Bike going slow then fast?

proximity · Crazy Courier Joined: 08 Oct 2004 Karma :

I posted not long ago about a bike i bought on the weekend, Yamaha SR 125.
I have noticed a new problem with it that i cant seem to solve myself. Basically sometimes it rides like a dog, and then sometimes it is really fast and i can accelerate quickly away and it seems quite powerful (for a 125). I have also had it ride on a dual carriage way like a dog then just kick in like someone is pushing me and it speeds up without any extra throttle, going fast again.
It seems quite random and isnt just when its hot or cold. Any ideas?

pipnet11 · Brolly Dolly Joined: 17 Sep 2004 Karma :

Sounds very much like the problem my mate had with his CB250. First make sure the plugs and throttle cable are OK. Almost certainly are but its very annoying when you have spent 4 hours cleaning things out to find out you only needed new plugs!

second, take the carb to bits and clean it. This is the most likely cause of sudden acceleration. You may have a blocked jet or something, this was my mates problem. Also make sure there isnt something in the fuel line.

If this doesnt work youve gone past the limit of my extremely limited knowledge Smile

Phill
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I love the girls that do, I hate the girls that dont, I hate the girls who say they will and then they say they wont, but the girl I like most of all and I think youll say Im right, is the girl who says she never has but looks as though she might. - Max Bygraves

proximity · Crazy Courier Joined: 08 Oct 2004 Karma :

Thanks for the reply, i have stripped the carb and it all looked clean as a whistle to me. I bought a new plug but it did seem to me that both plugs have like a lot of weak little sparks all over, rather than one nice big spark, but i am no expert.
The guy i got it off said he had just fitted a new coil, could that do anything to cause this?
In my haynes manual it says poor acceleration is due to the timing not advancing but i have no idea how to check this.

pipnet11 · Brolly Dolly Joined: 17 Sep 2004 Karma :

Playing with timings is a possible cure but way beyond my experiance. However Im sure there are people on this forum who could do it with there eyes shut!

Phill
____________________
I love the girls that do, I hate the girls that dont, I hate the girls who say they will and then they say they wont, but the girl I like most of all and I think youll say Im right, is the girl who says she never has but looks as though she might. - Max Bygraves

Sadie · World Chat Champion Joined: 14 Jul 2004 Karma :

Make sure that your HT leads are not shorting out on your engine. This can cause the symptoms you describe.

Easiest way to suss if this is the case is to use a long boot lace to go over your tank and tie the HT leads up away from the engine.

If it then goes OK, you need new HT leads.

Thumbs Up

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With experience comes age.

mr jamez · World Chat Champion Joined: 04 Aug 2003 Karma :

dodsi · Dirty Carny Joined: 06 Aug 2004 Karma :

I was riding behind him on my bike and one point we were going 45 and the next 55 no reason, but it could have been down too the fact he was running on fumes!

proximity · Crazy Courier Joined: 08 Oct 2004 Karma :

Nah i have 7 litres in there now and its still as random, i rode for an hour at dinner time and it was like a rocket, then after college it was a dog again. Could be electrical, however, someone on another forum pointed out that the fuel tank breather could be the problem and i noticed that it is usually fast straight after i fill up so i will check this tomorrow.