Another weird 'bike wont start' problem

[Bullet]

ok friends yamaha radian 600 wont start, never used to like hot starting much but now wont cold start either. have changed the battery, fuel, spark plugs e.t.c and took the carbs out gave them a good clean still nothing not even a splutter. except when we came to check there was a spark took one plug out rested it on engine casing to observe the sparkyness and turned it over there was a good healthy spark, but weird part was that it spluttered and almost started at this point with only 3 plugs in!! tried it on the other cylinders and the same thing but still wont fire with all 4? my guess is compression possibly head gasket? any ideas before i go buy a compression tester?
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nonstop unicorns vomiting sunshine pills into a rainbow toilet
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past: ZXR750 J1, K1 & H1, GPX600R, Yamaha Radian. present: ZXR750 J2, SP-1, CB350

[BFG]

Check the fuel and fuel/air mix.

Were the plugs wet? If so, mixture too rich would be something to look at. Blocked air filter? Bunged air-box hole? Will it start without the airbox fitted?

If plugs were dry check your carb settings inc float height.

[Bullet]

we did consider fuel or air starvation as the most likely thing after changine the plugs, but it all looks clear, plugs not wet but did smell of petrol air box and filter look to be fine, as did the carbs when we had them off the bike, there didnt seem to be anything wrong with them, but we gave them a good clean while they were off anyway just incase...
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nonstop unicorns vomiting sunshine pills into a rainbow toilet
-------------------------------------------------------------------------
past: ZXR750 J1, K1 & H1, GPX600R, Yamaha Radian. present: ZXR750 J2, SP-1, CB350

[BFG]

Will it start without the airbox fitted?

[ancientd]

Poor hot starting is a symptom of an over rich mixture, try to start it without using the choke at all, if you can. Yamaha's often run rich, I never have to use the choke on my Exup, even if the bike has been left for a while, and it always fires up first time.
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I ride 3 bikes, a Thundercat, an FZRexup 1000 Streetfighter and a Jawa 350 Combo, something for all seasons.

[finpos]

Sounds like reducing the load on the bike's battery helps, so I'd try starting it off a good (i.e. not that one you threw in the back of the shed 3 years ago) car battery first.

Then worry about the head gasket

finpos.

[Bullet]

didnt try it without the airbox fitted, but did try it without the backplate/filter in so effectivly not there, same result, for the hot starting trouble it had was it actually struggled to turn over the engine, would fire up right away when given a bump start. battery load doeant seem to be a problem at the moment, will spin over the engine more than happily, just no hint of a desire to start, tried jumping it when the battery eventually went flat after poking various things!, and even tried bump starting it down a big hill thinking it might give it more of a kick, that just resulted in the hard work of pushing the bugger home again!
____________________
nonstop unicorns vomiting sunshine pills into a rainbow toilet
-------------------------------------------------------------------------
past: ZXR750 J1, K1 & H1, GPX600R, Yamaha Radian. present: ZXR750 J2, SP-1, CB350