Low (and rough) idle after spark plug change

[potatobroxd]

I recently changed my spark plug on my cb125f (exact same type) and tightened it by manually (according to the manual 1/2 turb then remove it then tighten it 1/8 turn.

When the engine starts from cold, it idles at 1100-1200 rpm but really it should be around 1500 normally. The idling is also rough when cold. Once the engine is warm, it idles happily at 1500 or even slightly higher.

Is this because of how I tightened the spark plug or is it because of a faulty spark plug? Or is it like that when you just changed it?

[Guest Richzx6r]

Changing the spark plug shouldn't make it run worse, is the HT plug on the spark plug fully, did you gap the plug before you installed it, I know they normally say they are gapped correctly but I'd always double check it anyway it may be off, failing that it could be the carb is slightly out of sync with the new plug?

Have you messed about with the carb recently?

[potatobroxd]

Changing the spark plug shouldn't make it run worse, is the HT plug on the spark plug fully, did you gap the plug before you installed it, I know they normally say they are gapped correctly but I'd always double check it anyway it may be off, failing that it could be the carb is slightly out of sync with the new plug?

Have you messed about with the carb recently?

 

Thanks for the response; its a fuel injected bike and I havent checked the gap of the new spark plug, I'll do that next thing tomorrow

[Smithers]

Changing the spark plug shouldn't make it run worse, is the HT plug on the spark plug fully, did you gap the plug before you installed it, I know they normally say they are gapped correctly but I'd always double check it anyway it may be off, failing that it could be the carb is slightly out of sync with the new plug?

Have you messed about with the carb recently?

 

Thanks for the response; its a fuel injected bike and I havent checked the gap of the new spark plug, I'll do that next thing tomorrow

 

It only takes a slight knock to move the electrode out of tolerance, so checking with a feeler gauge against the bike spec is a wise precaution. If you've still got the old plug, just out of interest measure the gap to see what it was before the change over.

[potatobroxd]

Changing the spark plug shouldn't make it run worse, is the HT plug on the spark plug fully, did you gap the plug before you installed it, I know they normally say they are gapped correctly but I'd always double check it anyway it may be off, failing that it could be the carb is slightly out of sync with the new plug?

Have you messed about with the carb recently?

 

Thanks for the response; its a fuel injected bike and I havent checked the gap of the new spark plug, I'll do that next thing tomorrow

 

It only takes a slight knock to move the electrode out of tolerance, so checking with a feeler gauge against the bike spec is a wise precaution. If you've still got the old plug, just out of interest measure the gap to see what it was before the change over.

 

The new spark plug is 0.8mm and the old one was .89mm (worn). The recommended gap is 0.8-0.9mm so technically its still within range? should i widen the gap slightly?

[Guest Richzx6r]

Ok so cant be the carb....as it doesn't have one.

I'd suggest then that it's to do with the plug gap or the ECU adjusted the fuelling to the old plug tho I wouldnt have thought it would be that thoughtful

I'd say it's a magic and wizardry problem you have and because of this I'm no help

[winston smith]

Assuming the only thing different is the spark plug, that's the likely cause.

How old is the ht lead? These degrade with time and can arc through to the frame.

Sounds silly , but I've heard of the top of the plug unwinding and staying in the ht cap when you pull it off - worth checking the old ones not still in there.

Also worth checking that the inside of the ht cap is in good condition - no corrosion etc.

Maybe try putting the old plug back in and see how the engine runs - if it's ok with the old plug the new one must be dodgy.

Could be the ceramic insulator's cracked or maybe just a diff one.

For the sake of a few quid I'd just buy a new plug and get a refund on the diff one.

[BIKERDAD]

I agree with WS .. swap them back around and see if there any difference also I dont know how much a new HT lead be but I'd change It along with the new spark plug once you checked the old one again . .

[potatobroxd]

I changed the spark plugs back to the old ones and the RPMs are back to healthy range (1500). Checked the new spark plug and it seems to burn well.

The HT plug has to be new I assume as its a 2016 model. I dunno but I guessing the spark plug needs breaking in? If the problem persists ill probably just reorder another one to see if it runs differently

[dynax]

Open out the new plug to 0.85 and see if that makes a difference, often a bit of trial and error is needed for a good spark

[Gerontious]

they did send you the right one?

NGK CPR7EA-9 (or equivalent)

[Arwen]

My MSX does exactuly that when I change the spark plug and the gap is not right. Now I don't bother checking the manual. If the bike was running fine before I started servicing it, I simply match the gap in the plug I removed

I thought I had properly broken it when I first serviced it and it just spluttered and died when I pressed the button. That's when I learned how to use feeler gauges

[potatobroxd]

oopsies, even the old plug has rough idle now

I think it might be my valve clearence, oh well, not looking forward to it

[winston smith]

I wouldn't be jumping straight valve clearances just yet, not on a 2016 bike, unless it's done a lot of miles.

There are simpler things to check first - ht lead first, then fueling. You might try spraying some aerosol carb or brake cleaner around the inlet pipe and carb/ injection unit at tickover to see if the revs change(?)