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Tiny bit of assistance required Yamaha xt125 trials


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Hi Guys, most greatly appreciated quickie steer on a Yamaha 4 stroke fault.

Plenty of spark but no ignition and plug totally dry, replaced petcock in tank, replaced carb, (even removed carb and sprayed easy start direct into manifold while cranking), plug still dry with no ignition ?

I had removed the plug and filled the plug socket (in the head with tissue, turned the engine and it blasted out), hoping to indicate compression ?

I'm at a complete loss as to what to look for next, any ideas would be most greatly appreciated

Thank you very much

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1. Spark at the plug when out of the engine doesn't mean the plug is "good". I may not spark under compression. Try narrowing the gap!

2. Is the ignition timed correctly?

3. How old is the fuel? New fuel often cures all ailments. 

4. Engine needs spark, fuel and air to run. Spray easy start into the carb inlet. If it starts, then you aren't getting fuel. If it doesn't start, you know you've just given it fuel so it must be ignition related (ie: timing, plug, HT lead, coil etc)  ..... but you have spark so I'd go for plug, then timing.

Edited by handsome_al
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First off it’s an XT TRAIL bike, unless you have a special. Ignition is by CDI. You may well have compression but have you had the bike running before? 

You have fuel getting to the carb....you say you have blown out the carb and fitted another, or do you mean you took it off and then refitted it? To confirm compression, it won’t take much to blow tissue out, put your thumb over the plug hole and when kicking the engine over you should feel pressure that lifts your thumb up pretty sharpish. 

If the inlet valve is not working then that could be why the engine won’t start and hence dry plug. If you try another plug it must be the same length thread not necessarily the same grade but if the engine starts you must use the correct grade plug or you will create more problems.

 Stale fuel will prevent combustion taking place or if you could get the engine runni;g on stale fuel power will be very much down. It doesn’t take modern fuel long to deteriorate into a useless stale liquid....would you believe that in this modern day world of supposed technological superiority!!! 

The XT has a kill switch just ensure it’s in the run position, this is probably the case as the ignition gives a spark. That spark will have to be a strong spark to be able to initiate combustion in the high pressure atmosphere in the combustion chamber. Weak spark will not be able to cause combustion and that points to the CDI being at fault. 

This is unlikely but needs to be accounted for and that is the cam timing may have slipped, but this usually causes bent valves and noise.

Hope you can sort it, there’s quite a bit of helpful info on the net and this website, an hour spent researching could help you diagnose the issue more effectively. Good luck?

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Thank you all for your excellent mails, we have now managed to diagnose as badly damaged inlet valve, (as indicated by you guys), parts now on order, so should be able to resolve quite quickly

Many thanks to you all for your assistance, very greatly appreciated

Paul G.


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