Desperate help needed

[Jog_on_smudge]

Hi everyone, I'm new to the forum and you've got some great reviews so I'm hoping that someone can shed some light on my problem.

I bought my son a Yamaha jog RR to get backwards and forwards to college. The bike ran great but then started to get harder to start.

Then one morning it wouldn't start at all on the button or on the kick start.

I then cleaned the carb, air filter and put a new electric choke and spark plug in it.

It then ran great, started on the button for 2 days then this morning he went out and it wouldn't start again.

I'm now totally scratching my head as I don't understand why.

If anyone can throw some light on the problem I would be very great full.

Thanks Steve
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[arry]

Describe what's happening when it fails to start. I'm assuming it's turning over but not catching, in which case have you checked that there's a spark?

[Jog_on_smudge]

Hi thanks for your input.

Yeh it just turns over but doesn't start.

It's got good fuel to the plug, a good spark and clean air.
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[mitch99]

Whenever this has happened to any bike i've had, it's been valve clearances tightening up.

[Jog_on_smudge]

Hi

It's only s moped, no valve adjustment.
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[Rogerborg]

Specifically, it's a 2 stroke 50. You can infer that from the fact that help is "desperately" needed, although for some reason it's NOT CAPITALISED.

I realise that isn't helpful, but we're got nothing to go on. You've already done the right things and checked the right things.

It's not sparking strongly enough, or at the right time; the fuel-air mixture is off; or compression is coming and going. We can suggest a bunch of random causes, but it's going to come down to you testing each one to rule them out.
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[Jog_on_smudge]

Thanks for the comment.

It's desperate because I'm only home at the weekends due to work and the bike is needed to get my son to college so in my case iv ran out of tests and desperately trying to get a bigger audience of knowledge.

Can the cylinder compression come and go?
I would have thought that if the rings were worn or the bore was US it would always make no compression.

It's got a great spark and yesterday started on the button first time but this morning nothing on the button or by kicking it.

Thanks everyone for your comments and help.
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[jaffa90]

Did you find anything in the carb and what colour was the spark plug?

[Jog_on_smudge]

No rubbish in the carb and the plug looked a little black but nothing to worry about.
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[Suntan Sid]

Probably needs a de-coke, cylinder head, piston and more importantly the exhaust!
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[jaffa90]

[Jog_on_smudge]

Thanks for all the comments, I have put a new choke on it however when I tested the voltage I wasn't getting any.

Under the seat there is a module of some sort that is in series to the choke. Does anyone know what this module does?
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[jaffa90]

Module? Pass on that, any pics?

[Jog_on_smudge]

Sorry not until the weekend. It is in line from the wiring loom through the module to the choke.
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[Evil Hans]

Check the continuity between the (black?) earth wire coming out of the choke and earth to make sure it's earthing properly. Then try connecting a 12v supply to the auto choke and see if that helps?
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[jaffa90]

Voltage could be there when the engine is running and not just when the ignition switch is on.
Back to my second post really.

[Jog_on_smudge]

Yeh I thought that but if it's for the choke then surely you would need power for the choke? Or does the needle retract?

I feel it's got to be something to do with the choke also.
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[jaffa90]

The auto choke (fuel enrichment really) plunger is retracted when the engine is cold and no power to it and is pushed back out with power to block the passage (choke off) all the time the engine is running.

[-]

When was the piston and rings last changed?

All it takes is a cracked ring and compression can be low enough that you'll have great park, timing and fuel/air, but it wont start in a month of Sundays if it can't compress the mixture.
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[Jog_on_smudge]

But why would it start one day then not the next?
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[-]

[Analogkid]

Have you checked the battery?

Or could it be a dry joint or a faulty new choke?

[Andy_Pagin]

Since no-ones' asked the obvious question, has the bike been 'de-restricted' or otherwise mucked about with by previous owners?
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[andyscooter]

Simple thing what's the battery like

A lot of starting probs are caused by a shit battery

Not sure if jog has an immobiliser but if battery is dead it won't immobilise
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[Copycat73]

spark plug workin itself loose..

choke / mixture fouling plug ...
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