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

TT-R250M · Super Guru

A story of a jetting...

pekkal · Member

Hello all,

I have a 2001 TTR (USA model) with the following specs:

Piston replaced ~40 hours ago.
13-52 gearing.
A big hole in the airbox cover (by PO).
Standard header pipe with extra long (about 50cm) tailpipe with straight through silencer (Made by PO). The silencer has a smaller ID than the tail pipe. (this might actually be part of the broblem described below...)
Lightly oiled Twinair filter
Mitas C16 rear tire with studs (0.5bar) and Duro front tire with ice-racing spikes (0.8bar).
About -5 to -10 degrees celsius.
Riding on ice track (the only good thing about winter in Finland...).

I used to have the standard USA jets (50 and 137). On an ice track and about 500 meter straight the bike got to about 100km/h and felt really lazy.

I put in Mikuni 52.5 pilot and 142.5 main. The bike would not run properly. Way too rich. After removing airbox cover the bike ran OK in low gears but sputtered and run badly in high gears and would only go about 95km/h.

Tried a Mikuni 140 main. Allmost as bad as 142.5.

Put in Mikuni 137.5 (a fair bit bigger than Yamaha 137). Removed airbox cover. Now the bike feels that it runs and accelerates a bit better than with stock jets. Seems to pull even with 6th gear. However it still won't go much faster than 100km/h on the 500meter straight.

The acceleration is ok to about 85km/h and from there its just slow. I know the TTR is not the fastest bike on the track but it just feels really slow. My friends are riding 250 4-stroke MX bikes and when they pass me I feel like I'm standing still...

Does this sound right to you guys? Would a smaller rear sprocket be a good idea? Does it have enough torque to keep the acceleration? Maybe the small silencer is holding the engine back?

Thanks,

Pekka

pekkal · Member

Thank for the answer!

First three points: check.

If I snap the throttle open really fast then it bogs but anything slower and it rews up real nice. Mixture screw is open about 1.5 turns.

No boggin after 1/4 throttle. I think the main jet is close to optimum now.

Leaner needle position is something to try. I have tried to avoid it because it seems like a lot of work to get the slide out... :)

Good info on the sprockets. I'm also starting to think that 100km/h is actually pretty good with this gearing and the studded tires. I'll try to find a smaller rear and see what happens.

I'll check the plug next time I'm on the track. The silencer is also on the "to do" list.

Thanks,

Pekka

TT-R250M · Super Guru

No problems

If it is bogging down at 1/4 throttle i would suggest to go down in pilot jet size. Making the needle leaner won't change it all that much if the main jet is already good.

I thought my main jet was good at the third setting until i took it to the leanest setting & now it revs out hard & no flat spots.

I'd say most of it is caused by a lot of air being able to get in but cannot escape quick enough due to the silencer but try going leaner on the needle position & see what happens.

It is NOT hard to remove the needle & can be done without too much trouble. See here for more info on this.

TT-R250M · Super Guru

pekkal wrote:
The saga continues....

The bike ran OK with the current exhaust setup and 52 137.5 jets. Best so far. At WOT it seemed to rev rather high without "sputtering" and pulled rather good.

So then I decided to improve it a bit more. I opened up the exhaust. Now it has no internal baffles. Just a 35mm straight through perforated tube with about 28mm exit.

The I tried it with 137.5. Ran like ****. Did not rev high. Spit and sputter. Sort of random hesitation. Did a run with WOT and checked the plug. Light gray all over. Tried with 140 main. Just as bad.

Need to try with 142.5 but that has to wait for couple of weeks. Incredibly frustrating. I not even sure if it's rich or lean.

Also, I noticed that I have the "large round" Mukini jets instead of "small round"... I have these: link but I ques I SHOULD have these. Wonder if that makes a lot of difference? They do fit and clear the bottom of the float bowl easily.
And a bigger front sprocket is in the mail...
Thanks again,
Pekka

Sounds to me like you are running rich.

An easy way to tell if your running rich is to remove the air box side cover & see if it runs better. If it does your rich.

If you have to slowly wind the throttle on your rich.

If your plug is black or grey your rich.

..........................................................................................................

Now for lean symptoms....

If it's glowing red at the header pipe it's lean.

If it runs fine but doesn't have much power your lean.

If the bike stalls after some time riding & takes awhile before it starts again your lean.

You can also get detonation (pinging/knocking) if the engine gets too hot.

..........................................................................................................................................

What i would be doing is putting the 137.5 main jet in & move the clip position up the needle until it runs right.

Or put the standard #137 main jet back in & move the clip down until rich enough.

REMEMBER: Mikuni jets are MUCH larger sizes than Teikei sizes so in theory your running a Teikei #150 main jet when you have a Mikuni #140 main jet.

Not sure if the type of jet you are running is adding to this problem but it is likely given that it would not get as much air through the main. I can't see your current jets as the link didn't work but i'm guessing you have jets like THIS.

.............................................................

Different color plugs & there causes.

Black or grey means rich.

Brownish white means perfect.

White crust means lean.

See below for more info...

-- Edited by TT-R250M on Monday 18th of February 2013 01:56:28 AM

pekkal · Member

The saga continues....

The bike ran ok with the current exhaust setup and 52 137.5 jets. Best so far. At WOT it seemed to rev rather high without "sputtering" and pulled rather good.

