1979 YZ400

Jul 5, 2000
My brother has a 79 YZ400 he is going to work on, but the guy wants him to DEtune it so he can ride it.

We have't even rode it yet, but I thought I might get some sugestions.

Thanks Kevin
DEtune it?
Why would he want to do that?
How old is your brother?
If he is younger (under 16) the big YZ may be too much for him!
There isnt much i can think of to detune a bike.
Maybe some kind of throttle limiter....
I put a set of Boyesen 'power reeds' on mine to give it a bit more bottom end, and reduce the top end hit. (seemed to make it more user friendly)
But overall that gave it more power!

I've heard though that a YZ250 motor will drop right into the 400's frame though.....

PS: He doesnt happen to have a spare piston and rod laying around there,does he???
Jul 5, 2000
My brother does not own the bike, he is going to work on it.

The guy that owns it is over 60, he doesn't like the power delivery (hit).

dirt bike dave

Sponsoring Member
May 3, 2000
I think there was an IT400 that year - maybe parts from that (carb, cylinder, pipe, silencer, flywheel) could be found cheap.

Also you could add a tooth to the countershaft sprocket and a weld on flywheel weight.


Damn Yankees
Nov 21, 2000
Go to a junk yard and see if you can find a smaller carb with the same basic outside dimensions. Jetting shouldn't take too long to figure out. Wonder if there is a way to have a thicker base gasket. Does stuff from an IT bolt on? Anyone know Yamaha's enough to know? A heavier fly wheel would really help. Maybe an IT has one. Can always retard the timing a bit also. Putting a larger rear sprocket to gear it down will make it snappier but easier to ride in the tight stuff.

If you are in the Colorado area there are two junk yards with a LOT of that vintage YZ Yamaha stuff. One in Loveland and one in Longmont. Guess they were popular here.

May 24, 2007
keip82 said:
those yz490's run they r really fast.
He said it was a YZ400...not 490.

But Anyways, is an IT400 less powerful than a YZ400? My 79 IT400 has a lot of power, if that is the case, I'd hate to see what the YZ has. My IT has YZ parts on it, so I'm sure some parts are interchangeable.
Jan 3, 2007
I say put on a IT400 flywheel, pipe, and cyclinder. This will be sure to detunne your ride.

drgriffi1, My uncle also has a IT400 but it is a 77. It can pull triple diget numbers if its tunned right. I love the bike because its a mountain goat with the looks of sasquatch.

May 19, 2006
On the 400s, Carb is different. Exhaust is different. Piston is different (I would have thought cylinder porting would be different and piston the same, but here, piston is different and cylinders are the same). At least, accoding to parts listings for 1979. Later on, with the 490 series, it appears the exhaust, cylinders and carbs were different on the IT490s and YZ490s.