Sparkplug, whaddya reckon caused this?

[Gregor]

I’ve never seen a plug with an ashy deposit like this in just one specific area.

Anyone seen this before and know what the cause might be?

Yamaha XT660Z

60k miles, not using oil between changes.

[David]

You have a mouse with the shits in the combustion chamber.
Or crap in the fuel (not sure how often you should do that).

[David]

Seriously, check your valves (an endoscope would be useful).

[1913]

I could be wrong but in the dusty vault of my mind I seem to remember water, or too much lead.

[JackyJoll]

I could be wrong but in the dusty vault of my mind I seem to remember water, or too much lead.

I was thinking fuel additive, but coolant getting in is an interesting idea.

[Gregor]

Well I don’t seem to be losing coolant so wonder if fuel is contaminated with water, been filling up at the same ASDA station for ever.

I’m just about to check the valve clearances so will be draining coolant and removing the radiator will check to see the state of old coolant.

I have noticed the idle isn’t steady lately, very slight hesitation, not to the point of stalling just a slight drop in revs.
Riding on a constant throttle there’s a very tiny surge and hesitation, thought it was shagged chain and cush drive because these big singles do that when they wear but they’ve now been replaced so new plug, air filter and valve adjustment and see if that cures it.

[Taipan]

My guess work is the white-ish colour normally means overheating, probably due to running lean? Your uneven idle would seem to back that theory? AIr intake leak soemwhere?

[Gregor]

My guess work is the white-ish colour normally means overheating, probably due to running lean? Your uneven idle would seem to back that theory? AIr intake leak soemwhere?

I think you might be right, now when I blip the throttle it's taking a while for the revs to drop back to idle.

I was going to do a compression test ( burnt exhaust valve is what I'm dreading) but just found I can't screw the adapter for 10mm plugs on my Gunson gauge in the recess as it needs a 18mm socket which is too fat for the recess, need to figure out a way to screw that in and more importantly get it out again.

SO Tank off again and proper look for leaks and split hoses.

[Taipan]

Can't say I've ever tried it, but people say spraying wd40 around the intake will confirm a intake leak as the revs will rise notably when it suck in the WD?

[Gregor]

Seriously, check your valves (an endoscope would be useful).

Well I had a look inside with scope ........valves don’t seem to have any obvious damage but the piston crown has a lot of carbon/residue on it.Quite hard to see much detail with the old optical scope I’m using, no screen just an eyepiece.

Looks like the top ends going to have to come off for a proper look.

[Gregor]

Finally, after removing the AIS and coil I got the compression tester screwed in and its reading 120psi which is way higher than it should be on these low compression XT lumps, manual says between the ranges min 85 and max 99.

Next job adjust the valve clearances and check again but its looking like there’s a lot of carbon built up on the crown/ head and it’s been running very rich, or burning oil.

[JackyJoll]

Finally, after removing the AIS and coil I got the compression tester screwed in and its reading 120psi which is way higher than it should be on these low compression XT lumps, manual says between the ranges min 85 and max 99.

Next job adjust the valve clearances and check again but its looking like there’s a lot of carbon built up on the crown/ head and it’s been running very rich, or burning oil.

That 85 to 99 psi is remarkably low. Are you sure?

I am dubious about 99 psi being a maximum and 120 being somehow too high.

[KungFooBob]

Finally, after removing the AIS and coil I got the compression tester screwed in and its reading 120psi which is way higher than it should be on these low compression XT lumps, manual says between the ranges min 85 and max 99.

Next job adjust the valve clearances and check again but its looking like there’s a lot of carbon built up on the crown/ head and it’s been running very rich, or burning oil.

That 85 to 99 psi is remarkably low. Are you sure?

I am dubious about 99 psi being a maximum and 120 being somehow too high.

I thought it sounded very low too, so I googled it...

https://www.manualslib.com/manual/65649 ... l?page=103

I'd have thought well over 120 would be the norm!

[JackyJoll]

As that’s when cranking the engine, could the decompressor be in operation?

[Gregor]

Finally, after removing the AIS and coil I got the compression tester screwed in and its reading 120psi which is way higher than it should be on these low compression XT lumps, manual says between the ranges min 85 and max 99.

Next job adjust the valve clearances and check again but its looking like there’s a lot of carbon built up on the crown/ head and it’s been running very rich, or burning oil.

That 85 to 99 psi is remarkably low. Are you sure?

I am dubious about 99 psi being a maximum and 120 being somehow too high.

I know, Isn’t it!

I saw 120 and thought that’s ok, checked the manual and saw the above.

Low compression to allow it to run on shite fuel on your RTW trips apparently.

[JackyJoll]

The manual linked by Bob says it has a 10:1 compression ratio.

[Gregor]

Adjusted the valve clearances today, none were miles out but we’ll see if makes any difference when I fire it up tomorrow, need to fill it with coolant and get the tank back on first.Should be a little quieter now.

[Taipan]

No sign of anything that could cause an air leak?

[Gregor]

No sign of anything that could cause an air leak?

Not found anything yet.The hoses to throttle body and airbox are all ok, airbox is well sealed.

Need to check engine breather hose when I’ve got time to remove the exhaust, might pull the injector and replace o rings but again looks like quite a bit of stripping down to get that out.

Also I’ve got the Kev fuel mod fitted and I’m wondering if that might be have failed and not giving consistent resistance, it’s just a potentiometer in line with the air box temp sensor that richens up the mixture by fooling the ECU air temp is cooler preventing lean running with free-er a flowing end can.Still running standard airbox and filter on mine.So I’ll remove that mod to rule that out don’t really want to refit the original exhaust because these bikes run shite at lower revs with it.

It’s got MTC single exhaust fitted and a while back the baffle decided to part company, don’t know how long I was riding like that, realised one day when I wasn’t wearing earphones and later last year I also had a holed header pipe which could have been like that for some time so it could have been running too lean then but it never showed the symptoms I’m getting now.

Just gotta rule out the simple stuff before I bite the bullet and remove the engine and have a proper look inside see what state the valves and piston are in.

On the plus side it’s sounding a lot quieter now I’ve adjusted the valve clearances but no change in the slightly erratic idle.

and another plus I’m getting a new bike so I can sort this at my leisure

[The Spin Doctor]

Try an Iridium-tipped plug.

https://ngksparkplugs.com/en/products/i ... park-plugs

I used to use them in my big singles, and they made a noticeable improvement to running, particularly on the XBR500. That had a nasty trick of stalling as you opened the throttle, a moment after you'd got moving.

They're pricey, but one of the few performance aids I've ever felt worked, at least on those bikes in terms of smoother running.