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YZ250F YZ250F won’t rev past 1/2 throttle. Help needed please.

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Bought the bike as a non runner and was told all it needed was the barrel replated and a top end rebuild, had this work done and the bike won’t rev fully through the range, noticed the bike was getting very hot after just idling for about 20 seconds so tried bigger jets to richen up the mixture but still exactly the same problem, any ideas? I’ve tested the stator plug with a multimeter and that is within spec to the manual, tried new spark plug, any ideas please help. Driving me mad lol

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4 hours ago, Old Iron 1978 said:

What bike?  yz450f.... year?   Could be a whole bunch of things.

Yz250f 2005, I’ve tried a whole bunch of things😂

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Check the cam chain timing is correct after the rebuild.  Then put your carb back to stock and thoroughly clear all the jets with carb cleaner and compressed air.  Check what needle you have installed and the clip position and make sure the float is working properly.  Check for holes in the inlet rubber.  If that all checks out then it might be an ignition problem (like the plug cap) but that would be more intermittent 

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19 minutes ago, Ooshka said:

Check the cam chain timing is correct after the rebuild.  Then put your carb back to stock and thoroughly clear all the jets with carb cleaner and compressed air.  Check what needle you have installed and the clip position and make sure the float is working properly.  Check for holes in the inlet rubber.  If that all checks out then it might be an ignition problem (like the plug cap) but that would be more intermittent 

I’ve triple checked the timing and they are both flush with the head, tried the standard jets, needle clip is on position 4, no holes in the air boot or carb to head rubber. But I did notice with the standard jets that the bike would get red hot after only idling for about 20 seconds, so tried bigger jets thinking it could be running too lean, still no difference. Could a leaking valve seal cause these problems?

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Leaking valve would cause no start due to low compression. It really depends on how the engine behaves.

Does it run normal up to 1/2 rpm? Sluggish to 1/2 rpm and won't rev higher? Sounds smooth or popping and backfiring? Could be weak or bent or broken valve spring. 

Does trying it with choke on make it better or worse (lean versus rich carb)

Can you put a video up of how ot behaves?

Honestly the exhaust will normally get hot enough for it to glow in the evening light. But If its red hot in bright daylight that sounds a but warm, which could indicate late spark timing causing in burn in the exhaust pipe rather then the cylinder.

 

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2 hours ago, 3514vmach1 said:

Leaking valve would cause no start due to low compression. It really depends on how the engine behaves.

Does it run normal up to 1/2 rpm? Sluggish to 1/2 rpm and won't rev higher? Sounds smooth or popping and backfiring? Could be weak or bent or broken valve spring. 

Does trying it with choke on make it better or worse (lean versus rich carb)

Can you put a video up of how ot behaves?

Honestly the exhaust will normally get hot enough for it to glow in the evening light. But If its red hot in bright daylight that sounds a but warm, which could indicate late spark timing causing in burn in the exhaust pipe rather then the cylinder.

 

Runs quite rough up until 1/2 throttle some popping and generally not smooth, but idles smooth.. my choke lever doesn’t stay open I’m not sure if the latch has broken but it would be quite hard to try and hold it out and ride it haha! I can’t get a video as it only happens under load, it seems to rev okay in neutral or Atleast to a higher rpm, I’ve got it booked in for a leak down test on Wednesday, but failing that I think it could be something electrical? Possibly ignition coil breaking down. What’s the best way to check ignition timing? Thanks for the reply

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7 hours ago, Craig Piper said:

I’ve triple checked the timing and they are both flush with the head, tried the standard jets, needle clip is on position 4, no holes in the air boot or carb to head rubber. But I did notice with the standard jets that the bike would get red hot after only idling for about 20 seconds, so tried bigger jets thinking it could be running too lean, still no difference. Could a leaking valve seal cause these problems?

 

did you get the carb slide plate installed correctly?

mine ran lean and backfired when I put one in upside down.

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On a little trial bike I used a spark gap tester where you could watch the spark jumping the gap when you lut the tester between the plug cap and the plug.

Not sure if anyone makes one for a race bike as the plugs are burriws so deep into the head.

At higher speed it would be hard to watch the spark anyway, but that sure workes good at low rpm, when it missed you cool see the spark not jump and hear the snap when the spark does jump

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1 hour ago, SlowDinoDog said:

 

did you get the carb slide plate installed correctly?

mine ran lean and backfired when I put one in upside down.

Hmm, I’m assuming it’s the right way as I personally havnt taken it all the way out, how can I check if it’s the right way round? Thanks

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Well... after many more hours and a few more quid in the swear jar it turned out to be the ecu was faulty which I can imagine was throwing the ignition timing out, never stop trying😂👍🏻 Thanks for all the replies guys 

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