Topic: knocking under load

[Jeremy]

Bike runs great for about an hour, then seems to knock under load pulling away from a light, almost stalling . Never seems to happen any other time, have checked all the usual tune up stuff,timing, valves, plugs, even put on a new set of carbs. No change. What can this be???

[MRieck]

Are your carbs synched? Heavy knocking (mechanical) under load is usually a rod bearing.

[Justin]

Bike runs great for about an hour, then seems to knock under load pulling away from a light, almost stalling .

Does it start at an hour, then stop, or continue from an hour on?

Have you done anything to increase the octane requirement?
How clean is your combustion chamber (heavy deposits)?

Knock at low-rpm's, under load you say?

What is your current oil situation?
What are you running?
Could it be fuel contaminated, getting quite hot after the hour ride, with temporary levels of high viscosity loss?

[TwoTired]

A knock could be a rod bearing.

Or, it could be pinging, or detonation if it is running too lean.  They sound different, though. 
A rod bearing or detonation are sounds of impending doom.  Are your plug deposits, if any, white?

Check your timing.  Too advanced can cause pinging.

[Jeremy]

Noise would best be described as "lugging", like trying to pull away from a stop in third

[eldar]

well this may be a dumb thing but clean your plugs and ride. As soon as this problem starts to happen, kill the engine and check your plugs. This is usually a good indicator or what is going on in your engine.