1974 TY250 Trials bogs down upon acceleration

I have a TY250 (Yamaha Trials) -1974 that is in great shape. It starts up fine, revs, as well as idles just fine. When I place in gear and accelerate it bogs down. It's like it's choking for fuel. I can hold the choke down and this helps a bit, but eventually dies. Does anyone have any idea what might be causing this. I've been through the fuel system about three times already. Taking all advice as would like to get this thing up into the hills and start riding the ridges... thanks for any help you can provide..

It would most likely be in the carb, first make sure it is not restricted, and check installed jets as to what was stock,
With todays oxygenated fuel , you need to make the pilot system a bit richer, When it sits and idles and rev it, does it come down to a clean and steady idle?
Or if it revs and stays on a high idle its possible way to lean..
If there is a lot of blue smoke out, then its possible you might have a right hand crank seal leak.
Try a few of these things and re post, I know the old bike very very well,, will do my best to help.......
One other thing you can check... pull carb and make sure the float is working correct, is it a aluminum needle or one with the rubber tip.
If it has a rubber tip, take a Q-tip, put in a cordless drill and stickk it in the needle seat and go to town, go in and out and push the tip into the seat.
If its a aluminum you need a bit of valve grinding compound. do same,
clean out with carb cleaner and give this a shot...

Wait a minute! Something is ringing a bell here. Does the bike ever move!!!!!!!!!!!

Thanks guys.. Yes.. acceleration does get me down the road for a bit, but eventually bogging down. Will check the float, and follow through on the other advice.. Appreciate the help.. thanks

How about more info. Is this bike new to you. Is the tank clean. Does fuel flow. The carb cleaned completely.

yes, the tank is cleaned and the carb was boiled. It takes a couple kicks but starts right up. It idles great, and also revs, and derevs just fine when in neutral. When I put in gear and accelate it seems like it just isn't getting any fuel. It bogs down, barely idling with eventual shut off. Put it in neutral and it starts right up again. When in gear, if I hold the choke down a bit it seems to help a bit, but minor. Hope someone can help define a solution. I plan on pullling the carb, as well as evaluate the needle seat as previously recommended.. Thoughts

I remember now. A couple of trials ago, a bike (Yamaha) had the slide in backwards. I know it sounds odd, but worth double checking!

You've probably already checked, but is the gas cap vent clear?

Well.. no luck. Disassembled carb completely and recleaned. Inspected needle/slide..all good. Reassembled. However, I did move the clip on the needle to the lowest position. didn't work so well. Remove clip on needle to most upper position. What position is the best slot for the clip to be on the needle? Also, having issues getting the air mix/fuel mix screws set just right. Baby is blowing a little blue smoke so assuming rich. It's also sticking in high idle, de-accelerating very slowly.... thought she was going to blow for minute .. had to use choke to shut er off. If I screw them in all the way, does anyone know the best amount of turns out that would work the best? Elevation is 2600ft. Also, I'm assuming I have other issues other than the mix. Any ideas anyone? Appreciate all the help..you guys are great.

My Kids bike had some fuel problems as of late much like you indicated, would run but fad out after giving it some twist, turned out that my pet cock valve was the issue, would give me fuel but not as fast as the bike needed it, did a few open closes on the valve and it came back to life. Did this on two different days before I figured out it was the pet cock...

I need to pull it out of the tank and clean or maybe even change it out.

Not sure if this will help... good luck.

I have no TY knowledge but plenty of two stroke experience.

1) It's a good idea to always start in the middle position and make changes from there if needed.
2) I generally start at 1-1/2 to 1-3/4 turns out on the air screw and adjust from there.

What does the plug look like? Is it white, brown, black and oily, wet with gas, etc. This can help narrow down the issue.

How about the air filter?

The issues you are describing are what a motor does with bad fuel. before you chase this very far, I would get some fresh fuel.

I had about the same symptoms with my Ty250a a couple of years back. It was a barn find that had not been run for a long time. I had put a new mikuni vm26 carb on it and it started dying on heavy accelleration only to start right back up again. It would run fine in neutral but in gear under a load it would start sputtering and die. I posted the trouble on TC and someone said to change the points and condensor as it was probably the condensor. I had thought it was fuel related all along also but thought it was worth a try as I had no idea what shape the points were in. Thats what it was! Ran perfect ever since. If you do, put a new crank seal in it while you have it open.

appreciate everyones help.. going to replace the condensor and see if that helps.. just need to locate a puller.. thanks again

Good call, I forgot about condensers, I had a bad condenser in a 70s honda as a kid, as soon as it warmed up it would fail and the bike would die, as soon as it cooled it would work again. Took a little while to figure it out. Hope that gets you straight

Would anyone know an aftermarket carb available for the 74" TY250? Unfortunately I found out that the carb on mine isn't the original, and i've been trying to source parts without luck. Hopefully someone can let me know what I might be able to put on this to make 'er go. Everything else works great... thanks

A mikuni would be the vintage mod, but these have been around lately.
http://www.mid-atlantictrials.com/OKOGallery.html
http://www.mid-atlantictrials.com/OKO.html

Thanks for forwarding this information. Would the following be the best one for my bike? Sweet deal..and I certainly appreciate your help with this. Been struggling with this bike for a month now and looking to get it on the trail... thanks



OKO K-101 Carburetor for Yamaha TY 175 & 250
Item # K-101YamTY

B&J can fix you up with a new carb. www.bjracing.com

so is the mikuni of OKO the best one to go with. Not looking so much to restore the bike but to get me up and down the hills.. thanks