After having had my JT25 for a couple of weeks now i'm beginning to get things sussed. However, I am struggling to eliminate a problem with the engine that I call 4 stroking.

The engine runs really well at low speeds but as the engine speed increases to about half revs it begins to splutter.

So far i've stripped and cleaned the carb...

main jet 107

idle 36

starter 60

and whatever the other one is 270d

The needle is on the 2nd notch

Repacked the rear silencer

The front silencer is a sealed unit but seems very heavy to me. About the same as a bag of sugar. Can it be cleaned? If so how?

Having looked at the gasgas museum i can see that these are not the jet settings recomended but I find it hard to believe that they should be so wrong. After all, the bike was in everyday use before I bought it.

Any ideas greatfully recieved.

Fresh plug, of course.

At around 1/2 throttle, that could be the needle.

I like to get out on the road, get the problem to just begin to happen, and reach down an move the enrichener (choke) a little and see if the problem gets better or worse.

This tells me what direction to move in with jetting and needle position.

Good advise from Charlie. It's always a good check.

Sure sounds like it's rich by the description. Clearly the GG Museum specs show a much leaner main. If there is any restriction in the exhaust, similar symptoms will occur though.

Sorted now and what a difference !!!!

After many attempts to clean the front silencer I finally gave up and went bananas !

After checking with gasgas UK and being told that a replacement was about

Another way is to cut out an access "window" in the back of the tail pipe, and make a plate which you can screw onto the back to cover the hole, this allows regular repacking of your tail pipe without any cutting welding.