My generator start the breaker trips under load and stops engine but with out load no trips

I don't work on these, but it sounds like an internal breaker for the motor. Motor old? could be pulling to many amps, gets hot and trips the breaker.

No it sounds like the engine is losing power, as the RPM drops the voltage drops, this increases the current load and that trips the breaker.

Is it tripping under load? or not under load. If it is under load have breaker checked and unit tested on a load bank. If it trips under no load check for wiring shorts on the main AC lines to the plugs

A number of things can cause this. A sit worked before !
Basically:

Engines get older and can't hold HP which causes the rpm to slow and voltage dip, this causes a current increase and trips.

Breakers also get weaker especially if they trip a few times
HVAC and compressors pull approx 6 times the current to start as they do to run, once they have run a while, they build up higher head pressures which cause harder starting and more current draw, hence the breaker tripping.

It's hard to pin point which is the actual issue, but these all contribute.

1. Make sure that the generator limit is not being exceeded.
2. Try checking and raising the rpm. If the engine can not sustain the load from the generator, it can/will trip the breaker.
3. If the carb is dirty or the engine runs poorly it can trip the breaker. Tune up the engine, clean the carb, make carb adjustments to correct substandard running conditions.

The breaker itself is worn and weak. Replace it.

if no power on the plugs check for the cercuit breaker fuses and check for your load why it is tripping the circuit breaker the unit sbould not be overloaded

A tripping circuit breaker is an indication of an overload. You say there is no load on the breaker - how are you making this determination? Are you using an amprobe or some other meter?

A circuit breaker with NO load shouldn't ever trip. Likewise a circuit breaker carrying up to 80% of of the current it is rated for shouldn't trip either. Circuit breakers that carry more than 80% of their load will trip - if the load remains connected long enough. This is called "duty factor" or "service factor" The greater the load is in excess of 80%, the less time that the breaker will carry it before tripping.

An example of a 100 amp breaker with different loads on it (this 100 amp value was chosen for ease of doing the math):

80 amps - never trips
85 amps - trips after 48 hours
90 amps - trips after 16 hours
95 amps - trips after 8 hours
100 amps - trips after 4 hours
105 amps - trips after a few minutes
120 amps - trips after a few seconds
150 amps - trips instantly

This is only an example to show how a certain circuit breaker might trip under a load less than the rating stamped on the body or handle.

A circuit breaker that trips with no load or a load equal to or less than 80% of its rating is most likely defective. You need an amprobe or ammeter for amp readings. If the load is found to be 80% or less, there may be an issue of harmonics that is causing the tripping. You'll need to have special equipment and qualified persons to check harmonic problems. Most harmonic problems occur when the loads are not linear device (transformers). Examples of non-linear devices are "switching" (or solid state) power supplies like those in computers.

I hope this helps and good luck. Please rate my reply. Thank you.

check your load..the generator is carrying heavy weight when you are going to load a high wattage thats why its RPM will turn slowly and have the possiblity to stop it...It MUST be needed the the given wattage of the generator is greater than the load you apply.for example:when your generator's specification is 3500 watts,tehn the load you are going to apply is i watts to 3499 watts only..

If you haven't seen it yet, here is a page that may help with a manual or parts list.

This problem sounds kind of bad, generators don't do well when the engine stops under load.
Start by checking the circuit breakers in the distribution box (outlet panel) - you need ot do this with an ohm meter. After that check the power windings of the stator with the ohm meter, be sure to check between the 2 windings (should be open), and from each one to ground with the winding isolated.
Whining, a burning varnish smell, and breakers that trip all the time are indicators of long term overload problems. This could happen if you had most of the load on one winding for a significant amount of time, or if the one winding mostly powered motor-like loads with high startup currents.

You may want to look at this answer I posted some time ago.

Regards,
Carl