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

Sounds like it needs a good carby clean 

The hoses are the carb breather and overflow. They won't affect the running. More than likely you have a clogged pilot jet/passage. Most people think the pilot jet only controls idle but it actually effects how the bike runs all the way up to about half throttle. What's happening when you are cracking your throttle open is you are leaning out your mixture causing the bog because of your pilot circuit not providing enough fuel. I would bet if you got on and rode it you would have a mean lean decel pop coming off throttle.

I've done quite a few carb rebuilds. Taking it apart and blowing it out with cleaner just doesn't cut it most of the time. My process is this:

1. I always purchase a rebuild kit because of the chance of lost parts ect while working on it. I also like to replace everything while I have it apart because if you are going to take the thing off the bike why not freshen her up.

2. I take the carb apart and boil it in in acidic water. Remove all the plastic bits you can get to.  You can probably leave the slide in but definitely remove the float as you can ruin it. You can do this with lemon juice, vinegar, whatever. About half vinegar, water mixture. If you have a ultrasonic cleaner this is better but most people don't. This loosens the deposits / varnish inside the carb passages where you can't see or get to.

3. Blow out all the passages with compressed air. For the pilot jet listen to where the air is escaping and alternate plugging those holes with you finger to clean out everything.  Some people will try to run thin wires into the passages.  I've never had to do this after boiling.

4. Reassemble with new jets, needle seat, gaskets from the kit. I like putting new jets in because they are sensitive to contamination. A jet can be dirty and cause poor running and it is hard to tell just looking at it.

The carb guide on here can get you through all the specifics.

One more thing. If your engine to carb boot isn't tight tight it will run lean and you will run in circles rebuilding your carb trying to fix an issue that isn't the carb. Make sure that boot is really snug.

I re-read you post and your issue may also be caused by incorrect float height.  Check that too.

-- Edited by Rooster on Thursday 20th of September 2018 02:52:46 PM

I'm still staying with carby as Brian said the pilot jet is in play first third or so of throttle opening 

Also low float hight also not as common can cause a bit of grief 

It's a bit rare but the accelerator pump not working can cause a lag or flat spot but not that bad 

Just follow Brian's steps also be super careful if taking the float pin out I tend to use brake cleaner and compressed air to avoid taking the pin out 

Too cautious maybe 

Once all clean and back on the idle mixture screw should be at its best setting around one and a half turns out (if it's way off that now try setting it at one and a half turns out and try it first )