1976 IT400 Yamaha Drag Bike

[dennis]

I stumbled on this forum while looking for tuning information on my IT400 Yamaha Drag Bike.

The engine is a bone stock IT400.  I got the bike for $160 and it had less than 1000 km on it.  It had been left oustide for years so the seat was rotten, the carb and petcock full of goo, and the plastic all faded.  I fixed it up and it ran great as a trail bike ... gobs of low end torque.  However, trail riding is not my thing and I always wanted to build a drag bike based on a two stroke dirt bike, because I had seen some at the strip over the years cracking off really good times.

So far I have only modified the engine by making my own expansion chamber from a snowmobile.  It is not performing well as a drag bike.  I am looking forward to lots of mods to up it's performance in the quarter mile.







It has only been to the track once.  It kept starving out after the 1/8th mile.  I had a tiny petcock and a tiny fuel filter.  I have changed those since.  I have been researching all kinds of two stroke tuning tips.  I would like to find a top end from a 1976 YZ400 which had more high end.  If I can't then I will likely get out the dremmel and started carving on the ports.  I have a wad of jets to play with.  Then there are reeds, squish shapes and tuning of the expansion chamber... and sprockets.  I have 15/45 but I think I can get 18/40.



I'm looking forward to learning as I read through a zillion threads on this forum.

Peace & Grease, Dennis Smith
Prince George, British Columbia, Canada

[twosmoke595]

WOW! way cool project!! please keep us updated on it, that is as cool as it gets right there! a shame you had to hack up a old bike, but from the sound of it, there wasn't much left to use anyway. good use and resurrecting an old monster 

[SachsGS]

I suspect you may need a much larger air filter. I've had 2Ts starve for air due to problems with the air filter/air box design. As well, that IT 400 is running pretty wide gearbox ratios,not sure how suited they are for the 1/4 mile. I'm not entirely positive but I think a YZ 490 motor should bolt in and that will have you running 12's.

 Cool project and love to see some video's of that air hammer in action!

[_X_]

next mod a cr 500 motor. that is all.

[TMKIWI]

Thats a cool looking bike right there.

[dennis]

Thanks for the comments.  I actually wanted to do this with the largest air-cooled two stroke I could find and really wanted a YZ490.  However, the donor bike came my way at such a good price, I just decided to go for it. I built the frame and motormounts, so if a big 490 or CR500 came along for the right price, I'd probably swap it in.

The build pictures are mostly on a buddies photobucket so I can't graft them in here. However here is a link to our club forum with the build.
http://www.princegeorgevintagemcclub.org/forum/index.php?topic=717.0

I actually have one video of it running.  Pretty short.  It was made the night we fired it up for the first time.


Its the end of summer up here in Canada, and there is only one more drag night.  I don't think I will be able to do much fine tuning after that until next year.  That should give me a long winter to port, polish, squish, jet, time and tune my exhuast.  And play with sprockets.

Peace & Grease, Dennis Smith
Prince George, British Columbia

[_X_]

good luck definitely a cool bike.

[citabjockey]

That is an interesting project you have there.

You might try to get an account on www.yamahaenduro.com -- there are some guys there that have tuned these engines plenty.  Right now there is a DT175 based salt flat bike project ongoing:


With respect to the enduro vs MX tranny, I think for drag racing you are better off with the wide ratio. With the close cogs you will either run out of RPMS at the trap or be slipping the clutch like mad off the line and have a tough time launching. That 400 should pull the wide ratio's just fine with accurate shifting and some mods to give it a bit more rev out.

Between carb mods (bigger?), pipes and porting you should be able to make that IT into a real screamer. One thing -- the pipe must be closely matched to the port timing of the engine. That pipe you have on there probably is less of a match than the stocker and might actually perform worse? That said, if you have detailed pipe measurements and it is setup for appropriate peak HP rpms AND the ports can be made to match it you may have a win on your hands. But this stuff is really complex -- more that you would think.

Do keep us posted! More pics and video!