XJ900 running lumpy off idle - no. 3 cylinder

Hi all

Newbie here to the Forum - I have had an XJ900 Diversion that ran perfectly before a longish storage (9 months) over winter here in Ireland.

Pulled it out 3 weeks ago and it started and idled fine. However, now it runs lumpy off idle and performance is down, fuel consumption up and oil consumption now noticeable - slight smoking from RH pipe.

No. 3 clylinder is the culprint - see plug chop below


The bike runs fine at idle and just off idle and the lumpiness starts at abut 1/8 throttle opening onwards, but it still goes ~80% power and can get to 100 mph or so tops (previously 125).

So, given the bike was in storage my hypotheses are

1) Stuck rings on No 3 cylinder - can only confirm with a compression test (coming soon).

2) Gummed up No. 3 carb or stuck float valve leading to a too-rich mixture (which burns the oil off the bore) - have tried STP petrol additive to clean carbs out but no joy yet. Might have to strip the carbs which I am not looking forward to. (Still better than changing the rings....)

Am I on the right track or any other ideas?

Cheers
Mike

 

Sounds reasonable to me. Compression test woul be my next bet too, I'm not entirely sold that #3 is rich. That looks like oil to me. Fuel consumption is probably just up cuz your bike is (inefficiently) running on 3 cylinders.

No external leaks anywhere? Weeping gaskets?

 

Nope - engine is clean
Doing compression test this weekend

 

That is definitely oil on the #3 plug.

 

Put a new spark plug in

 

Also, look at your carbs. I had these exact symptoms once, didnt put my pilot jet for that carb in tight, found it in the bottom of the bowl.

 

OK problem solved!! Phew!
It was the rings on that cylinder. I took out the plug and added some STP Petrol Additive down the plug hole, let it sit. Then with the bike in 1st gear I rocked the pistons back and forward using the back wheel. Turned her over to blow the excess STP out and put the plug back in. BINGO! Full power restored!
Here she is ready to roll last night - you can see the excess STP under the sump.

Drove 105 miles from Dublin to Belfast straight after the "treatment" and back up to the full performance - felt like it had been turbo'd as I'd gotten used to the reduced power.
Thanks all for the tips!

 

I'm still thinking the plug.... Maybe not the plug itself, but the cap. In pulling the plug and then putting it back on, you broke the oxidation the built.. Or at least in the jiggling of the wires, you regained the electrical flow.

Do the compression test anyway and let us know....

 

Hi hfs
Not the plug I can assure you - I had the plugs in and out a few times before to check the mixture on that cylinder, plus I put new plugs in when it first started running lumpy. She's sweet now - just did 700km up and down Ireland and she ran so sweet - cruising at 100mpg no probs again. When I get a chance I'll do the compression test but the seat of the pants test tells me it's all good

 

What beautiful outcome!

Awesome views, must had been an awesome ride too (with full power)!

kinen1

 

Hi Kinen1
Was indeed a beautiful ride - recommend that coast road in Antrim to anyone!
M

 

Hi mate, have the same issue here. Which STP additive did you use?

 

Wow that looks like a nice ride, I'd love to so that...except you drive on the wrong side of the road...Lol seriously nice pictures of coast post some more please ..