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

Medic5916 · Member

Help Uncorking the TTR250

Medic5916 · Member

I know I have a 1993 or 1994 (my frame number is 4GY-002345) so I'm going to try to go with these parts:

Mikuni Pilot Jet 52.5- http://www.motorcyclecarbs.com/MIKUNI_PILOT_JET_VM28_486_52__P10654C28.cfm

Mikuni Main Jet 140- http://www.motorcyclecarbs.com/MIKUNI_MAIN_JET_N102_221_140_P2414C1443.cfm

T-4 Slip on Muffler (says its for a '99-'06 but hope it will fit my '93-'94): http://www.amazon.com/99-06-YAMAHA-TTR250-Circuit-Slip-On/dp/B000GVFCB0

If anyone thinks there is a better alternative for a '93-'94 ttr250 then let me know. I'll probably order the parts tomorrow and give it a shot.

TT-R250M · Super Guru

Hello Medic5916 & welcome

Regarding the uncorking of your TT-R250...

First of all you won't have any trouble with the parts that you mention as these are the same from 1993-2013 with the OE models. The Raid runs a different compression ratio so it's different (not sure about Raid fitment).

1.

The best muffler (in my opinion) would be a Pro circuit T4 or FmF 4Q. I choose the FmF 4Q & worth every cent i payed. The FmF 4Q slip-on muffler is very quiet until the throttle is twisted. It then becomes loud but not too loud for the ''oldies'' & has the sweetest note ever! aww aww aww aww With the spark arrestor removed with a GYT-R twin air filter it runs GREAT! aww aww aww aww Even with the spark arrestor it's a handful in the tight stuff. Maximum decibel rating for this exhuast is 96 decibels (not sure about the Pro Curcuit T-4)

Not to make the Pro Curcuit sound worst but i have heard that they are too loud,not enough power gain ect but don't take my word for this.

Link to Fmf 4Q slip-on muffler at FmF.com HERE

2.

The air-box is a controversial subject as some do things different. One idea is to cut a rung or two from the top of the air duct/snorkel but this may not suit you in you go through water as deep as two foot. Another one is to do the same to the inside of the air duct/snorkel. I did this on my old TTR & it did help a little. Another is to remove it completely(not recommended) but some say that removing it completely is no help? However, no mod has been made to my current TT-R250 air-box but i have plans to either ''mod-up'' a cold air induction pipe that runs from the front of the bike (yet to find out where this would fit). Or buy an aftermarket air box from Vortex (somehow). It is not a necessity in my case & it's goes off like a cut snake!

3.

Jetting is one of the most hardest things to judge as there are many factors involved. Some of the main factors are elevation,fuel quality,average temperature,humidity & then the biggest....JET SIZES. Most manufactures make their sizes at a different flow rate. For example #147 main jet made by Mikuni will be different to Yamaha's #147 main jet & same with Sigma. That said,what i bought was Sigma jets from ebay & asked for a #150 main with #52.5 pilot hoping the size difference was not too much. Luckily for me it turned out perfect but the Sigma #150 main is so large that i had to set the jet needle to the last setting (not pointy end) to make it perfect. Once i modify my air-box (in some way) i will be dropping it back down to en-richen the mixture so I'm happy with the sizes chosen.

I could not choose a size that will be perfect for your bike unless all factors are taken into account. I have never bought Mikuni jets (for a MC) so can't tell you what size to buy. However,#150 Sigma should go nicely if doing all mods mentioned,including opening up the air-box as i ported & polished my cylinder head which would defiantly change things.

Some people say that a Mikuni size #140 is the same as a Yamaha size #150 but i don't think that it would change ten sizes for the same flow rate. It pays to go richer on jet sizes & work your way down until it's too lean,then go up a size.

Link to Sigma jet kit on ebay HERE

REMEMBER: It is better to be running rich than lean,as lean can damage the engine.

Hope that helps & if you get stuck,I'll be here to help.

-- Edited by TT-R250M on Tuesday 12th of February 2013 09:28:49 AM

Andy_C · Veteran Member

Hi
Just finished the exact same upgrades to my '94' oe, these were taken from the "uncorking your TTR" post.

I ran into a few snags...

1/ I used a FMF powercore4 silencer, first findings was it was very loud,too loud for me riding and other people in the world.
overcame this by buying a baffle from ebay which was the same diameter as the inner pipe of the silencer so it sat nicely inside and then I purchased some repacking material from "silent sport".
Took all the old exhaust wadding out and repacked it,the result was much quieter but still had a good through flow for exhaust gas.
I may make some more mods on this just to quiet it down a touch more.

2/ With the better breathing exhaust and the mod to the air intake snorkel and the jets all fitted the bike started great,ran great from lower throttle to the upper range with no problems but on wide open throttle it hesitated like a pig!
I thought WOT (wide open throttle) ahh that must be the main jet,got more main jets sized either side of the mikuni #140,fitted and test rode them with no improvement.
I spent many hours trying different jets,moving the carb needle up and down in the grooves to make it richer or weaker,I could now strip that damn carb down with my eyes shut! All had no improvement on the problem.

Scratched my head for a while,refitted the #140 jet and took the air box side cover off and went for a test ride.
Result! very noticable improvement in performance! All the time it was lack of air into the carb. obviously the air snorkel mode of cutting 2 rungs on top and 1 below didn't allow enough air in when at WOT. I think it was because of the powercore exhaust having a lack of restriction the exhaust gas flowed better but the air box mod just could'nt cope with the WOT when air flow would be at its greatest.
I've still got to do a proper mode on the air box but its my intention to enlarge the hole in the top of the air box and make my own snorkel type of thing for it to reduce water entering.

Hope some of this may help you.
Andy

-- Edited by Andy_C on Sunday 24th of February 2013 03:15:14 PM

davecambo · Senior Member

hey andy C, I took out the snorkel, enlarged the hole, and then ran a bead of silicone around the hole on top, then another on top of that. it turned out perfect, it acts as sort of a rim that is higher than one of my fingers if i put it next to it, and i have removed the seat after doing creek crossings etc and if any water did get under the seat, it cant get over the silicone rim.

verticaltwin · Member

davecambo wrote:
hey andy C, I took out the snorkel, enlarged the hole, and then ran a bead of silicone around the hole on top, then another on top of that. it turned out perfect, it acts as sort of a rim that is higher than one of my fingers if i put it next to it, and i have removed the seat after doing creek crossings etc and if any water did get under the seat, it cant get over the silicone rim.

Thanks for that idea... I have a Pro-Circuit T4 slip on muffler and the bike runs great. The air box has yet to be modified, so I know I can get more power out of it, but was hesitant to remove the snorkel due to water and mud here in Kansas. Been trying to think of a good way to open it up and still try to keep the water out.

Right now the bike runs great and is geared good for Kansas riding... Lots of clay/mud, twists and turns, ruts and roots, etc. But may be moving to SE Colorado or Arizona later this year (if all works out) and then will definitely want to add a tooth to the front sprocket and open up the intake.

-- Edited by verticaltwin on Tuesday 26th of February 2013 02:15:26 PM

Andy_C · Veteran Member

Thanks for that Davecombo thats a innovative solution....Thats well worth considering and would work with the exact shape which is cut to enlarge the airhole. Out of interest dave how large did you increase the hole size?

I was thinking of cutting a more rectangular hole keeping a 15mm margin all the way round the edges of the top of the airbox,where the original snorkel sits.

Cheers Andy

davecambo · Senior Member

that is about how big i cut mine funny enough.