Not firing on cylinders 2 or 3

[Goose]

Hello All,


Further to my recent post.... I am having an issue where my bike doesn't seem to be running on cylinders 2 or 3 at idle.  They seem to sometimes join in at higher RPM, but this seems to be hit and miss.  Sometimes they run fine and idle without issue, but this seems to be increasingly rare.

I have changed the coil packs for new items, which unfortunately has made no difference (I've checked the resistance of the primary and secondary windings and everything checks out ok), I have also changed the spark plugs.  I have checked the resistance of the spark plug caps and they are all OK.  I have disconnected the HT leads one by one and used an old spark plug earthed to the engine block to confirm that there is a spark at all cylinders, although it's possible (really hard to judge) that 2 and 3 are perhaps slightly weaker - but I'm not convinced, and there is definitely a spark.


Pulling off the HT leads one at a time shows that when I pull 2 or 3 off it makes no difference, whereas if I pull 1 or 4 if it won't run at all.


Does anyone have any ideas? I am now at a bit of a loss as to what to try next.   I am a little worried that perhaps the head gasket has blown between cylinders 2 and 3 (has anyone heard of this before?) but that doesn't really tie up with the fact that at first this was an intermittent issue.


Any help would be greatly appreciated!


Cheers!

[Disorderlypunk]

cant really blow a gasket between 2and3 as the timing chain is in between so would have to blow twice
can you do a compression check (no dismantling needed)
start her up and let her idle and check downpipe temperatures to make sure its 2+3 (they will be cold)
after a while of just tickover try having a look at the spark plugs to see if they are wet
might just need a tin of fuel system cleaner ran through the tank cause it sounds like blocked idle jets to me
failing that its a carb cleanout time

[darrsi]

Head gasket failure would be noticeable in your oil i would've thought?
In winter it would be normal, but look for any milky/frothy creamy stuff in your oil level glass.

[Gnasher]

This could be fuel related ie blocked pilot jets.  Check it there's a spark on 2 & 3 take a plug out or use another don't matter which use a plug.  Put it in one of the caps lay it on the cam cover so the nut area is touching the cover, now start the bike if there's a spark repeat with the other cap if both spark, it's a fuel related issue.

[Goose]

Thanks for all the replies!


I had forgotten that the timing chain lived between 2 and 3, that likely rules out it being a head gasket issue then.


Annoyingly I can't currently do a compression test, as my compression tester doesn't have an adaptor for 10mm spark plugs, but I've ordered one.   


I've used an old spark plug to check each lead is firing, and they all are. I'll check again tonight to see if the sparks are better on 1 and 4 vs 2 and 3 - I wonder if a weak pulse on the inputs to the coil pack would result in a weak spark?


Other than that I am guessing it's going to be a fueling issue.  Seems odd that carbs 2 and 3 would develop a fault simultaneously though, but I can't think what else it might be.


Cheers all!

[BBROWN1664]

It may not be a simultaneous failure. It may be that #2 was OK and #3 was poorly but you didnt notice it so much until #2 went too.

[daviee]

does the coils not  operate 1and 4 then the other run 2 and 3 i would suggest checking to see if your coil has taken a dump try swaping coils about
 

[Goose]

I've already tried swapping the coils for different ones, unfortunately it made no difference.


Tonight however whilst tinkering I noticed the fuel pump does not seem to be pressurising, as it keeps ticking (slowly) more than it ever used to before with the ignition on.  Further inspection showed that there is definitely fuel leaking out of the carbs somewhere, as it's dripping off the bottom of them.  I suspect that one or more carbs is leaking fuel somewhere, so it looks like they'll need a rebuild.   I've removed them tonight which was fairly straight forward.


Ah well, all fun... or something!

[Disorderlypunk]

i think you may have found the problem
just be carefull with the fuel pipe ,remove from the pump end not the carb end - that little T-piece seperator cost 35-40 quid if i remember correctly
i was a little shocked after it snapped on my hand and even more shocked when i saw the price.

[Goose]

Cheers for the heads up! I'll be extra careful.