Will not rev up

Hello, new member her.

Got a FC501 to sort out, its my sons.
2000 model, with the hi / low selector on the handlebars.

It starts a treat but will not rev up.
It will tick over all day without a problem, but when I give it a tiny bit of throttle, it will hesitate then die.
If I take the throttle off at the crucial time, it wll tick over again

He got it as a non runner.

To date I have :- new plug, ground the valves in, checked the piston rings, set valve clearences, set valve timing, replaced stator, replaced carb, checked earths. All the usual things.
All the above improved starting, but the main problem is still there.

Suspect it may be the coil, can anyone advise on how to test this or suggest another solution.

Dave L

Taffy,
My bike has has a "Y" in the frame number so from the web site, its a year 2000 model FC 501.

I have tried a new stator from the earlier model (5500), would only spark every third kick & would not fire at all, nothing.
So, sent it back and got another stator from the later model (5410), checked it with a meter & the resistance values are spot on for this model of stator.

Put it on the bike & its the same problem, starts easy, ticks over no problem but will not rev up.

Have decided that it is not stator related, because its the same problem with 2 different correct stators (later model)
I dont think its carb related because its the same problem with another carb (Kehin off a 460 Yam in place of Dellorto)
Prior to this, I have had the head off, ground the valves in & set the clearences.
I am sure its not sucking air in anywhere.
I have had the flywheel off & the key is OK.
The cam profile does not seem to be worn away.
Moving the ignition timing does not seem to effect it, when its running it just gets a bit worse at extremes of adjusment.
It has got some petrol in the tank & both taps seem to flow plenty when opened.
I have set the cam timing up with both cams pointing up at same angle in relation to the head at TDC - is this correct ?

I am running out of ideas.
Could it be the coil ? I have been told that its starting & running so it wont be that.
Could it be the auto de-compressor as it seems to be rattling a bit when the engine is running ?

It starts easy, even from cold, when warm it starts first kick, when I open the throttle a little bit past tick over, no matter how carefull I open the throttle, it just dies.

Any advice would be appreciated.

Thanks for the suggestions to date, Latest up-date on my problem of FC 501 starts easy, but will not rev up

Tried another new stator, putting a meter on it before fitting & the resistance values are spot on - no better.
Tried another coil from Taffy that had the diode mod. - same probelm.
New HT lead & plug cap - no better.
Tried making the plug gap bigger & smaller - no better.
Bought one of those plug caps with the indicator for the spark.
Kicking it over, it flashes OK even if the engine does not start.
At tick over (engine running) it flashes really well, every time the engine fires (approx 800 rpm)
If I open the throttle slowly to approx 1200 rpm is still OK engine running & flashing OK.
But if I slowly open the throttle a bit more approx 1300 rpm the plug cap stops flashing & the engine stops.
If I quickly close the throttle, it will start flashing & the engine picks up & will continue to tick over.

So, to me, the reason it will not rev past 1200 rpm is the spark not working past 1200 rpm ??

I put a meter across the red / black leads from the stator, when at tick over (approx 230 MVA ) & when I open the throttle to approx 1200 rpm, the reading does go up slightly (260 MVA - bouncing about a bit) , but if I open the throttle a bit more, the engine dies, the plug cap stops flashing, but the stator is still generating approx 260 MVA when it dies, so I dont think its a lack of electricity - should it develop more as it revs more ?

Perhaps its loosing its trigger signal when it revs, is there any way of testing this ?

The only part of the ignition system that I have not replaced is the flywheel, but that seems OK.

This week end, I am going to try another re-conditioned stator, instead of the 3 that I have tried from the current supplier.