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

This week I have mostly been fettling the spare carbs I put back for projects that I am not now going to do!

The first two just needed cleaning up and testing and they both worked fine. 

The first is on 147-50 jetting and the second on 137-50. These jets were what were already fitted so I didn't need to pull the nozzles to check their numbers.

The third carb was a bit of a puzzle as I had obviously robbed it of its nozzle and jets back along 

I had a spare V95GO nozzle which is the one fitted to the blue model TTRs so assumed the jetting would be the same as on the UK blue models i.e. 147-50

However, when I fitted the rebuilt carb to my TTR it only ran properly at low revs. As soon as I gave it some throttle it just bogged down and died. I abandoned my test ride to the Post Office and did something else whilst I thought of what best to do and why it ran so badly.

I had removed the carb before I remembered the simple test I advise everyone else which is to remove the airbox cover and see if it ran better 

Anyways up, I decided to try the simple remedy of putting in smaller jets. I put in 137-48 jets, replaced the carb and took it for a test run. What a difference! The TTR accelerated well with good top end. However, it backfired a little and didn't run well at part throttle - it felt "lean".

So off with the carb again (getting quite quick at doing this now!) and put the 50 pilot jet back in. Another test ride showed the problem was solved and the TTR ticks over nicely and pulls cleanly through the rev range.

I confess that I didn't think such a small change in the pilot jet (going up from 48 to 50) would make such a difference.

Lessons learnt. Every day's a school day here 

I would love to know what goes on with TTR carbs and why all the different combinations of nozzles and jets for what seems to be the same carb body. And, before anyone asks, the part number for the needle is the same for all model carbs!   

Looking through the Yamaha fiche, the part number for the whole carb assembly for the blue models is 4GY-14301-24 and for the OE and Raid white metal-tanked models 4GY-14301-00. I have no idea what the difference is but clearly the smaller 137 or 142 main jets work well in the older carb bodies and the 147 is needed for the later carb body.

Brian