I have a DR400 in the UK that I've used for rallies
It's got a 32mm SP370 carb as I couldn't get the jetting remotely close with the 33mm DR 400 slide carb. If I swap the needle/needle jet it may work. The 370 needle was made just for that motor and is no longer available, though UK Mikuni specialists say there are needles fairly close that might work with the needle set in a different position. The standard carb works ok with just a free flowing exhaust but once you cut away the snorkel base on the underside of the airbox trumpet it all goes haywire. [ it works ok with just the snorkel removed]. The 370 carb, though only 1 mm smaller uses a narrower slide. The 370 carb produces noticeably more power everywhere and only needs the needle dropped one notch and the main jet dropped one or two sizes. These motors [standard internals or not] always seem to need the jetting leaned out on the mikuni slide carbs when running less restrictive exhaust and/ or inlet. I run the DR 400 idle jet in the 370 carb. The 400's is two sizes leaner which the derestricted engine needs
I have the intake weir ground off [a la Dirt Bike mag] and run a Protec copy street and track cam built by Phil Joy. I've had that cam for well on 25 years. It fills the mid range hole that the standard cam has, but you need to use aftermarket springs or the followers take off coming off the lobe. If I were just trail riding I'd be inclined to leave the intake weir as is as I think it would be less inclined to occasionally die when dropping back to idle and would have a touch more oomph straight of idle.
I use Protec aluminium keepers and Ford Cortina springs [there are a few different types] These were the same spring rate as Protecs but not made out of knicker elastic. Mega Cycle have cams and spring kits available
As for suspension I'm running 1981 38mm RM125 forks and yokes. This was a mistake as I've had to change the wheel and I'm running the TLS RM500 1984 brake. This uses the same hub as the 1981 RM.
It's not good and I'm going to get it skimmed and relined using forklift track braking material. By the way this a brand new hub and brake plate and it's still not a vast improvement on the old DR brake
The 1980 forks have a smaller wheel spindle. With hindsight I would have waited for a 1980 fork to turn up.
Prior to that I had PE 175 36mm forks which weren't stiff enough, the front end wandered at speed.
The rear shocks are 16.5 eye to eye to get the head angle back in sync, That,s as long as you can go using the DR swingarm
I've got Falcons, 4.5 inch stroke single spring slightly wider body but not piggyback They work a lot better than Hagons
I've used the PE175 swing arm which fits everywhere except the swing arm pivot. You need reducer washers to allow for the smaller spindle. I think the PE 250/400 ally swing arm has a bigger pivot. These swingarms are maybe a couple of cm longer which seems to help. A friend of mine said the DR was known to be a bit light on the front end but he's big and has to sit well back on a bike. I haven't had a problem but I'm skinny. However the shock placement is further forwad so one needs a shorter shock to boot and that means it's working harder. so I stuck the steel DR swingarm back on/ Maybe a PE 250 swingarm with the shock bolt placed a la the DR would be the best ticket.
It's interesting to hear that there isn't much weight difference between a RM and DR frame. Previously people have said that the DR frame was overbuilt. The other thing is that there not much difference between a DR 400 engine and a XT 500 in weight either!
White Bros used to have a 420cc high compression piston, a Wiseco I think. Any more than that and the tuners had engine problems, though they didn't specify what. The clutch plates spring don't seem to last long, though on measuring there seems virtually no wear on the friction plates so maybe it's more the springs, which do sack. Nobody currently seems to have found a piston which works. Increasing capacity has come from stroking the engine. The first test of the DR 500 found no increase in power over the 400, though they did place well in the Paris Dakar, so they must be tough, again Mega Cycle have stuff, and a lot of the running gear is the same as the 400.
The worst thing about the bike is the rear wheel and cush drive. The bearings don't last long in mud and it's fiddly to refit after a puncture. I'm tempted to sling the lot and convert to a rear disc
