Hi there, I’d really appreciate some advice about my small Suzuki GS125 bike.

I was happily cruising in top gear when I completely lost power and had to stop. The bike now starts and idles but won’t rev above 3500 rpm. When you try to exceed this, the revs drop and it just dies.

I’ve checked that it’s sparking ok, there is no blockage in the fuel filter and I have completely replaced the carburettor but still the same problem persists!

Does anybody have any other ideas about what may be wrong? I assumed that it was some kind of fuel feed problem but surely replacing the carb with a brand new one would solve this.

Any ideas for me to investigate will be massively appreciated!

Many thanks,

Rob

Will it rev above 3500 in any gear or in neutral?

I’d check the coil and reg/rec – sounds more like an electrical issue rather than fuel.
Have you tried raising the revs slowly to see if it will pass 3.5k?
Don’t just open the throttle.

Check fuel flows freely from the tank. All GS125s are pretty old now so rust inside the tank partially blocking the fuel tap could be a problem. The bike wouldn’t have had an external fuel filter when new so maybe a previous owner was aware of that as a problem. Drain the carb float bown and check for water in the fuel too.

Thank you for the ideas!

The revs will struggle all round. The rev limit mean that it’s nigh on impossible to ride away in first gear. I’ve bump started it into 2nd gear and it dies very quickly.

Yes, I’ve increased the revs very slowly and still, trying to exceed 3500, the revs will just drop. A reg/rec test is a good idea and looks fairly easy to do.

I drained the tank from the tap and the fuel flowed freely. Checked the filter that lies above the tap too and no blockage…. I then filled with some new fuel before trying the new carb and so there shouldn’t be any water in the float bowl.

Check your earth connections.
Single cylinders are a bit vibey
I had a similar problem on my Buell (also vibey).
Was a loose earth connection on the battery – once the revs/vibes got enough it wouldn’t make a connection and acted like an extra rev limiter

Does the headlight get brighter as the revs increase?

Thank you, that’s something I hadn’t even considered but you’re right about the vibrations.

jota180, I’ll check. I seem to remember that was the case when the bike was in working order

If the headlight dims and brightens with revs, it points towards a faulty rectifier/regulator

A few things you can check:-
1. Fuel tank breather blockage – try undoing the filler cap, sometimes the beather pipes can get blocked usally on dual purpose mx bikes.
2. Spark plug cap, connection to HT lead, some have resitors which can break down!
3. Blocked air filter
4. Block exhaust – shouldn’t really be an issue with a four stroke though.
5. Once I had an intermitent fault, when the fuse suppling the CDI would shake open, but would test OK, a nightmare to locate !!!
6. Is the throttle openning OK, streched cable, throttle assembly ?

Good luck.

Check the throttle cable hasn’t slipped.

Thank you all for the suggestions. Looks like I’m going to have a busy weekend!

The throttle cable and assembly is ok as I can see it move the relevant things in the carb.

When it’s ticking over does the exhaust smell rich?

How new is the spark plug? And HT lead?

Check under the seat for anything blocking the airbox inlet – rag, gloves, waterproofs…
Take off the fuel cap. If that fixes it, it’s the breather.
Stick a new plug in it.
Take the fuel bowl off, remove the main jet, blow it through.
Remove air box connecting rubber, see if that ‘fixes’ it. While you’re there check the slide moves up when you open the throttle.

All the above is a good start. Previously for me this has been because of reg/rectifier failure, old spark plugs and coils.

Viagra?

Check the spark plug first.

Problem found, and from an unlikely source! Had a look at the bike again today and found that one of the tappet adjusting screws had come off. Really, completely off! Of course, this was the last thing I checked. I’m fairly certain that wasn’t helping matters but haven’t repaired yet. Thanks again for the help