Yamaha YZF-750R that suddenly refuses to start......

[graham247]

Good evening all,


I have a 1997 Yamaha YZF-750R which I bought in August this year. It’s run sweet as a nut since then, been regularly ridden and has started right on the button every time. It had a brand-new battery fitted as part of a service just before I purchased the bike.

I last rode it last weekend and left it filled up with a full tank of (fresh) fuel. Today I went to start her up to go for a quick spin. So, choke on, ignition on, engine run on, pressed the starter button as normal and the following happened;


1. Engine span up, she immediately tried to fire (for about 1 sec) unsuccessfully but did not start

2. Tried to start again, starter motor turning the engine no problem - but not a hint of her firing up

3. I left bike for about 5 minutes and tried starting again. Same result – plenty of juice in the battery, motor turning but no hint of firing up


Most surprised as over previous 2 months she’s started immediately every time. (One difference today possibly relevant is air temp. - around 6 degrees, probably 8-10 degrees colder than when I have started her before)


So guessing either the fuel system or ignition system at fault, given she’s always started first time before. I am thinking common components like fuel pump fuse or pump relay or the fuel pump itself might be the cause. I’m not suspecting Spark Plugs/Coils as I think if one of those had packed up she’d still have tried to fire up on 3 cyls?

Before I start investigating this this problem further next weekend just asking for any ideas or best guesses as to what might cause a sudden failure like this?

Thank you

Graham

[hoist1]

Hook up an old plug to save taking one out to check for spark. You prob right to suspect fuel pump related, shop may have disturbed something that has hung on for a while, look around under tank etc for anything out of place.

[marsbar350]

check you haven't put diesel in.its happened before

[graham247]

Nov 16, 2019 21:30:00 GMT 1 marsbar350 said:

check you haven't put diesel in.its happened before

Er no.....defo not. Rode it back last weekend 30 miles after filling up with super unleaded  

[Tobyjugs]

Can you hear the fuel pump working when you switch it on?

[graham247]

Nov 16, 2019 22:00:18 GMT 1 Tobyjugs said:

Can you hear the fuel pump working when you switch it on?

Hi - when I put my ear to the tank when I switched the ignition on, yes there was a whirring sound for about 10 secs. Pretty sure that noise came from inside the Tank and wasn't the Exup valve self-cleaning. 

[rich]

If it's like an FZR, the exup valve doesn't self clean like a YPVS does so you won't hear it.

[headcoats]

side stand switch faulty ?

[graham247]

Thanks for ideas and thoughts guys.
Done a bit of studying on the maintenance manual this afternoon. Established there are two coils that take care of two spark plugs apiece. If one of those had failed I would have thought the motor would at least have tried to fire on the other two pots but perhaps not. Planning to take tank off and investigate further next weekend. Will update thread with findings. cheers.

[graham247]

Promised I'd come back with the findings which were.....
Battery voltage had dropped to 11.9v
After putting it on an Optimate Charger this went up to 12.5v and held steady at that level disconnected for 3 or 4 days
Last weekend I put the Battery back on the bike, Choke On and pressed the Starter and hey presto she started right on the button.

Checked and cleaned all the various loom connectors, fuses etc and then took her out for a 60 mile spin with no problems.
So unless there is an underlying other problem it would seem that the bike is very sensitive to a small voltage drop to just below 12v.

Anyway.....Thanks for all replies, appreciated

[chris356]

When cranking over it probably dropped below 10 volts you might have a cell down on the battery

[nobby62]

I wonder if the new battery, fitted as part of the service, is a cheap or incorrectly specified item... worth checking, the problem appears to be battery related..