Atlas help

'67 Atlas idles and revs like a champ. Magneto rebuilt 1 year ago, new clutch, carbs just gone thru and tuned, brand new plugs, newer wires. Sounds amazing and starts easily. However, if push it beyond 4000 rpms, in all gears when warm ( 10 minutes into a ride) she misses horribly. What gives ? this is my first magneto bike. Timing at idle is perfect. Any information would be greatly appreciated. Thanks

Check main jets ... should be #350 per manual. Smaller jets can cause fuel starvation at 4000 plus rpm where the main jet kicks in.

Also check screens in banjo at carb, and screen on petrol tap in fuel tank.

Slick

Don't jump to the conclusion that it's a magneto problem.

Are you sure it's rpm, not throttle position?

What colour are your spark plugs ?
Filthy black, or ghostly white.

If you are careful - and keep your eyes on the road - you can try to tickle the carbs while motoring along.
If this works - and keep your eyes on the road - this can tell you if the bike is starving for fuel.
If this drowns the engine with fuel, it may also tell you if the carb(s) are flooding...

Check also that the breather hole in the fuel tank cap is clear.

You say that the ignition timing is correct, but what degrees are you timing it to, fully advanced.
Also, have you checked that both cylinders are getting the same spark timing.
And have you double checked that the carbs are synchronised - BOTH carbs getting equal slide lift when you open the throttle.
hopethishelps.

Just a silly thought but ise the fuel tap supplying enough fuel to maintain the correct fuel height in the carbs at large throttle openings?
I once, in my even more ignorant days of youth, had a carburation problem on my Dommy for a long time TILL i wondered if it was a fuel flow problem. i had previously changed the fuel tank and tap..... I calculated very roughly that flat out I needed a pint in slightly less than 2 minutes......I measured the flow and found it gave a pint every 2 minutes 40 seconds......I changed the tap to one with a larger bore and the problem was solved.
i have heard of fuel taps where the neoprene seal has expanded partially blocking off fuel flow.......
Personally after my experience I only used solid brass gas tap type ones which are still available and are manufactured in the UK by the original manufacturer although the prices I have seen being asked for them on E Bay are, being VERY polite, somewhat high ....but I suppose while people are prepared to pay such prices.........

J. M. Leadbeater said:

…solid brass gas tap type ones which are still available and are manufactured in the UK by the original manufacturer…

Click to expand...

What make?

Fritz

Another silly question...is the choke on?

Remember, with English bikes you pull the choke off - the lever pulls the air valve out of the airstream. You'd be lucky to get 3500 RPM on a warm bike if those choke plungers are in the way.