Ok, i recently purchased a DTR 125 R 2000 model with running problems The bike will start with the choke on, then die with the choke off, the fuel/air mixture screw was mashed up so i purchased another 2nd hand carburettor , cleaned the reeds, cleaned the carb , fitted a new spark plug, new air filter Fitted the 2nd hand carb and still the same problem. can only start the bike with the choke on, and when taken off it just loses power and cuts out if revs drop below 3k i dont know what else to try ?

I would say lack of fuel. Choke or enricher not working properly. Motor needs retimed.the fact it started means ur getting spark. Weather or not at the right time. Try a shot of brake kleen down bore&try again. If it starts right up the timing is close, if it dies again, ur not getting enough gas.

could be you spark plugs need replacing

fit a bigger main jet

Chances are that some trash has gotten into the float bowl and is intermittently plugging up the main jet. When the bike dies the suction on the jet dies, the trash is no longer blocking the jet and the bike starts right up.

Being you did'nt say what kind of bike you have Yamaha has been making motorcyles since 1967 for sale in the United States and their still making them now. they make them in all sizes and some of them are fuel injection and they make 1, 2 ,3, 4 and 6 cyclinder bikes Then they make street bikes and trail (offroad) and mixture of racing bikes the answer to your question depends heavy on Make year and model. And if any equipment has been changed away from stock. If the bike is fuel injection it will not have a choke. If the bike with carbs has sit along time the air bleeds and even thefuel jets in the carboraters may be stopped up. some models have to warmup a fairly unreasonablely long time before they'll take the gas with the choke off. It's not wise to ride the bike around until this is fixed as the engine will run on the wrong mixture of fuel but it will put gas in the oil and wash the cyclinders down and generally ruin the engine.

sorry, dont have a solution, but was curious if you ever figured this problem out, i have the exact same issue

my 2001 just had that problem. Take off your air cleaner and look in the carb. There is a plastic venturi there that sticks. it will free up with your finger

Sounds like your flooding it out with the choke out. Try the next time you go to start the bike when it's cold with the choke out and when it starts push the choke all the way in and keep it running with the throtle. If it runs better after doing this then your flooding it on start up. Use 1/2 choke to start cold and see what it does. Good Luck

HI

It is clearly that you have poor mixture.
Every engine needs more fuel in starting, thats the reason for the choke circuit.
But yours doesnt work as you said
Fix your choke circuit in your carbs and every thing will be OK
If you had problem with your fuel pump your bike never run ok cold or hot it dies in any throtlle response

i'd start with removing the carbs and giving them a thorough cleaning as when bikes sit for so long, the carbs begin to get gummy/varnished from the old gas.

check for blocked holes in the pilot jet & main jet and look for "hard & dried up" gas in passages. be careful of the float bowl gasket as to not tear it or you will need a replacement.