Hi all, a couple of days ago I noticed something is not too well with my machine. It starts fine, runs up to 40 fine, and if I gently accelerate all is fine. But if I try to accelerate quickly (twist the throttle like I mean it!), especially if I'm going over 50mph, the engine will sputter. If I back off and accelerate gently all is fine and I can build up speed to 80+. It was fine with this until a couple of days ago.

I gave the bike a major service about 1,000 miles ago - oil+filter, air filter, plugs, final drive oil etc - and it was running sweet as a nut. Now this happens. I have good technical knowledge (and a workshop manual!) but before I lose another weekend in the garage dismantling anything & everything is there something obvious? If not, what order of attack would the wise ones here suggest?

It may sound too obvious, but what if you got a tankful of dirty gasoline? that and the quality of the fuel seem to impact my machine a lot.
I had to stop filling up a a couple local stations because their fuel made my engine knock a lot.

Does anyone know how to check the fuel filter?

I would suggest a fuel additive to clean the fuel injectors and engine.

Two things I'd try first would be a Sea Foam treatment and sparkplugs. Bad plugs often tend to cause problems with rapid throttle operation. Sea Foam fuel system cleaner is one miracle in a can I've had good luck with. I use it as a preventive once or twice a year in my car, truck, Goldwing and Silverwing.

I suggest you look at the basics to start. Since this came on suddenly after running fine for 1k miles after your tuneup, keep it simple. I've had a spark plug crack months after installing it. Check both spark plugs first. Since they're easy to remove, pull them both and have a look at their tips for colour and their insulators for cracking. Report back.

In addition to something like a defective plug, check the plug wire connection. On my Yamaha, I had a similar problem to the one you have described. The wire was in contact with the spark plug but not securely in place for a good connection. The scoot would run fine under light acceleration, but would sputter and stall under hard acceleration or with a pillion. After firmly connecting the wire to the plug, the engine ran fine in all conditions and has done so ever since.

I would also check the TYPE of spark plug. If they were some other 'compatible' brand selected off a conversion chart, this could happen after 1000 miles, especially under hard acceleration. I know... I did this many moons ago and have used factory plugs ever since.

I agree with checking the plugs and the connections too. Also, maybe a fuel or fuel system problem..i.e. fuel filter, bad gas etc. It seems that is the general comments here. I'm sure it is something very simple though.

I've checked the plugs (NGK CR8EHIX-9) and both have a dusting of soot, so I would guess the plugs haven't been firing properly. I could troubleshoot this on a carburetted system but FI is trickier. There are no errors showing up from the FI system when I turn the ignition on.

I've tried fuel from different petrol stations bit that made no difference so I don't think it is bad fuel.

I'm changing the plugs (they are gapped correctly), and checking that I've got the leads fully home on the plugs and see what happens. Feel free to add some more suggestions since I don't have much faith that this is going away with new plugs (I'm a glass half empty kind of guy!)

whats the milage of your bike

My book calls for plug number CR8EH-9. Are UK plugs different, or are these platinum plugs or something?

I believe he's running the Iridium plugs.
If both plugs are the same colour then I would move on from plugs being an issue. Next I would have a look at both the ground connections and the spark plug leads at the coil pack.

Is anyone else running Iridium plugs, and if so, are you experiencing any problems like Coops666 is having?

The spark is actually not wasted, it fires on the exhaust stroke for improved emissions.
When both spark plugs appear to be covered in a black "soot" this indicates 2 possible problems. The first being spark plugs that are eithe faulty or the wrong heat range for the engine. Or secondly the the fuel or supply is the problem. Could be bad fuel or the injector is sticking open slightly. As Big Bird said you should always start with the basics first.

There is always unburned fuel on the exhaust stroke. The air introduced to the cylinder on the exhaust stroke is to cool the cylinder down to reduce emissions.There fore the spark on the exhaust stroke, helps burn excess fuel.

Could the soot on the spark plugs be from a bad fuel presser regulator or a vacuum leak

Wingmann,

Your right. The fuel pressure regulator could be the problem. The diaphragm inside the housing, will from time to time, develop a small hole and gas will get sucked in to the vacuum lines causing the mixture to be to rich with fuel. I have seen/heard of this happening on other Silverwing scooters.

Big bird, I am not going to argue with you, 31 years as a Master Automotive Technician, I know what the purpose of a spark on the exhaust stoke is. Maybe a motorcycle engine has a different method of handling emissions, but in a Modern internal combustion automotive engine that is basic theory!