So what do you do with TTR250 with poor carburation and a serious flat spot when its as it came from the factory ... I mean seriously bog-standard! 

1. Change the pilot? ... nope, waste of time with a slight improvement in performance but still flat!
2. Adjust mixture? ... nope, absolutely no difference (which on its own tells me there is something wrong).
3. Fit smaller main, needle and pilot jets? ... nope, that's ridiculous. We all know that as a rule of thumb flat spots at bottom end imply weak mixture and you certainly don't want run the risk of running weaker still and overheating, do we? ... or do we??? Well that is exactly what I did!

How did I come to this conclusion? Simple; Everyone I spoke that had anything to do with the later UK models (generally the blue models) had experienced similar problems; excessively hot exhaust, to the extent it glowed in many cases, Major flat spot and general poor performance. On the other hand, the earlier models were known for great performance with plucky engines. The simple answer; get your hands on an older model carb and compare the two. I poised over eBay and as soon as one popped up I pounced.

Having got my hands on the carb I put the later and older on the bench and stripped them to their component parts (taking care not to mix them up). Here is what I discovered;
Carb body - No difference
Slide - no difference
Needle - no difference
Needle setting - no difference
Choke - no difference
Float - different height setting
Needle jet - longer on the hex on later model
Main jet - old; 145 newer; 147
Pilot -old; 48, newer; 50
In addition, there was a plastic shroud around the jets on the later model.

I tried running the bike with the old carb as it arrived, knowing that it had been taken from a good runner. Ran as sweet as a nut.

I then removed the jets from the old carb and put them in the 04 carb, took out the plastic baffle, swapped the floats over and stuck it on the bike. Imagine my despair when the bike ran like a tin of nails. Stripped the carb again and found the needle washer had slipped out of its locator slot. Threw it back on and BINGO; no flat spot, no overheating exhaust and much more peaky and smoother running. I took the bike out for about 30 miles to test it out properly and its a different bike all together. Running like it should with no signs of excessive heat.

REMEMBER - you need to change the needle jet and adjust the float level to gain the improvement!

Please bear in mind that I'm not suggesting everyone should rush out and do this and I am only providing a guide for possible solutions. Only consider this if you are familiar with carbs and their settings or have a learned to person look at it for you. I can't accept responsibility for any mods that others make to their bikes.

These are the bits out of the later/04 carb.


Mike Ashton - a great free forum to discuss trail bikes and trail bike rallies. It would be good to see some more TTR250s in the great UK Rally events!