Thunderace stuff here….


Some thunderace articles:
Ace_Cat Advert
scrapheap challenge 2003
Yamaha thunderace to R1 article
Thunderace and thundercat owners forum

Mounting LM-2 wideband lambda sensor(o2 sensor) to thunderace.

the exup has been removed, so I made a plate that has lambda thread to the exup mounting hole.

The engine has had problem with hesitation at high rpm. it first had Arrow muffler and 133.5 main jets + K&N filter. It just never run right. With exup or without it. I started to go back to find a working baseline, I installed 125 main jets, to all, even normally main jets are staggered, 125 and 127,5. so the center cylinders have different jets. Also main air jets are different 45/60. Some say it’s because center cylinders need richer mixture, but usually that is not the case with water cooled engines, I think the best explanation is that airbox is limiting airflow to some cylinders and that needs to be fine tuned with jetting differencies.
But without wideband lambda it’s very hard to decide is the hesitation rich or lean. Or it could be some other issue with the ignition or something else.

After installing LM-2 it became immediately clear that the full throttle is rich, it’s close to 1:11 mixture, and that causes the hesitation. I also noticed that 3000rpm area is little lean and it causes some running issues in roundabouts and cross roads(this is probably because there is no exup). (this was tested with stock muffler and stock paper airfilter, 125 main jets)

I then tried ARROW muffler to determine if the race muffler would lean out the full throttle area so it would work with 125 jets. And it worked, below 5000 the bike is lean with arrow. 1:15 and sometimes worse, so needle has to be raised or exup installed back. But at full throttle it goes very well, mixture is still 1:12 or so, so it’s on the rich side, but works without hesitation.

I think the reason for the fact that the bike cannot handle stock size main jets with stock muffler is because the exhaust collector is longer then normally, it has been welded longer. This could improve the 3000-5000 area, and cause the lean situation there, but at high revs it cannot handle the 125 jets anymore. I’m going to try the 122.5 jets next with stock muffler and raise the needles.

EDIT 05-2018: With 122.5 main jets it works great and fuel mixture is very good. Done couple of trackdays and the it’s goes very well with full rev range. The 3500 lean area was because of missing exup valve, this are is very narrow, so it’s very hard to fix with jetting. So i reinstalled the exup valve and now the lean spot is fixed and it’s much easier to ride as the 3500 area is very common in normal traffic situations. Lesson for the day. Keep the exup. Don’t jet it up unless you have some major modifications that would need it.



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