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Ok this is starting to get on my nerves,I've got a standard SX150 and for some mad reason it seems to over rev on the flat like a screaming banshee. It's not the clutch, it pulls like a train up hill in 4th but on the flat it's a different matter. I've inspected the clutch, sprockets, chain adjustment and cush drive and all good ( I made sure that I didn't have a series1 spring in the crush drive/front sprocket set up) the one area I haven't looked at is the gearbox- could worn bearings / cogs cause this? It seems to be working ok. Had the engine on the road for the last eight years but in the last couple it hasn't been running well, been slowly working my way around the engine, it's like it's been under-geared, any idea's? Paul

'Screaming like a banshee' made me think instantly of a weak mixture/air leak, but you can't mean that, can you?

If your carburation is sound (verified by plug chops) have you inadvertently gained power so need to gear up? Ha!

If you mean a 'shriek', as in metal to metal contact, it could be any number of things, especially the gearbox & bearings, but also a sticky clutch can squeel. It doesn't happen up hill, under the most load, so try (carefully!) applying brakes whilst holding throttle wide open to see if the noise diminishes. Is it a noise like the Police helicopters hovering overhead? Seriously, their gearboxes are noisier than the engines.

You may need to analyse it further to give us more clues

Nope, the engine runs smoothly at lower speeds, text book plug chops etc, it just over revs in 4th where previously it would storm away quite happily. Had the clutch apart, all corks, steels, springs, basket etc are A1. Paul

Still think it could be gearbox related but want to ask about before pulling the bugger apart Paul

Worn front cush drive.

Maybe you need a taller gearbox?

CHRIS in MARGATE wrote:Worn front cush drive.

I did inspect the cush drive and it did have a few worn components which I replaced, I didn't have a good look at the sprocket sleeve, will have another look. Paul

[quote="ToBoldlyGo"]Maybe you need a taller gearbox?[/quote.

That did cross my mind

If your carburation is sound (verified by plug chops) have you inadvertently gained power so need to gear up? Ha!

is another way of saying

Maybe you need a taller gearbox?

You've been riding this 8 years and it's recently started doing this; is that right? No modifications recently?
You haven't got a different crash helmet have you, allowing you to hear more?
At 57 mph SX 150 would be doing about 6000 rpm and it does sound busy !

CHRIS in MARGATE wrote:You've been riding this 8 years and it's recently started doing this; is that right? No modifications recently?
You haven't got a different crash helmet have you, allowing you to hear more?
At 57 mph SX 150 would be doing about 6000 rpm and it does sound busy !

Hiya Chris, nope no changes to the spec of the engine since the build, yes the SX150 engine is no targa twin but when its revving out at half throttle when it would previously zoom away then there's something not right, like I mentioned before- would worn gearbox components cause this? Paul

Not gearbox I shouldn't think. Most likely clutch or cush drive UNLESS gear selector and sliding dog are falling apart and not keeping 4th gear engaged. That is an unlikely scenario though.