White Bros used to do a 36 mm Mikuni carb for the DR400
I have the carb body and most of the internals brand new. What I've lost is the jetting specs and White Bros has been sold. However there are jetting specs for the XT 500 for this carb to be found on the internet and they may not be too different. For this carb to work I reckon you would have to have a fairly modded motor or it would be likely to be slower [on tight tracks at least].
This carb was used in conjunction with a race cam, ported head [intake weir ground off], 420cc high comp piston and a megaphone!
Past tuning attempts with the XT in standard capacity found the 36 to be too much carb, with power down virtually everywhere against the standard late model 34mm. Apparently the late model XT OEM34 produces good power but is difficult to rebuild, thus most people stick on the aftermarket 36 which is a very very simple carb. The aftermarket 34 has been said to not run cleanly for some reason, not for the XT at any rate.
Now the standard DR400 comes with a 33mm carb, has a bore 1mm more than an XT, but a shorter stroke.
However the jet needle on the 400 is a single taper and as soon as I did any more than take off the intake snorkel inside the airbox the jetting went haywire , or in standard english Jens it went lean to rich or the other way round all on the jet needle. Idle was close and so were main jet and slide cutaway.
The single taper needle profile stopped working when I opened up the airbox intake.
I had a spare SP370 carb so a I put that on and it was waaaaaay faster. So I dropped the needle one position, took the main jet down two sizes and later put in the idle jet in from the DR400 carb, and it carburates very well.
With hindsight I should have upped the main jet back to standard after I put the two size smaller idle jet in, but by the time I found this out it appeared to be running happily still, so I've left it alone. I now have bought two more 370 carbs on UK ebay, as the 370 has a two taper needle which was just made for the 370; so they come with the carb or not at all!
The standard jetted 370 carb also works fine [ just down a size on the main jet] with just a K&N airfilter and no airbox on a bog standard motor. I ran this combo in a DR400/Maico sidecar combination for several 1000 miles 15 years back. What I haven't done yet is switch the needle/needle jet from a 370 carb to the 1mm bigger DR carb to see if that works. The carbs use different diameter slides so it is not 100% that it will.
As the DR400 was only sold in the USA for one year there is almost no internet knowledge to be had from the USA.
I've trawled and trawled the Internet seas but never found any helpful engine tuning info.
