Big single won't start - backfiring through airbox

[loply]

Hi folks,

I have a Yamaha SZR660 which I don't ride. I start the engine once a month or so and give it a few handfulls for 10 minutes to keep it running.

Normally its a bit hard to start but after a few mins comes in.

The other night it started then died right away (this is normal), but it won't start again after that.

The engine turns over fine and once every few seconds a "puff" noise comes from the airbox. It's like a pop of air coming up out of the engine.

Some investigation revealed the airbox had been popped off the carbs. I put it back on but it still does the same noise (albeit without the airbox coming off).

Any ideas where to start? it has a spark, fuel is going through, starter is turning over nice and fast...

Thanks for any advice folks.
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Yamaha SZR660 Caution to the wind, the throttle pinned!

[LeeR]

I owned a 650cc 1cyl and only had it back fire on start-up a couple of times, it was down to a bone dry air filter on both occasions. I appreciate this may not be the fix for you problem but it could be worth checking the condition of the air filter.
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My claim to fame: Austin Vince nicked my pen...

[ssray]

Try a brand new sparkplug, if its started and not run the plug will not get hot enough to burn off any fuel residue on the plug.

The only times i had a single spit back was timing out,not very likely
on a szr or the cam timing had slipped.

look for thumper club on google

[2006 _p1]

if you keep trying to start it and its gettting a spark and fuel on the plug it has problary gone. try a new plug and see what happends. happend to me when i kept trying to start my bike.

[loply]

This is an SZR660 btw.

I'll pop the plug out and give it a good clean, then replace it if it's no different.

Thanks for the tips thusfar.
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Yamaha SZR660 Caution to the wind, the throttle pinned!

[Odie]

Had similar happen on a CX500 i had laid up, turned out to be stale fuel!
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You are so ugly when you were born the doctor said "I?m gonna drop it, if it falls is a rat, if it flies is a bat."

[2006 _p1]

[thegubner]

Has the fuel in the tank been there a long time?

[loply]

It's been there for quite a while (since September or so), but having said that it did usually run fine once every three or four weeks.

I accept that it could do with changing though

These engines are supposed to be able to run on anything that's vaguely liquid in form though...
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Yamaha SZR660 Caution to the wind, the throttle pinned!

[tutton]

People say that,
but obviously will run better on fresh fuel.
bet that fuel in there has no vapour left at all
hence the shit starting qualities of it.
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Past: '05 Peugeot XPS '99 Aprillia RS125 '94 FZR600R
Current: '02 ZX636 A1P '51 CCM R30

[thegubner]

[loply]

Okay folks I popped the spark plug out and it did look a bit gummed up but it was sparking. I have little or no experience with spark plug problems but to me it looked intermittent.

I went out and bought a new spark plug and now it won't spark at all! Even with the old one back in... Because of the design of the HT lead you can't try the lead against the chasis for a spark.

As far as I can see the HT lead is permanently fixed into a black cylinder (which I guess is the coil).

Any suggestions what to try? Is there a way I can verify if the coil is working or blown?

I guess the HT lead or spark plug cap could be at fault, but it generally looks in good nic, how do you normally disconnect it to test/replace?

Thanks for any help.
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Yamaha SZR660 Caution to the wind, the throttle pinned!

[tutton]

You do have the plug in the ht lead out of the cylinder when putting against frame to try see for a spark right?
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Past: '05 Peugeot XPS '99 Aprillia RS125 '94 FZR600R
Current: '02 ZX636 A1P '51 CCM R30

[Ariel Badger]

I take it that there is some electricity in the battery?
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Bikers make great organ donors, get 115 on your licence today.

[loply]

yeah there's plenty of battery, I hooked up a booster battery just in case too.

I have the plug out of the cylinder, does part of the plug body need to be grounding on the engine/frame?
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Yamaha SZR660 Caution to the wind, the throttle pinned!

[Ichy]

The threaded bit that screws into the cylinder head should be earthed.
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[Ariel Badger]

If the plug is not going to earth it will not spark. If you can not earth it put your finger up the plug cap, you should get a high voltage low ampage shock when you turn the engine over. It will tickle a bit but it will not hurt you seriously.
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Bikers make great organ donors, get 115 on your licence today.

[Irezumi aka Reuben]

Try cutting the end off the HT lead.

They corrode and exposing a new bit can make it spark a lot better.

[loply]

Hi folks,

Oddly when I first removed the old spark plug I tested it without grounding it on the frame and it sparked intermittently, hence it never occured to me to ground the new one on the frame either.

In any case the spark was fine after I did this but it still wouldn't start.

Popped off the carbs, took them all apart and cleaned and wiped everything. Nothing was particularly dirty but did it anyway. Sprayed them with loads of WD40 followed by copious amounts of compressed air.

Put them back on, put fresh fuel in and it fired up within a few minutes.

Thanks for the help guys!
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Yamaha SZR660 Caution to the wind, the throttle pinned!