DR/GN/SP400/ 370

[cloggy]

Has anyone else done any tuning on these motors, or knows of any source material?

[Jez F]

Hi, I bought a GN400 lump( v.cheap) with the intention of dropping it into a Morini frame. As they only kick out 27 HP from new I decided to investigate a bit and see what could be done. There is what appears to be some sort of dam in the inlet throat, possibly to limit power for various markets, so out with the dremel and it soon disappeared! The state of the inlet and exhaust ports can best be described as 'rough as a badgers a***' so a bit of smoothing out would help. I was then going to put a 38mm dellorto on it and blend in the inlet, but a DR750 bargain (ha ha ha) came along so thats as far as I got so the 750's going into the Morini instead and the GN is in a box under the bench. Standard compression is quite low so there's room for improvement there, did think of resleeving the barrel out to 90 or 91 but I doubt if you could go much bigger as you would break into the cam-chain tunnel , try Wiseco or Omega for pistons and Joy engineering for cams. A mate sent me a picture of a racer with DR500 head (4-valve), bore is the same but longer stroke, best of luck,
cheers Jez

[guest295]

It's a Suzuki dirtbike lump so there should be heaps of stuff for it. Keep in mind, though, that the starter gear will not be happy with any significant increase in compression.

[cloggy]

They stopped selling them in the very early 80s. There's precisely nothing available of the shelf.
Phil Joy does a very nice Protec copy road and track cam grind, you have to use aftermarket springs and keepers. I use Protec keepers and kent clubmans springs for the 1600 cortina lump. This cam eliminates a  hole in mid range
MegaCycle do a couple of cams and springs. With the exchange rate that's peanuts at present. One does indeed grind off the inlet weir, to very little detriment straight off idle. They bored them out to 420cc max. Any more than that and apparently the drive train gives problems. The piston was a high comp Wiseco. I've not managed to source one. Possibly a big bore XT 500 piston might fit. The clutch plates don't last long even on a mildly tuned bike. White Bros did a Mikuni carb kit but I've lost the spec and they've packed. It was a 36. Since the standard is 33  and an elongated shape at that I would imagine much bigger than that the bike will lose power. The stock 33 carb does not respond well to tuning though replacing it with the SP 370 needle/needle jet may work as the 370 32mm carb works great. You have to jet down on the needle and main once it's tuned, which is not what one expects. The 34 Amal Mark 2 doesn't seem to have a slide that works, though apart from a slight hesitation off idle it runs fine with a KN. 27bhp is what Suzuki claim so that's pony power. Real horsepower is around 23. The 370 was faster with the same gearing. I always thoght it was due to changing from points to primitive electronic advance/retard but the staggering difference in the carbs have caused me to rethink. I'm running two sets of wheels with my road gearing, the same as the GN 400, using a locally made Supertrapp and pipe, the Protec cam and gubbins, cleaned up head and modified airbox. The standard airbox , like the standard exhaust, is incredably restrictive.

[MrFluffy]

I have a sp400 with the 370 slide carb on it, k&n and a openish pipe. Its still gutless...
I used to have another motor with the exact same combo on it, and it would pull 90 on the speedo with that lump in, but a camchain tensioner slide snapped, and I left it at a shop to have the engine swapped for a spare gn400 one and it mysteriously was stolen out the storage area before I had chance to crack open and see what had been done to it. By I suspect someone who wanted to know what was done to the innards too...

Since then she went to another shop to be rebuilt in the engine dept, and fitted with a new 1mm oversize piston and stuff, and is still completely gutless...
I have plans afoot for a higher comp ratio, some porting and perhaps a bigger piston to see what can be done to the old girl to warm her back up. I have a spare head, and barrel to play with, and a handy assortment of machinery to perform the deed when I get around tuit, but currently Im off playing with a turbo bike and my xt motard amongst other things...
 

[MrFluffy]

It's a Suzuki dirtbike lump so there should be heaps of stuff for it. Keep in mind, though, that the starter gear will not be happy with any significant increase in compression.

Your thinking of the drz there
SP/DR400, poxy 6v electrical system, kickstart only

[cloggy]

Dead easy and cheap to convert to 12v lights
I've got my engine set up as a trail engine. If I wanted it to produce more power further up the range I'd play with the aftermarket 36mm mikuni and get a race cam. 4 valve head sounds intereesting but as far as I know the DR/SP500 never got across the english channel

[MrFluffy]

I turned down a whole DR500 in a sorry state about a year back on the basis that they were so rare and I had enough projects already. We were sizing up the motor to go into my sp as the original frame had snapped, and its physically taller and wouldn't go. The whole engine had a completely different appearance so I would be unsure the head would fit straight onto the sp bottom end. Thats the only one Ive ever seen in the metal, and until I got to measuring that one up I thought it was just a overbored sp400. I didnt even realize it had a 4valve head to be honest.

