Why does engine"die" when throttle is opened?

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Why does engine"die" when throttle is opened?

Postby David123 » Thu Mar 15, 2018 9:49 am

I am trying to help a young relative who has a problem with his scooter, which is a two year old AJS Digita 50cc. The bike starts fine, but as soon as you try to open the throttle (or even drive off) the engine just "dies." He has taken it back to the dealer where he bought it, but even they seem to be struggling to work out what is wrong with it.

Can anyone suggest what might be wrong with it?

Thanks for your help.

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Re: Why does engine"die" when throttle is opened?

Postby SPRIDDLER » Thu Mar 15, 2018 3:11 pm

Welcome, David.
Very few of us have Chinese AJS or other modern Chinese bikes; maybe ten (?) out of a couple of thousand Club members.
It's a very poor show when the dealer can't sort it out! My first hand experience is that many of the Chinese bike sellers are just 'box-shifters' with little or no real mechanical expertise and the established traditional dealers with proper mechanics are not keen to work on Chinese bikes since they realise that they've been bought originally very cheaply from a 'Pop-Up' dealer or online. Few have any real hands-on experience of fixing these bikes.
I'm assuming the scoot has run o.k. for him at some point and he hasn't just bought it in that condition. If it has been standing unused for a while petrol will evaporate leaving a gooey deposit inside the carburettor/pipes/filters.
It sounds to me like fuel shortage or a weak air/fuel mixture due to a blocked jet in the carb.
I haven't been able to find out which carb is fitted to these bikes but most carbs have a Pilot jet for starting and small throttle openings and a Main jet for general riding. I suspect that a jet (probably the Main jet) is blocked or restricted. In this case the carb should be stripped and thoroughly cleaned. Surely the dealer would have done that? Caution! If you dismantle the carb take photos at every stage and make a note of the 'settings' e.g. positions of adjusting screws and how many turns 'open' they may be so that you can restore the original settings after carb reassembly.
From the spares listing on the AJS importer's website I see that the fuel tap is vacuum operated so perhaps that isn't opening fully, restricting the fuel supply.
It also has an automatic choke which may not be functioning correctly.

There's an outside chance that it's an ignition fault. Even though the bike starts it could be a duff sparking plug which functions at tickover but fails a higher revs when the compression 'tends' to be greater or when the plug is hot. It's worth trying a new sparkling plug.

A decent dealer should've tried ALL of the above suggestions. :roll:

From the AJS website parts list I see that this bike has an EGR (Exhaust Gas Recirculating) valve. This valve routes exhaust gases back into the carb intake in order to meet emission regs. If this unit is faulty or the rubber pipes to/from it are perished, split or detached excess air would be fed to the engine resulting in a weak mixture when the throttle is opened which would/could cause the engine to die. A dealer should know more about this than I do.
I poke badgers with spoons.

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Re: Why does engine"die" when throttle is opened?

Postby mick h » Sun Mar 18, 2018 6:03 pm

my mum had a Yamaha passola years ago from new with the same problem. It is/was the automatic choke, you need to place the bike on the mainstand and start it up, but do not rev it, the revs will rise slightly the back wheel will spin. After a few minutes the revs will drop, the back wheel should stop spinning and it should be ready to ride as normal, hope this helps, mick.

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Re: Why does engine"die" when throttle is opened?

Postby David123 » Wed Mar 21, 2018 11:01 am

Thanks for your help guys.

I'm glad to say that - although it took more than two weeks - the dealer has now fixed the bike. They say it was a carburettor problem.

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