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Gord
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Joined: 21 May 2012
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PostPosted: Tue Jun 11, 2019 6:27 pm    Post subject: YZF 750 not firing on No3 cylinder Reply with quote

My YZF started running on 3 cylinders last week, I replaced the spark plugs, took it for a run then checked the plugs. No3 plug was still as new and clearly had not ignited any fuel, I found a set of spare coils and checked all coils, leads and caps with a multimeter to make sure they weren’t faulty and that a spark was present, to cut a long story short no matter what combination of coils or plug leads I tried No 3 cylinder is not firing.
I also checked fuel was getting into the carb bowls, took the carbs off to check the rubber mounts for splits, checked the diaphragm for splits and sprayed the jets with some carb cleaner (not a full strip down).
Could this have anything to do with valve shims and would a compression test show up any issues, any help is appreciated.
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Teego
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PostPosted: Tue Jun 11, 2019 8:00 pm    Post subject: Reply with quote

Won't be valve clearances. If you have a reliable spark at the plug it's a fuel issue.
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Fatdog
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PostPosted: Wed Jun 12, 2019 12:08 pm    Post subject: Reply with quote

I would agree with Teego, it's a fuel issue.
Could be a faulty float needle valve.
I've had one go before.
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andy88
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PostPosted: Sat Jul 06, 2019 7:18 am    Post subject: Reply with quote

A compression test would help... Do one to rule out a a goosed piston...

I've seen holes blown in piston tops..

Have you done a physical check on your headers whilst the bike is running.. Touch them,,, carefully to see if it's hot on no3..

If it is hot, strip your carbs, check and clean everything and reset float heights etc.

Also check your wiring to coils.
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Stig
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PostPosted: Sat Jul 06, 2019 8:59 am    Post subject: Reply with quote

Stuck inlet valves on no 3?
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Gord
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Joined: 21 May 2012
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Location: Teesside
PostPosted: Tue Aug 13, 2019 10:16 pm    Post subject: Reply with quote

I haven't had time to do much with this problem recently, I done a compression test yesterday the results are;

cyl1 dry 120 / wet 235
cyl 2 dry125 / wet 225
cyl3 dry 125 / wet 210
cyl4 dry 130/ wet 210

Cylinder 3 is cool to the touch and the plug is still as new (fitted 4 brand new plugs), so it hasn't ignited any fuel.
I have taken the carbs off and will strip them down and give them a clean next.
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