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2005 Sy250 Spark Plug Problems

trevor lucey

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I have a SY250 2005 and every couple of months it will just go dead and will not start again until I change the spark plug.

I am using an 80 to 1 mix two stroke mix, I have started to use the NGK Iridium plugs and they last a little longer but not much. Yesterday morning I started first Kick as usual was riding about an hour and a half practicing hill climbs so I was doing a lot with clutch and throttle but nothing more then should be expected from the bike, anyway half way up a particularly tough hill the bike stopped, so I had to turn it and free wheel back down.

Just to answer the obvious before we start.

Yes there was plenty of petrol in it.

No it did not overheat or flood infact I tried it a few hours later and still nothing.

I know the bike will start first kick after changing the plug.

I have tried a few different two stroke oils with no difference

I am currently changing plugs every 12 or 16 hours of of riding. This is the only problem with the bike, does anyone have any similar problem or a solution for it?

Any suggestions would be greatly appreciated



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to suggest what to look for, more info is needed - suggesting a cause without info would be simply a guess

what does the plug that comes out look like? (nose and electrodes) - can you do a decent close-up photo of it?

what petrol are you using?

does it run well when it is running?

has it always had the problem?

how long have you had the bike?

how much use has the motor seen?

has the exhaust system ever been cleaned out?

has the motor ever been decoked?

what is the ID on the plug are you using?

are you doing a particular type of riding each time it cuts out?

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At some point in a normal ride, I usually try to perform a good clean out of the motor. Get some heat in it as compared to putting about, and rev the crap out of it a few times under load. This tends to blow out excess oil accumulation in the bottom and clear the muff as well, but it takes heat!, so a couple of good drag race runs through the gears at least.

Motor should rev to top cleanly before you are done, plug gets clean and happy, requires less replacement due to fouling.

Start out slow, then get some heat init! May smoke like hell from the muff if has not done in a while!

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Thanks for the replies,

Feetupfun: I don't know to much about the history of the bike but everything is pretty original so I presume it was not abused everything else is perfect runs like a dream. I assume it has been serviced regularly before I got it but other then that I don't know. I only use the recommended filters and plugs and starts first kick when the plug is ok. I know its hard to tell just from a quick description of what's wrong.

Copemech you may be right because where I practice it is often only in small sections because I am practicing for an hour or two on a specific thing with no real milage being clocked up or without any opportunity to open it up. As there are no summer trials where I live this is what most of my summer outings are spent doing.

Thanks for the replies a bit of drag racing will be on the cards on Sunday.

Thanks again


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My one used to do that too. It would stall in sections at low RPM - the plug would be fouled - black & sooty The mixture screw was too far in and it was running rich, it needed a plug change every two trials.

I cleaned out the exhaust after it caught fire internally - the smoke was foul and had the split in the centre box welded up. It seems to be a lot better now.

Think I set the mixture screw 1 3/4 turns out, it was way off when I checked.

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Will Keep you posted, Changed the plug and set the mixture screw to 1 3/4 last night and it started first kick.

Gonna give it a good workout at the weekend and hopefully that will be the end of it. If it happens again I will let you know.

Thanks again


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I also stripped the exhaust down, de-coked the header pipe, plugged one end of the centre box and similar with the silencer and filled them both with white spirit. I left it to soak for a long while in the hope it would liquify all the oily sludge, it did. Runs a whole lot better. While I had it all apart I filed down the sharp corner that the engine number is stamped on - it pokes a hole through the underside of the middle box if this isn't done.

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