SR125 starts then cuts out when I try to open throttle

eastendborn · L Plate Warrior Joined: 21 Jun 2022 Karma :

SR 125 cuts out when I start it. It starts -then as soon as I go to give it revs- it dies.
I spoke to a motorbike mechanic today and he asked me what I put in the tank. I told him some Redex (as I had read was ok)
He said it is the Redex causing the problem -but my motorcar mechanic says it wouldn't be that. Any thoughts please?

Ariel Badger · Super Spammer Joined: 02 Dec 2006 Karma :

Why did you put that snake oil in your tank? What ratio did you mix it at?
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jaffa90 · World Chat Champion Joined: 06 Apr 2016 Karma :

Sounds like the choke jet is clear but the others not.
Suggest a carb service and fitting an inline fuel filter if possible.

eastendborn · L Plate Warrior Joined: 21 Jun 2022 Karma :

I put it in as it misfired a couple of times, and I read somewhere that a guy with a motorcycle said he kept his clean with Redex so I thought it was ok. I put about 40-1 ratio in. I start it without throttle and choke on - then as soon as I try to pull away with a little throttle it cuts out. Some guy said he he thought that was because the air was getting through but no fuel to mix it with.

jaffa90 · World Chat Champion Joined: 06 Apr 2016 Karma :

eastendborn · L Plate Warrior Joined: 21 Jun 2022 Karma :

Ok thanks for your help,, I'll enquire about getting it cleaned. Cheers Smile

jeffyjeff · World Chat Champion Joined: 02 May 2020 Karma :

I would start by making sure your idle air screw is adjusted properly; 1 1/2 turns out from fully seated according to the yamaha club forum. If that does not help, your symptoms suggest a restricted pilot jet. That would be the smaller of the two jets you see when you remove the float bowl. Remove it and clean it out (but don't ream it or you'll likely affect the jet size). Since you have the carburetor apart, best to check the float height is within specification, since float height affects carburetor performance across a range of operating conditions.
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eastendborn · L Plate Warrior Joined: 21 Jun 2022 Karma :

I removed the fuel filter today -( the bike was stored in a garge for 10 years before I bought it). As the sponge filter (that goes before the metal one) looked shabby - I put it into warm water. It thereby dissolved into like a black ink. As there was a hole in it when I removed it- I guess some of it could have got into the carb. Not good. Anyway, I'm addressing all advice so thanks again blokes.

MCN · Super Spammer Joined: 22 Jul 2015 Karma :

Fit a new spark plug.

Or at least remove the plug and look at the condition of the electrode, the side wire and the ceramic insulator.

The engine needs three basic things to run.

Fuel, oxygen and a big phat spark.

If any one of those three is weak then the engine will stall.

(Too much fuel can also kill an engine. The choke is possible still 'on'.

The start procedure:

Chock on. Not 1/2 on but fully on.
Crack the throttle open a tiny amount. 1/8 of a turn. That lets some air in to burn the fuel.
Turn the engine over on start lever or start button.
When it fires you may need to phannie a little with the throttle for a stubborn pig.

Let the hoowur run on full choke until it stabilizes. It 'should' start to idle at a higher speed.
Try to switch the chock off. Maybe hace to phannie the throttle again.

It should then run smoothly but possbly fartanfuk until it is warm enough.

No real need to let an engine warm up for much more than a minute or two. Ride off and just don't give it handfuls of throttle until it is warm. (Circa 5 minutes)
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Disclaimer: The comments above may be predicted text and not necessarily the opinion of MCN.