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

Maxim_X · Member

I can't get the jetting right. Or something else?

TT-R250M · Super Guru

First of all I would suggest that you familiarize yourself with the TTR250 carburettor. A blocked air jet or passageway could cause all sorts of dramas. It is most important that the carb is spotless and is functioning correctly before re-jetting!

Make sure that the accelerator pump is working correctly, the jets are clean (including the air jets) etc. To assist with this see here- http://ttr250.activeboard.com/t55374798/rebuilding-a-ttr250-teikei-y30p-carburettor/

IMO- cutting the airbox snorkel is the worst mod anyone ever came up with. All it seems to do is make it run rich, so rich in fact that full throttle was hardly reachable without a lot of splutter.It is better as is, removing it completely or modifying it as I have done. See here- http://ttr250.activeboard.com/t53318498/air-box-snorkel-modification/

Things to try:

Try pulling the airbox snorkel/air duct out from the airbox. In my experience it makes it run too rich and performs badly.

Take the air filter side case off and go for a quick ride to see if it's any better, if it is, it's running rich.

Questions:

Are you using Mikuni jets or Yamaha/Teikei?

What other symptoms does it have and when do they happen i.e. idle, 1/3 throttle, from 1/3 to 1/2 or full throttle?

The throttle stop should not be removed, nor does it need to be removed. All that is needed is adjustment. If it is removed you run the risk of damaging the slide from over extending it.

Jarrah

ttboof · Super Guru

agree with jarrah make sure the carby is clean .

I've left the main jet standard and went up one on the pilot

As for the surging I'd check for air leaks between the carby and head and maybe the float level

And put the throttle stop back in and adjust it correctly

Maxim_X · Member

Mikuni jets

It feels fine until 1/2 - WOt especially when the motor is in lower RPMs. Running all the way through the revs isn't bad.

The stop isn't removed, it's re-aligned to allow true WOT.

I'll try removing the snorkel when I get a chance. Work has been crazy lateley so I don't know when I will get to ride next. But, from that thread it sounds like it's my exact problem. You guys are the best!!!!

TT-R250M · Super Guru

Maxim_X wrote:
Mikuni jets

That is your problem...

If using Mikuni jets you have to remember that they do not have the same flow or taper as Yamaha/Teikei. It has been suggested that Mikuni jets are roughly ten sizes larger than Yamaha/Teikei. That said, you are now about 13 sizes too large on your main jet and 12.5 sizes out on pilot.

Unless you fit an aftermarket exhaust/air filter there should be no need to enlarge the main jet anyway and at the most the jet needle clip position may need to be adjusted towards the sharp end 1~2 settings. However, the pilot can be upgraded to a Yamaha/Teikei #52.5. This would in theory be a #43 Mikuni pilot jet.

I suggest that you try either a Mikuni #42.5 or #43 pilot jet with a #128 main jet, use either a #42.5 or a Mikuni #43 pilot jet , or stick to the original jetting and adjust the jet needle clip position until you want to upgrade the exhaust, air filter etc.

The ''cutting the snorkel mod'' is not recommended IMO and could be contributing to the rich condition. I would suggest to remove it and find a suitable cold air induction pipe to make the snorkel larger (or similar).

Jarrah

Maxim_X · Member

I don't want to argue with you. I am sure your right about it, I don't know where I got the info to buy these specific jets (maybe tumpertalk?) I just went off what google said. But the problem existed before any jetting change and is almost no different since them. I changed them because I thought it was a lean condition, but it turns out to be the opposite. I will try removing the snorkle and if that doesn't fix it I will replace the main with the stocker to see if that does it.

TT-R250M · Super Guru

Not arguing, just explaining so I hope you don't think I sound like a nagging wife wink

As mentioned, check the carburettor inlet stub for cracks also.

I don't want to swamp you with too much info at once so that will do for now. wink

Let me know how you go.

Jarrah

ttr250slw · Veteran Member

check your bowl height mine was off and i had massive flat spots and flooding issues and it took me ages to find the problem but it is very easy to check :)

there are instructions on how to measure the float height in the service manual.

Maxim_X · Member

Well Finaly got around to ripping the snorkel out. That was it. Night and day. The rideability is perfect now and it will do 70mph! I'm super pumped to start using the bike to commute. Thanks for all the help!

ttboof · Super Guru

i found just thing to open up the air box at the local hardware shop

its a pvc gutter fitting .that has a convenient flange to pop rivet it on works well