Bike going slow then fast randomly?

[doctor_prox]

I bought a second hand bike on the weekend, Yamaha SR 125.

I have a 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 (like my mates SR). 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?

I have cleaned the carb, and bought a new plug, however both the new and old plug seem to have quite a weak spark as far as im concerned.

The haynes manual says that timing could be a problem with weak acceleration but wouldnt that be a permanent thing unlike this?

[Rave]

Have you checked the fuel lines to the carb? Might have a blockage. It sounds like a fuel starvation problem; if it's not the fuel lines did you check that the float and float needle in the carb can move smoothly?

Rich :¬)

[doctor_prox]

Checked the fuel lines and the float moves well.

[TeePee]

Carb Icing? Some Silkolene Pro FST fuel additive would help, give it a try and see if it cures it. Long term a carb heater is the solution.

Sticking fuel tap? Maybe it's gunged up with something like:

Corrosion? Inside the tank. Have a look inside to see if the inside is rusty. Fixing this is possible with a rust treatment.

If you suspect the weak spark is definitely the cause, first thing to try is a new battery.

[doctor_prox]

I have two plugs, both with a weak spark. I also tested the voltage on the battery, it didnt come out like the manual says, it was
12.4v with engine on, the book says 14?
Could the bad spark produce these problems, and is it worth me getting a new coil / ht / regulator - rectifier to try and solve that?
Bear in mind there is a problem with the lights that is also weird, i rev up and they dim. Bit backwards? I am yet to try that when its running well.

If i take the fuel pipe off the carb the fuel flows out well, what is carb icing?

[doctor_prox]

I doubt there is ice in the carb, as it isnt that cold here and when it take it apart its nice and wet, no ice.

[TeePee]

The low voltage does suggest the reg/rectifier is worn, but I doubt this would be the sole cause of your problems.

Carb icing is caused by the temperature drop in the carbs caused by the fuel vapourising (Which can be as much as 30C below ambient) causing moisture in the air to freeze.

[Urban Terrorist]

Have you checked the air filter? That can cause problems.

[doctor_prox]

We retested the voltage and its fine. We have checked all the simple things like air filter, weve been at it for 3 hours today. It sounds choked like its the exhaust but sometimes becomes unchoked and revs nice.

[doctor_prox]

There was some rattling coming from inside the exhaust but when we took it off it didnt seem to do it.
I have a fuel filter and there is no muck in the carb.
Coil has been changed by previous owner and is not the standard SR one from the looks of it. While we were fixing it, it just revved really high and nice one time which it does when it is working right. Then it went back to being bad. Its liek you have your hand over the exhaust. We loosened the bolts and it performed the same, im going to clean it later. Its not a carb problem im pretty certain. Spark is weak too.
The voltage seems to be ok now, i measured it again and its like 13.8v

[TeePee]

Sticking power valve?

Some 125's have power valves designed to restrict the exhaust at low RPM for smooth running, and open at higher revs. Don't know where it is or if you have one though.

[doctor_prox]

Im pretty sure thats only on 2 strokes, mines a 4.

[TeePee]

Indeed, it is only on two strokes. Not sure what an SR125 was!

[Jiff Lemon]

Sounds like an Air leak to me. Get yourself a peice of tubing and listen for Hissing around the inlet manfold/carb gaskets.

[big dave 1979]

Hi i have a sr125 and mine does the same as yours no power then loads of power i have done all the same things as you have any more help????????? Regards Dave

[big dave 1979]

i have the same bike and mine does the same any help anyone?????????