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

JCW · Veteran Member

Carb adjustment question

PlumasDude · Senior Member

I have trouble telling if a 4 stroke is too lean. If its extremely lean I've noticed that it can "bog down" more easily (have to down shift earlier), maybe. Rich is easier -- if its too rich you get little backfire pops when you cut the throttle, especially on a slight hill (extreme engine braking I don't think you notice the pops as much). So I try to get it to not pop, just barely. After a dusty day or two I'll get that little pop letting me know its time to clean the air filter. But I'm an ignoramus on this stuff, here's some more elevated discourse on the subject: ttr250.activeboard.com/t57613161/opinions-please-confused-jets-popping-exhaust/

Sorry if I'm repeating myself -- I have 2 bikes, one with the T4 and one stock, both with the airbox cut open. With the stock pipe I run one setting to the lean, with the T4 I'm at the middle setting, 2000 - 7000 feet elevation. I'd suggest testing it at the middle setting and move from there. I can't imagine you'd need to set the needle any richer unless you increase the size of your main jet (and then you can test each jet at each setting and spend your winter pulling that carb out over and over, being careful not to break the cheap arse pin ... -- I gave up and went back to stock).

JCW check out the Carb Stuff page on this forum, it may take a while to sort through but just about any question you could ask about jetting, needle, mix screw, etc. is answered in there somewhere, usually with good pictures. But remember most of the bikes on this forum are the UK or AU versions, which have slightly different exhaust and jetting (why? emissions? DB levels?).

BTW my wife says the loud pipe doesn't do anything but trigger testosterone and its all in my head.

katana · Senior Member

I drilled a 20mm hole on my stock exhaust and opened up the snorkel after cutting it shorter. To get it running well I had to drop my needle - leaner. The only trouble now, is that it bogs in 6th gear at max revs. Rich or lean I don't know.

JCW · Veteran Member

Thanks for the info guys. I took it out today after making the opening in the air box bigger and cutting 8 ea 3/8” extra holes in the stock baffle. That was a big help. It has a little more power, and much smoother acceleration past 50mph. The jetting feels great. My only issue is a few light backfire pops (when the choke is on and it is cold) in the carb that don’t even make it out the pipe. You can feel it if you are touching the choke, but the sound is very quiet. It stops after it warms up. I think I will leave it alone for now.

JCW · Veteran Member

After a few weeks of the above modifications, my bike runs great...with one exception. It cranks very quickly, but seems to take 4-5 minutes to really warm up. During that time, I get the little carb backfires that don't seem to make it out of the exhaust pipe with any noticeable sound. However, you can feel the boot between the carb and motor expand lightly each time. After you get it out on the road and open it up, it is perfect from that point on...no more little carb backfires. I see where others have faced this issue. Any suggestions on how to cure this? Thanks!!

ttr steve · Super Guru

That is a massive bit of rubber glued to the metal innards. Maybe it has become unglued from the metal component and letting in a little air giving you the running problems and also allowing the rubber part to expand a little, not sure how else it would do this.

Hope that helps

Steve

ttr steve · Super Guru

Forgot to add, the only way to know that for sure is to un bolt it and inspect wink

TTRfan · Super Guru

When the inlet stub "goes bad" - very bad in this case! The rubber part that attaches to the carb should be firmly fixed to the aluminium part that is bolted to the head!

Inlet stub fallen apart.jpg

JCW · Veteran Member

I can certainly take it apart, but if it was a rubber issue, wouldn't it do that even more after it's warmed up? Obviously, the choke is on when it is first started, and I push the choke in about 50-75% if the way after it runs about 10 seconds. But, it still does the little back fires until I get it out, run some higher rpms, and warm it up. I will inspect the rubber intake boot while it happens, and take it apart as well, but I think it is fuel related. Thanks for the replies!!!

ttr steve · Super Guru

Cheers Bri

Always nice to know you or Martyn are around for a photo call biggrin biggrin

JCW · Veteran Member

Awesome support. I will just order a new boot and replace it when I break it down. I will reply with the results.

JCW · Veteran Member

JCW · Veteran Member

I took it apart today. The problem was that the boot was not fully pressed against the cylinder. While the outside edges of the boot had broken where it slips over onto the cylinder, the boot was not cut and the surface area was good. I reinstalled it with heat proof sealant and made sure it was fully slid on against the cylinder (w/heat resistant silicone). That appears to have resolved the problem. I will repost if any issues arise. Thanks again everyone for your help!

ttr steve · Super Guru

Good luck with the repair, not heard of anyone doing that before and a good idea, if it works.

JCW · Veteran Member

It worked great! No more little backfires. You have to make a real special effort to get the intake boot fully onto the cylinder when putting the carb back on. You can’t see up in there and it would be really easy to not get it on fully....leading to those little annoying backfires.

TTRfan · Super Guru

I am sure you did a better job of the repair than the owner who did this one wink

Carb inlet stub repaired 1.jpg

Carb inlet stub repaired 2.jpg

JCW · Veteran Member

He had the right idea. This really is a design flaw that most owners learn the hard way like I did. Yamaha should have locked down this boot to cylinder connection point. Thanks!