cant get it to fire it will seem like its goin to start for couple of seconds then it just keeps cranking until battery runs dead,just cleaned carb also did oldman mods,spark plug looks dry after its been cranking ,we also but his cdi box on my bike and it fired in a second so i know its not that he is running full rotax exhaust , stock cdi ,uni filter,175 main 3turns out, needle on 4th grove lid on, 3950 elevation,im thinking of puting my carb on his, thanks for any help.

 

The DS needs to have the battery almost totally charged.
It draws so much voltage to the starter that there is nothing left to go to the ignition.
If you do not have time to charge the battery, then do a pull start. With your quad pull him up to speed. Key on, gas on, 2nd gear and drop the clutch.

You did check to see that fuel is in the float bowl!!

 

oldman thanks for info but i just swithed my battery and carb to his bike and it still does not fire, i also put his carb on my bike and my bike fires right up, i know his cdi is ok so im gussing coils, he replaced his plugs with champion plugs but it was doing that even before that what do you think.

 

Pull out one plug and with plug installed in cable, rest plug against block and turn over motor.
Need to be in the shade, can you see if the spark is bluish or is there no spark at all?

Pull out wires to rec/regulator and make sure they clean, maybe put alittle dielectric grease on the connectors.

 

just checked both plug they both have a lot of spark. ill will check rec, regulator and conections , will take any more ideas

 

When you disconnected the fuel line at the carb and then turned on fuel, fuel came out the hose adequately.

 

Fuel, spark and compression all at the right time....Maybe time to check valve clearances, Intake valve may be to tight and stuck open a little?????

 

i just gott, er done,it finaly went just had to use a battery charger and jumper cables from my bike to his and fired ,but whats really weird is that my battery will start my bike but it wont start his ,it needs cables plus my battery whats the diffrance. thanks for help.

 

does anybody have a clue y my brothers bike needs so much more juice compared to my bike, my battery will start my bike then ill put my battery to his bike and it wont crank as fast as my bike so i have to put charger plus jumper cables to get it to start, after it starts the first time it will start all the time until it sits for a couple of hours then i have to do this all over again. thanks.

 

It could be low compression as Eagle suggested. If you havn't had the valves adjusted, it would be a good time to check them. It is common for the intake valve clearance to close up in the first 50 miles or so. After the first adjustment however, they seem to hold tollerance pretty well, at least that was my experience anyway.
The other possibility is a starter issue. If you have the meter, perform a amperage draw test on his starter and compare it with yours. Also check all battery cable connections - both ends.
That should about cover it.