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
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
different height setting
Needle jet - longer
on the hex on later model
Main jet -
old; 145 newer;
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.
http://trailbikeandrally.freeforums.org/ - 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!