Corsa low revs misfire

H

husky981

New member
Tech advice please on a low rev / small throttle opening misfire.

The bike is a Corsa, standard apart from Jota pipes, Witt + Dyna coils and Mikuni RS36s.  I do quite a lot of miles on it in all weathers (well almost all...) and it has been running sweet as a nut until this morning.

After about 10 miles on the motorway, I slowed for the remaining 4 city miles of my commute and the bike was running rough, but in a consistent way that sounded and felt like one cylinder wasn't firing.  If I opened the throttle it would pick up and accelerate normally on 3, then revert to the misfire when I closed the throttle.  Wasn't keen to idle, not too surprisingly...

I'm thinking one idle jet may be blocked, but will check all the ignition wiring too just in case something is loose - does anyone have any other ideas or experience of this kind of problem?

Thanks in advance  :)
 
A few things to check:
plug cap to HT lead, good connection?
spark plug itself
vacuum hose link between carbs, any leaks?
rubber hose connection between carbs and head
ignition pick-ups LHS of the crank. There should be equal air gaps from the trigger to the 3 pick ups

Try these for starters, might be that, or could be something else but worth checking
 
Thanks guys, I will try those things tonight and report back.

Cosi, sounds suspiciously like the same issue, and I don't have filters apart from the plastic strainers on the top of the fuel taps, so a pair of filters would be a good idea in any case...

I will see if the misfire is consistent (downpipe that's slow to get hot) when I start up - that should indicate where to start looking.

Cheers, Nick
 
Centre cylinder is the culprit.  I limped home on 2 last night and the pipe was barely warm when I got off the bike. 

Checked all the ignition circuit LT and HT connectors - all seemed OK apart from the centre plug cap which was loose, so I changed the plugs (centre one black and slightly oily looking, others fine) and tightened the little spring d-clip inside the plug cap.  I also pulled the centre idle screw to see if there was dirt present - just slight discolouration at the tip, but I gave the idle screw port a squirt of brake cleaner in case there was some debris up there.Still misfiring, so I checked the centre spark with a loose plug touching the head and there were sparks present, but they looked a bit weedy, lilac/yellow colour, not the big fat blue jobs I was expecting with a Witt and Dyna coils. 

I stopped there as I get up pretty early for work, but will compare the sparks from left and right with the centre one tonight, do a compression test just in case it's a burnt valve and see what turns up.  With the colour of the plug and the consistent nature of the misfire I strongly suspect the trigger circuit or the coil, so swapping LT coil trigger wires should tell me if there's a HT or LT issue - but who knows?!

More to come...
 
Trim 7-8mm off the end of the HT lead and replace the offending plug cap. I had a very similar problem very recently with my RGS..... :-[
 
Thanks to all for suggestions so far.  I shortened the centre plug lead by 8-10 mm and replaced the cap, had good sparks on all 3, checked compression - spread of 3-4 PSI across all so not that (phew!) but the misfire is still there so it's carbs off and strip/clean tonight  >:(
 
Thanks again for help guys!  It's sorted now after a short delay while I let other crap get in the way...

The cause in the end was very simple - centre slide lever screw had come out of the main throttle shaft, so the slide wasn't moving.  Thankfully it was the first thing I looked at once I'd taken the pipes from the airbox off, so it was then fairly quick to remove the carbs, retrieve and fit the screw and refit. 

It's also good to have dealt with the loose centre plug lead / cap (thanks Alan) and to get the prompt to fit some fuel filters (thanks Cosi) so fingers crossed I've got lots more trouble free miles ahead (145k kms so far...)

:D

 
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