Yamaha SR 125 starts but won't take throttle

Byker 1127 · L Plate Warrior Joined: 04 Nov 2020 Karma :

Hi all in need of a little help, i have a 1998 Yamaha SR 125 that will start and idle although roughly but it won't rev. The cylinder head has been rebuilt valves lapped in, new valve springs, stem seals the works. Head fitted back on the motor with new gaskets, valve timing is spot on. compression tested (105psi) Has had a replacement carb fitted (chinese ?) this has been stripped and cleaned (although new you never know) and an original carb has been fitted as well just in case the new item is somehow faulty. Brand new battery also fitted, has a good spark (New plug D8EA) pulse coils checked (238-240 ohms). I am at a loss to find the problem, any help would be appreciated.
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Suntan Sid · World Chat Champion Joined: 07 May 2009 Karma :

Wrong carb or incorrect carb settings would be my first guess!
I've had mixed results with "new" chinese carbs, the same with carb repair kits. Try using the old needle in the new carb and see what happens, try the needle in the different positions.

Having said that, we all know that most fuel problems are actually electrical problems and vice versa.

Good luck!
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jaffa90 · World Chat Champion Joined: 06 Apr 2016 Karma :

If you have a good spark and give it more revs (air) then it needs more fuel. What about the choke operation or part blocking the air intake to give it more fuel?

steve the grease · Crazy Courier Joined: 26 Jan 2018 Karma :

My mate spent ages looking for a similar fault. The clip holding the need le had failed and the needle wasn't coming up with the slide in the carb. Result - weak , wouldn't take throttle.
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All the above is my personal opinion, you can see my lips move, but I'm talking out of my arse.
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Byker 1127 · L Plate Warrior Joined: 04 Nov 2020 Karma :

Thanks for the reply guys, Suntan Sid, Steve and Jaffa i know Chinese carbs can be a problem that's why i sourced an oe carb but had the same results, both carbs were cleaned and checked needle set at factory spec mixture also, i was thinking more on the electrical side so pulse coils checked ht coil also both read ok new spark plug etc. I've played with mixture settings starting off with the factory recommended 11/2 turns out and going in and out but still no joy. Choke appears to be working correctly as it needs it for initial start up and then i can clear it after a few seconds as it starts to run worse if i leave it on. Real head scratcher for me this one !!
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jaffa90 · World Chat Champion Joined: 06 Apr 2016 Karma :

Are you sure about the 105 psi, it should be 150 +.
https://www.manualslib.com/manual/882832/Yamaha-Sr125-1997.html?page=56#manual

Suntan Sid · World Chat Champion Joined: 07 May 2009 Karma :

Are you absolutely sure the valve timing is correct and is not 180 degrees out?
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Byker 1127 · L Plate Warrior Joined: 04 Nov 2020 Karma :

Hi guys all sorted, it was a blocked exhaust pipe !! although there was smoke coming out when it was running ?? once i got the exhaust off and compared it to another exhaust borrowed from a mate the weight difference was quite noticeable, banged it on and hey presto idles and revs out no problem.
Thanks everyone for your input Thumbs Up

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