So then I decided to improve it a bit more. I opened up the exhaust. Now it has no internal baffles. Just a 35mm straight through perforated tube with about 28mm exit.

The I tried it with 137.5. Ran like ****. Did not rev high. Spit and sputter. Sort of random hesitation. Did a run with WOT and checked the plug. Light gray all over. Tried with 140 main. Just as bad.

Need to try with 142.5 but that has to wait for couple of weeks. Incredibly frustrating. I not even sure if it's rich or lean.

Also, I noticed that I have the "large round" Mukini jets instead of "small round"... I have these: link but I quess I SHOULD have these. Wonder if that makes a lot of difference? They do fit and clear the bottom of the float bowl easily.

And a bigger front sprocket is in the mail...

Thanks again,

Pekka

pekkal · Member

Thanks for the reply Andy. I tried with #140 and no side cover but I think I did not warm up properly and went home cursing and swearing the bike to the lowest hell... I'll try it again and see how it goes.

I did read that topic that you linked but did not see that last post. It sounds exactly the what I've been going through! My airbox is cut already. About 5 x 10cm hole. I'll be drilling holes to the cover next I think ;)

Pekka

-- Edited by pekkal on Sunday 24th of February 2013 04:47:12 PM

pekkal · Member

Ok. More testing done. Mikuni #142.5 main. Air box cover off.

Tried air box with cover on and also tried to tape 1/2 of the air intake to the air box. These made the bike run worse. So I'm guessing it not running lean.

First it seemed that there was not much improvement over the #140 or #137.5. However, the bike seemed to run a little bit better than last time so I decided to do a few laps just to get my heart pumping. After couple of laps the bike started running a lot better. Still not perfect but close.

I think that last time I did not have the patience to warm up the bike properly. It seems to take at least 15 minutes of real riding to get the lump up to proper temperature in below freezing weather. I guess I'll put in the #140 and see how it goes from there.

Pekka

Andy_C · Veteran Member

You tried #140 with no air box side cover?

read this....http://ttr250.activeboard.com/t52609744/help-uncorking-the-ttr250/

I had a similar problem.

pekkal · Member

Ok. Now I've done a few laps with Mikuni #140 in about +5degC weather. First the airbox side open. Few warm up laps to get the lump to operating temperature...

Some sputtering and hesitation but not as much as with #142.5. Did rev rather high but had to roll the throttle open slowly.

Then I tried with the airbox cover on. Now the bike did not rev as high but had a lot more torque at low revs! And I mean a LOT! It actually was faster like that because I could now shift up to 6th gear instead of fifth at the end of the straight...

If I only could get it to rev higher with the airbox cover on. Ideas? Back to the #137.5 jet maybe?

Pekka

Andy_C · Veteran Member

With your mikuni #140 fitted and I assume the mikuni 52.5 where in the power band or throttle position did this hesitation or stutter come in, when you tested it with the airbox cover removed?

If it came in in the mid range band then it may be a case of setting the needle circlip into another groove, to make it richer.

I can only go by my findings that if you've fitted bigger jets you will need to increase the hole size in the top of the box where the snorkel fits. you could try it with the snorkel removed and see if thats enough otherwise you need to enlarge the hole.

From all the information I have read online the Mikuni #140 should work,plenty of people have done this mod and been successful therefore I can only assume your problems are air intake related. The #140 is as big as you should need,the yamaha jetting is #150 max and the mikuni equates to that size.

Test ride it more and diagnose where the troubles are in relation to the throttle position.

Andy

pekkal · Member

My best guess at the moment is that the #140 is a bit too big. My reasoning is this:

With no side cover the top end power was good but midrange to upper midrange was bad. Adjusting the needle might help. But I'd like to keep the side cover on in the future so no sense in tuning without the cover.

So, it seems the idle jet, needle, needle jet, slide, accelerator pump combo works good with the air box. Everything from idle to upper mid was good. This brings me to the conclusion that the main jet is too big. I cant make the hole on the box any bigger and do not want to punch holes to the side cover. So the "only" option is to try smaller main jet.

The ice road racing season will be over in couple of weeks. Then I'll be riding enduro trails mostly. Maybe I should just be happy with the increased low/mid power and forget the top end... ;)

Pekka

Andy_C · Veteran Member

HI pekkal

I wasn't suggesting you leave the side cover off all the time, I removed mine when I rejetted just for testing purposes so I could understand what was going on which lead me to discover my problem was lack of air at full throttle even though I had done the snorkel mod.

I'm sure you'll crack your problem with trial and error,as you suggest maybe the #137 is the correct jet and you may be able to tweak it by moving the needle, sorry I can't give you a diffinitive answer. Hope you sort it soon,its annoying when something not running right.

please post your findings when you got it done,I'd be interested in knowing.

Andy

pekkal · Member

Sorry for the delay. I settled for #140 Mikuni jet (actually and EBC "precision" version of Mikuni). I also drilled a few holes to the side cover (mistake maybe?). The bike now pulls ok in low and mid revs but does not rev up all the way. Opening the side cover a bit by losing one screw and putting a ~10mm wooden stick between the cover and the box helps a bit in the high revs but makes the low end worse.

I think the #140 is too big but for some reason the #137.5 Mikuni I have (an actual Mikuni not EBC version) is worse. Also the #142.5 (EBC) is also worse than the #140. Need to get the "small round" Mikunis one day and see if that helps. For now I'll just ride...

Pekka