSX-8S

12-11-20, 05:59 PM	#10
Seeker Member Mega Poster Join Date: Jun 2016 Location: NE Lincs Posts: 1,069	Re: Misfire at idle only! The only plug failure I've ever had (other than 2 stroke plug fouling) was on a curvy SV. It didn't like low revs but worked better at high revs (but not perfectly). NGK have had such a problem with counterfeit plugs they released a tech bulletin https://www.ngk-sparkplugs.jp/englis...ake/index.html __________________ 2016 SV650AL7 2023 GSX-8S

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