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Hi whats the most likely problem with jet sx200 as it tends to jump out of 4th gear. Scares the living shit out out me at times when the engine revs increase
I dont think it has done many miles (american import).

It could be a worn selector deep in the gearbox, but it would be worth checking the cables first, cos there is nothing more annoying than stripping the gearbox, rebuilding it only to discover the problem lay outside the engine all along.

So carefully adjust cables, then think about the gearbox strip if this fails. Have you stripped a gearbox before?

Hi no i have never stripped a gearbox. The furthest i have been is to replace the clutch. I dont think its a cable problem as it all seems ok lined up at the handle bar end. Is there anything else cable wise i should look at?

A classic mistake is to trap the loose end of the gear cable in the rear shock nut, or behind the shock itself. This is common on the clutch cable, but not uncommon on a gear cable. Always worth checking that this is not stopping cable pulling the selector fully into 4th gear.

If incorrectly set up, the gear slector are and rod can hit upon the rear shock lug at the back of the engine.

If you can, undo the gear selector arm from the connecting rod on top of the engine, and then rotate the gear selector to ensure that swivel moves from one extreme position to the other.

Once all options with gear cables have been excluded, it's time to buy Sticky's Manual and dive into the gearbox.

Does it have cable gaitors on
Could be this

No it doesnt have any gaitors on Will have a look at the weekend to check these things out. Thanks all

If it's apart check the internal gear selector pinch bolt, if its fitted with a 10mm headed bolt instead of a 9mm then that can prevent fourth gear fully engaging as the bolt head catches on the inside of the engine casing.
"It probably isn't this" but if someone has recently rebuilt the engine then they might have used the wrong bolt.

Scot. I'm not saying some of the advice above is not good advice. But I read from your post that the engine is in the frame and still complete. so do the simple, easy things first.

If an engine slips out of first or fourth only it is generally the cables and or the cable or swivel mechanisms (not always, but concentrate on the easy things). These can easily be eliminated.

First thing to do is check that you have free movement of all, I'm assuming you do as you can obviously get all gears and it is only jumping out of fourth. If you do then next thing to do is tighten up the outside gear cable on the adjuster, if you need to slacken off the inner cable by a similar amount. Sometimes one cable being too tight restricts the other from doing its job. Do this and see how it goes.

If this works, move on and enjoy! If it doesn't then ensure there is no obstruction in the headset stopping the full rotation of handlebar. This could be a bent rod, a clutch cable or wiring loom incorrectly routed etc etc. You might need to disconnect the cables at the engine end to fully check this. Whilst they are disconnected, manually move the gear swivel with your hands (if you raise the rear wheel and rock this as you do so then it will be easier). If you can get them to engage without difficulty then reconnect cables and adjust. If this works, move on and enjoy. If not we will need to go into the engine, but do the simple things first.

hope this helps

Thanks Berniek, have adjusted cables and done a couple of hundred miles and all seems well. Thanks for your input. Saved me a hole lot of trouble

Follow the excellent advice given by Berniek plus check the tie rod for wear. I had the same problem and it just required the cables tweaking.