Backfiring / carb adjustment

[proximity]

Hi i just got my new bike (K reg Yamaha SR) and had to clean the carb before it would start. After this my dad adjusted the carb so that it would run better, but it has started to backfire like when i throttle off for a junction etc. Sometimes it can be quite loud. I think the problem is too much fuel in the mixture, am i correct?
Next question, to reduce the fuel in the mixture do i want to turn the mixture screw in or out? The default is 1-1.5 turns out. Does the idle screw make any difference or is that just the speed that it idles at.

Any help is appreciated.

Jay

[Scooby]

Idle screw is just that, the idle speed. Does not affect the bike in any other way. I would suggest putting the mixture screw back to the factory recommended settings. Has your dad had any mechanical experience or is he just poking around? He might be doing more damage than good, even though his intentions are good.
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A big enough hammer fixes anything!

[Finglonga]

Sounds like you have an air leak somewhere on the exhaust.
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Andy Sez....
F.Y.Y.F.F.

[proximity]

There is a hole on the bottom of the exhaust, its like a really small one that looks like it has been drilled, i would say about 3mm diameter. It is just before the pipe starts to cone out.
Any idea which way the screw goes for less fuel then?

[mr jamez]

You have the default value, so tighten the screw as far as it will go (don't force it round, when you feel it pinch stop turning) then wind it out to the specified amount.

Fill the hole on the exhaust with something like 'metalweld' you can get it from halfords, its just like a glue really.

[proximity]

I will do that, say 1.25 turns, thats between the defaults. So should the hole not be there? Can anyone with an SR confirm this?

[Jrod]

[proximity]

Good job you cleared that up before i went out and filled it in the morning
I know you all recommend the default setting but im sure thats what my dad set it to when it was backfiring, hence wanting to know which way it goes for less fuel.

[mr jamez]

If the screw is on the engine side of the carb then it is a mixture screw, turn it anti clockwise to richen the mixture, other way to lean it off.

If it is on the air box side then it is an air flow adjustment screw, anticlock wise to to lean it off, clockwise to richen.

[proximity]

I have attached a diagram i drew of my carb, looks to be definately the engine side one but just to make sure.

[Dom_]

You sure it was a backfire mate??
Not just popping and fuel igniting in the exhaust on the overun?

[Jrod]

[proximity]

[swampy]

Backfiring sounds like a large bang often accompanied by a lurch in the bike, if it goes popopopopopoopop when you let off the throttle then thats over run and should be ok
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"I hate to advocate drugs, alcohol, violence or insanity to anyone, but they've always worked for me." Hunter S Thompson
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[proximity]

Ok good, there is no lurching like you said so this is normal and nothing needs adjusting? I think it only does it when its cold.