A forum for owners of Yamaha TTR250 trail and enduro bikes!

Sounds like a problem with fuel flow 

Blocked tank vent 

Blocked fuel tap strainer 

Blocked fuel filter if you have one 

Blocked strainer in carby inlet 

Last resort 

Dirty carb/ float height 

Still the same do u think need bigger jets

Is the airbox all standard? air filter clean? snorkel all clear?

 

I think your problem is fuel flow into the carb, as has been suggested. If it runs flat-out to begin with, but then starts to falter, the fuel in the bowl is not being replenished fast enough. There is a tiny filter inside the carb, above the float valve which might be partially blocked. (As ttboof states - blocked strainer in carby inlet. Have you gone through the rest of his checklist?) It is difficult to get at because the float pin is easy to break. Read all the carb rebuild threads Brian has put together first.

There is no reason to change jets if nothing else has been changed on the bike.

I clean carb out and seems to run ok now fingers crossed

Ask the seller Tony - they should know what is needed else they shouldn't be selling them 

If the exhaust is much freer flowing than standard, you will almost certainly have to fit bigger jets (pilot and main) to make sure it is getting enough fuel. It is all trial and error, so unless someone has your exact setup, nobody will be able to suggest anything other than "bigger than what you've got"! Running too lean will do damage to your piston, so if it's farting, popping, banging and stuttering, don't leave it!

I would certainly make sure your bike is running properly BEFORE you start modifying it, otherwise you will never get the carb set up correctly.

Once you have it starting properly without choke when warm, and with choke when cold (meaning your pilot jet is about right), accelerating without stuttering and coughing (meaning the needle height is about right) and running comfortably at or near WOT - wide open throttle (meaning your main jet is about right..... then start making changes like new exhausts.

Depending what age your bike is, and what previous owners have done you might have a 50 pilot jet and 137 main. A lot of guys fit a 52 pilot, and there are main sizes up to 152 I think. Check the carb jetting info thread- there is heaps of info on this forum but you have to dig a bit! I suppose 52/147 would be a starting point after fitting a free flowing end can.

ttr250.activeboard.com/t49951533/carb-jetting-information/

 

Moving the clip down the needle has the effect of lifting the needle up out of the main jet allowing more fuel. This is NOT a substitute for fitting the correct jets, as when you are near WOT, the needle has no effect on fueling, so if the main jet is not big enough, the bike will run lean leading to burnt valves, melted pistons and deep unhappiness!

-- Edited by mossproof on Thursday 9th of July 2020 08:35:35 PM