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

Sounds like a blocked or partially blocked pilot jet. They are very difficult to clean thoroughly and, even after putting through an ultrasonic cleaner, they still sometimes need a careful poke with a bit of copper wire.....

Well i removed the carb today to have another look. Removed the float bowl and the pilot jet and gave it another clean, i spent a fair bit of time makings sure it was clear. The thing that puzzled me was the float bowl had a lot of debris in the bottom and i only cleaned it out a few weeks ago ! Then while i was messing with the carb on the bench i could hear a trickle, i looked round and there was petrol running from the fuel tap and it was in the off position ! I set about taking the fuel tap apart and was shocked to find what a state it was in, it was full of like a sticky tar substance and other debris. The easiest thing to do is to fit a new tap, iv'e had poor results with repair kits over the years.

Sent you an email Brian regarding a new fuel tap.

I looked for a filter in the carb but couldn't find one ! How do i get to it ?

Whats the best direction to get the float pin out ? Either ?

I will blow through first and if ok i'll forget about that filter.

-- Edited by locky on Saturday 24th of January 2015 11:53:08 PM

Excellent news!

Neat idea with that little remote fuel tank 

Time to get it dirty now then??!!

Pics to follow then hey

Did you end up modifying your air box?

I thought i'd sorted this but it's still there, if i hold the throttle slightly above idle it's still slightly hesitant. It's only in this throttle position that it happens.
 When the engine is hot and i give it the slightest amount of choke it runs perfect. Any ideas ? Would it be worth raising the needle ?

-- Edited by locky on Friday 27th of March 2015 06:35:05 PM

I should have updated the thread. I had the carb off last weekend and gave it another thorough clean out again. it now seem to be running perfect and ill test it on trails in a few days .
Renewing all the rubber parts in the carb would have been my next step.

Well my carb trouble came back again, i noticed it while riding a few local trails a few weeks back. It's just the same as it was at first !
I stripped and cleaned the carb again last weekend, it all seemed nice and clean but it ran the same when i put it back together.
I decided to order a new diaphragm and the carb side plate 0 ring that turned up today (thanks again Brian).
I installed these tonight but it runs just the same. I'm now at a dead end !!
Just to explain the problem again, when i hold the throttle just a tiny bit above idle it will run eneven and sort of hesitant. On the very odd occasion it will backfire through the carb.
It idles very well and i can turn the idle down quite low and it ticks over like a sewing machine. It revs through the gears as it should and pulls well.
The problem is just above idle. HELP :)

I'm sure I got the cam timing correct when I rebuilt the top end but will check it again to make sure.

Looks about right to me. That's not the cause of your problem then.....

Ran it again after another carb check (nothing found) . Been thinking how to describe how it runs just above idle, i can hold the throttle slightly open and it sort of runs rough, hesitates and will eventually backfire through the carb if i hold the revs the same. If i just rev it from idle its fine and it idles very well. When it backfires through the carb it cuts the engine. 

Carb off again and also removed the rubber inlet to check it. It was fine when i rebuilt the engine. This is what i found 

Yes the rubber has come away from the alloy ! But has this been caused by it backfiring through the carb or is this whats causing my problem ? 

Ive ordered a new inlet but also repaired the old one incase its the backfiring through the carb that blew the inlet apart .

Ive used this to repair the old one 

http://www.dortechdirect.co.uk/window-installer-tools/dow-corning-895-structural-glazing-silicone-sealant.html?gclid=Cj0KEQjwmLipBRC59O_EqJ_E0asBEiQATYdNh474w5pFpBVA_7e1I7lFY7ao1gI1jBXdY1xEDXw4ggUaAg9a8P8HAQ

It's a bit like the sealant they use to bond car windscreens. 

I'll refit the repaired inlet tomorrow and see how it runs, fingers crossed 

 

 

Fitted the repaired inlet rubber and it ran just the same ! It does seem a good repair with that sealant i used but will still fit the new one when it arrives just to be on the safe side.

So at this point i was 100% out of ideas. Stripped the carb yet again just to see if id missed anything, blew everything out with the airline and refitted, guess what , it ran just the same!!!

I have the jets incase i decided to do the airbox mod (52.5 pilot and 140 main Mikuni) so i thought id fit the 52.5 just to see if it made any difference, and guess what, it now seems to run perfect with no hesitation and no backfiring through the carb  . I can't believe that it's made such a difference, but i wonder if it will run a bit rich ?

The pilot screw also seems to make more of a difference when wound in or out compared to before.  

 

-- Edited by locky on Thursday 16th of April 2015 07:07:56 PM

I don't reckon it will run rich locky, just check the plug after a bit of a rag around

Fitted the new rubber inlet tonight and tested how it starts with and without choke from cold. It still needs choke so thats a good sign. Ill take it for a ride tomorrow to see how it runs but im expecting it to be fine.

Since my last post ive not been out on it , just been to busy with work and other stuff. When i go out on it next ill post my thoughts.

That's good that that niggling issue has gone now huh, look forward to more pics on the next outing locky, come back sun!

 

I'm guessing the plug is looking a nice tan colour? Not too black...

 

-- Edited by peteBLUEttr on Saturday 25th of April 2015 07:07:38 PM

If your going to stick an exhaust on at some point it might even work without touching anything. As to the reason why your bike was running lean, I'm not really sure, you still got the restrictor in the back of the header?