Carbed SV backfiring at low revs - standard fueling etc

G · The Voice of Reason Joined: 02 Feb 2002 Karma :

I've got a pretty standard curvey road SV.

Since I got it second hand it's 'backfired' a bit at low revs.

Seems to be fine higher up, but especially from the front cylinder it seems to pop a bit below 3-4k. Does this while riding generally, not just on the 'over run'.
I stuck some new (well new to this bike, were in my minitwin) plugs in and the old ones looked like both cylinders are running rich.

Taking the air box off and starting it up, it was actually properly bike firing a little at low revs (through the front carb at least - didn't play too much though!)

The air filter looks ok.

Exhaust is standard as everything else seems to be.
Exhaust seems to be secure without leaks.

Any ideas what's likely to cause this?
I was thinking maybe float height set too high, but I still don't have a great understanding of carbs.

finpos · World Chat Champion Joined: 13 Sep 2005 Karma :

Backfiring is more likely to be caused by an electrical misfire - have a closer look at that horribly exposed from ht lead / cap / plug before you go tearing the carb to bits Smile

finpos.

G · The Voice of Reason Joined: 02 Feb 2002 Karma :

Already torn the carb to bits Smile

.

Well, the front one at least, which looked clean and in ok condition.
Did seem quite a bit of petrol in it, one of the reasons I wondered about the float height.

Someone who had a similar problem had a problem with the low tension lead for the coil, so will check that, the HT lead seems to be ok.

Kickstart · The Oracle Joined: 04 Feb 2002 Karma :

Hi

Popping back through the carbs tends to suggest that the inlet valve is not sealing when it needs to. Check the cam timing. Also possibly a burnt valve.

All the best

Keith
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the grim reaper · World Chat Champion Joined: 29 Jun 2005 Karma :

All SVs backfire because of water in the front HT lead. My missus had one and that did it, I've worked on about five others that all did it as well. Fit a fender extender and put grease around the HT cap to stop water ingress.

Cheers

Grim
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finpos · World Chat Champion Joined: 13 Sep 2005 Karma :

Innit. Later ones have a high-tech rubber flap dangling down from the tank somewhere to protect the plug+lead.

finpos.

G · The Voice of Reason Joined: 02 Feb 2002 Karma :

mr.z · World Chat Champion Joined: 04 Feb 2004 Karma :

I'd have thought an air leak (both these wont take 3 minutes so worth a try to elminate them, may be wrong)

on a cold engine (have an extinguisher standing by just in case) try spraying a little easystart/wd40 arround the inlet rubbers, if the revs rise at any point theres your leak..

also when its warmed, put a rag over the end of the exhaust, if it stops your exhaust is leak free, i had a previous bike pop and fart on occasions when there was a leaky exhaust on it (even crush washers worth a try).

Maybe valve adjustments?

Good luck fixing it, i hate niggley stuff thats difficult to pin down like this..
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Barry_M2 · World Chat Champion Joined: 09 Sep 2004 Karma :

I'd run a compression test, this will show up a valve problem.

I was also going to suggest an air leak on the inlet side (as mentioned above) between carb and head.

Does it ever miss on tickover?

You might just find that your carbs need a bit of a clean, more so the pilot jet(s). Main jets are not really used until higher up the rev range (further than you'd expect).
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G · The Voice of Reason Joined: 02 Feb 2002 Karma :

Thought I mentioned, but seems not - had a look in the carbs and all looked pretty clean.

Don't have a compression tester, though an excuse to get one Smile

.

It misfires on tickover a bit as well.

Barry_M2 · World Chat Champion Joined: 09 Sep 2004 Karma :

When you say 'looked clean', did you remove any jets and check?

What I'd do is, get a can of carb cleaner, drain petrol, remove carbs, take off float bowls, remove jets, spray through the jets, and spray into every single hole and crevis you can see in the carb. Also remove the float and be carefully not to loose the float needle and check that too.

It could also be an electrical problem, but if it dissapears after a certain rpm, I'm not so sure, I'd expect it to be consistant.

Thumbs Up

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bish777 · World Chat Champion Joined: 11 Nov 2004 Karma :

Ditch and replace the HT leads and caps. Replace the air filter as you cant always tell if one is getting to the end of its life, they may look clean but may still be blocked.

look over the airbox for any holes that arent meant to be there. Ive found black silicone sealer is great for inlet tract repairs and sealing.
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the grim reaper · World Chat Champion Joined: 29 Jun 2005 Karma :

If you know about the front piston problem, I take it you have re-routed the petrol cap drain hose as well? If not, the hose can become kinked round the tank hinge and then collects water when it rains, eventually the tube fills up and then you get water sitting round the petrol cap, when you open the cap, water gets into the tank.

If it's not this, I would also check the carb mounting rubbers, SVs can push their carbs off over time, exoecially if they backfire a lot (due to the front cylinder problem - DOH).

Cheers and good luck.

Grim
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G · The Voice of Reason Joined: 02 Feb 2002 Karma :

I took out the main jets and looked over them properly.

I've swapped the front coil/HT lead and will try that out tonight.

Carb rubbers were fine.

I haven't checked the drain hose, nor did I know about it.
However the bike is mostly garaged / covered. Will have a quick look, cheers.

Cheers for the ideas. Will report back after poking tonight.

Barry_M2 · World Chat Champion Joined: 09 Sep 2004 Karma :

G · The Voice of Reason Joined: 02 Feb 2002 Karma :

Yea, I appreciate that. Was a bit rushed at the time.

Was taking them apart to get an idea of the general cleanliness of the carbs.

If the HT lead doesn't work, I'll get it apart again and go over everything in a bit more detail.

mr.z · World Chat Champion Joined: 04 Feb 2004 Karma :

https://img238.imageshack.us/img238/7325/carbjetusage1ly5.jpg
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FreshAL · Sir Crashalot Joined: 04 Jul 2005 Karma :

G · The Voice of Reason Joined: 02 Feb 2002 Karma :

H'okay, a bit of stupidty and lack of observation on my part.

Was going over the carbs, making sure jets were clean etc.
There was some deposits around the butterfly valve, so cleaning those off and what's this... a vacuum tube with nothing attached. Hmm (just by the rubbers).

After slapping myself a few times, I pop over to my 'spares' SV and grab the cover that goes on this one, stick it on and it seems to be much, much better. Could swear I still heard the occasional little pop, but not nearly as bad.

Will take it out for a ride later to see.