Single cylinder when accelerating - in the dry!

Hey dudes,

Bit of an issue ... '04 SV-S (40k miles, last big service @ 36k). Suffering from some bizzare behaviour that's just started happening (it seemed to start after plug replacement following a drop in September, and has got steadily worse).

When rpm is around 2-2.5k and I accelerate away, she seems to cut out , and only one cylinder is firing (not sure if front or back). Idling is fine, cruising at that RPM speed is fine, accelerating from higher rev range is fine... it's just on acceleration from between 1.8-3k (used to be sub-2k only!). To reproduce: Cruise at 28-30mph in 4th gear (around 2.8k rpm). Open throttle hardish and try to accelerate to 35/36 - struggles on one cyclinder and gets there eventually. Second cylinder kicks in at around 3.2k. If I roll off at any time, the second cyl kicks back in. This is particularly annoying at roundabouts / starting off from traffic lights as stalling frequently occurs It feels almost like fuel starvation to one of the cylinders.

My initial thoughts are:

1. Spark or lead need replacing as are fowled / knackered... However, I replaced the sparks less than a month ago (so about 1200 miles) and lead *seemed* OK. Plus, I'd have assumed that if it was leads/sparks it wouldn't idle fine, and wouldn't be limited to exhibiting this behaviour at such a specific rev speed.
2. When I changed the spark last, I think some crap fell into the combustion chamber and this could have fecked the engine?
3. Oil needs a change (clutching at straws here!) - due within 2k miles anyway
4. Fuel filter needs changing / injectors blocked?

I've had a search, and can't find any related threads to this kind of problem... Any advice would be very welcome!

Cheeses,

Al.

[maviczap]

Feel the header with the back of your hand, whichever is not firing properly will be cooler

Could be fuel or sparks

[the_lone_wolf]

Originally Posted by maviczap

Feel the header with the back of your hand

Yes, if the skin that peels off blackens immediately it's hotter than if the skin just sizzles like a bit of KFC

[maviczap]

Originally Posted by the_lone_wolf

Yes, if the skin that peels off blackens immediately it's hotter than if the skin just sizzles like a bit of KFC

If you feel it with the palm, then the reaction from the nerves is to grip even harder, which is bound to smart a bit

Alternatively use a piece of wood[bit of "broom" handle],header pipe to your ear and listen------only the wood gets burnt.

[the_lone_wolf]

Originally Posted by maviczap

If you feel it with the palm, then the reaction from the nerves is to grip even harder, which is bound to smart a bit

Just a bit, there's still a little bit on the KTM header where I brushed it almost immediately after stopping...

[maviczap]

I used to have an imprint on my thumb and forefinger from picking up a square bar which I'd been heating to put a twist in it, and forgot it had been red hot just prior to me picking it up. Doh!

Thanks for the replies

With the wood trick, what would I be listening for? Mind you, the idea of riding along with my head on one side and a piece of 2x4 sticking through my helmet into my ear is quite an amusing one - as long as there are no speed bumps!

On a more serious note, this is proving tricky to analyse as idling and higher-rev riding is fine. Sigh.

Where does anyone suggest I should start first (and no reg-rec comments just yet )? I'm thinking replace both HT leads and sparks and see what happens? It's just bizzare trying to work out what would cause it to only happen under low-rev acceleration.

Al.

can you not recreate in the lower rev ranges whilst the bike is static/in neutral? if so then you would be listening for the sound of the cylinder running (or not)

[yorkie_chris]

Drain the carbs and check for sh*t... it could be some oddness in needle circuit causing this farting about. Could also be coils and/or leads.