Hello All!,

Got a 1988 NC23, standard jets and setup on the carbs and an aftermarket set of downpipes (larger diameter downpipe) and straight through carbon can, standard air filter.

Bike runs fine on mid - full throttle but once it's warmed up its hunting on idle and tends to stall plus it bogs down heavily when pulling away. If you let off the gas at high revs there is no overun whatsoever as the bike runs down

Mixture screws are set to 2.5 turns out and the carbs are balanced.

Any suggestions on how to stablise the idle (Currently set to tickover at 1100rpm) and improve the pick up on primary jets?

Cheers in advance.

C

Get the carbs balanced first.

Wozza wrote:Get the carbs balanced first.

c7ll wrote:Mixture screws are set to 2.5 turns out and the carbs are balanced.
C

Silly question but are the carb clean and properly secured??
i would have thought it was running rich with the symptoms.

I'm no expert but I recall the Haynes saying idle should be 1300 plus or minus 100RPM, also how old are the spark plugs, my NC23 used to be really lumpy on idle and after I put new plugs in it was much better, made a difference to the performance of the bike too, they must have been ancient plugs though.

Also might be something to do with valve clearances?

Yeah my guess was it leaking air but it only does it when hot.

Plugs are an unknown so going to change them before changing anything else.

If it's running rich then am I correct in saying the mixture screws should be dialled IN to lean it out. Half a turn perhaps?

Cheers,

C

Chris, from the symptoms you describe and what you told me the other week I would suggest its probably running lean, lean problems generally become worse as the engine heats up and with the system you have on there larger diameter pipes are very inefficient and usually have to run richer to make the same results which makes them very rich low down, you need to turn the idle up a bit 1400-1500 ish warm up the engine and blip the throttle if the revs hang above idle for a few seconds before dropping back that's a good indication your lean, if it drops below idle then settles back up that's a good indication its rich, screw in to lean off out to richen on these carbs, have a play with it and see if you can make it better you can always put it back, but check also your part throttle/cruise isn't affected to much that's the important thing to get right, any problems I finish work at 17:00 on Sunday if your around in the evening and still want to bring it round?
Simon

Cheers Simon,

Fitted new plugs last night and after about 3 mins of sitting on tickover I could hold by hand round the downpipe of cylinder 1! - can't be right, other 3 downpipes where up to full temp. My fault finding plan so far is:

1) Check for a spark - the bike seems to run fine from mid - full throttle so I don't think it's the spark but will look for this first. Do coils ever partially breakdown so you get no spark at low revs but then it wakes up at higher revs?

2) If the spark and coil tests out ok then it must be a blocked primary jet, carbs off clean jets out and try again

When I took the old plugs out, they all looked fine but the plug for cylinder one was black - does this suggest unburnt fuel or a spark with no fuel?

If after 2) the problem remains, my money is on carb balancing.

It's unlikely be a coil (doggy earth or plug cap maybe) if its a spark issue it will likely happening at high revs as that's when the spark is under the most load, check your jets and remove the a/f screws as well and give it a clean and blow through, I wouldn't worry to much about the colour of the plug just yet.
Simon

Ok spark is fine, coil and ht lead test ok, cleaned jets and no blocks. Re installed everything and still no heat on downpipe 1, screw the mixture needle out and no change but screw it all the way in (ie as lean as it will go) and instantly heat in downpipe 1 and tickover rises and stabilises!

Must admit I don't understand why leaning it in is causing the primary jet to operate but it seems to be the case, will take it out for a ride tomorrow and see how it affects the performance on the road.

In the meantime any opinions would be welcomed!