I have a 6v solid state regulator/rectifier from paul goff and a 6v 60W halogen bulb in the headlight + led taillight, it all seems to work fine so I dont think Ill create more work converting it yet. I didnt want to risk converting it to 12v and the electronic ignition circuit suddenly get robbed of power by the extra load (nicking the magnetic flux, deprived of webers, theres a joke there somewhere). Ive seen other bikes with electronic ignition magnetos have weird effects when the lighting circuit was converted to 12v.

I have a spare xt500 carb that I can use, since i used a 38mm del-orto on the XT (it has a 600 kit). If (when) I get carried away, Ill probably put a cam in it too. Im trying desperately not to convert my sp into yet another winter project right now, theres already 5 on the go in the shed
I read it as phil joy did your cam. Whats special about the protec keepers and why are they needed with the cam?
Im not after amazing power out of my sp, Ive just had the old girl so long I feel like she should be improving as time goes by, so I try to keep doing bits and bobs, but Ive sort of run out of things round the outside to tackle so maybe its time for internal surgery.

Id love to know what was inside that other motor. That still rankles me 14 years on...

[cloggy]

The stock 400 has a bore size of 88mm. This is probably to try and reduce it's height, being a wet sump motor. XT500 is 87mm. TBM has not only got the details wrong for the DR400 [they state it's got 4 valves etc], they've also got the year of manufacture several years out!
The standard cam is pretty mild and has a hole in the power just where they probably did the noise test.
The Phil Joy Protec copy has slightly more lift and a lot more duration. The extra duration causes the valve to be slammed shut quicker than standard and  thus standard springs allow valve float, ie the valve stem rocker briefly loses contact with the face of the cam, which doesn't do perfomance or the cam much good.
The Protec springs were wider and thus needed different seats and keepers. Fortunately, though the Protec springs were made of knicker elastic and never made it to 7000 miles without sacking, the Kent Custom springs fit the Protec keepers, never seem to wear out [on this engine at least] and used to be dead cheap. They also have the same spring rate
 Megacycle do two cams and recomend their own springs on both
The Protec keepers aren't just wider, they are also made of aluminium. I'd be worried about this  but I must have done something close to a 100000 miles on them so....
The stock SP370/DR400 carbs don't have the sophistication of the XT carb, but seem to work fine with a stock airbox, whatever is done to the inlet weir and exhaust. However once one 'derestricts' the airbox only the SP370 carb is close in terms of jetting. You can play around with needle position and mainjet forever, and not get the 33mm DR400 carb quite right. Quite apart from the annoying jetting glitches the engine is just plain sharper and much faster with the 32mm SP carb. Of course it's possible that the DR float height was off but it was ok on my standard engine.
I haven't tried swapping the needle jet and needle over as the SP carb was only one needle notch and one mainjet size off as stock. The DR carb didn't idle as reliably either. I suspect that when Suzuki did the original developement work they jetted the carb and then restricted the inlet and exhuast as a matched pair till they'd got it quiet . I think they used those restrictions as a starting point when they put together the 400 so the carb doesn't respond so well to the restrictions being subsequently removed; but it's only a theory and I'm no more than a back street butcher.
The GN and SP400 varients use a vacuum 36mm carb. After I got a watchmaker to drill out the mainjet mine ran fine with a no airbox and a K&N filter. You just don't get the initial whoomph of a slide carb.
Mead and Tompkinson, 24 hour endurance racers, got hold of a 370 engine and I think sleeved it down to 350 for the new endurance rules. If I remember right they used a 32mm Amal mark 2. The rules were then changed before the new season so they never raced it. They had loads of experince with the BSA B50 unit motor so I bet that little Suzuki engine went like stink.

[MrFluffy]

Ah so the keepers are just different because theyre lighter (aluminium) and hence raise the rev limit a bit, and are a different size. Bit of 7475 grade aluminium alloy should do the same job...
You wouldnt happen to know what kents part # for those springs are? or the spring rate/dimensions ?
Ill have a peek at how much the cams are now, it may indeed get done this winter...

[cloggy]

I think they are called clubman spec springs and they are for the 1600 crossflow ford cortina lump. They are shorter than the 2000cc pinto lump springs. I can find out exactly but don't want to bother the engineer until he's found my parts of 14 years ago....
Quite frankly the dollar is so low at present that I'm tempted to get my next cam and springs from megacycle. I'd get the milder of the two grinds. I doubt there's much difference between that and what I already have. They have an excellent online catalogue but no apparent way of accessing via internet

[#88]

Hi there,
  I am quite interested in tuning a SP370 as I recently bought an old Hejira SP370.Only had a brief ride during practice and ran into brake problems.Still it felt perky enough but could do with a wee bit more top end.I have raced a couple of singles over the years had a Yamda(TZR125RR yamaha with a XBR 500 motor) built it myself and finished second in the championship that year,a couple of years ago had a Honda Tigcraft with a Dominator motor was fine till it went bang.anyway back to the Suzuki been surfing the interweb but not much information to be had.Was looking through an old mag from 1980 and found an article about the Hagon SP370 Racebikes.Found out that Hagon would take the motor out to 480ish by stroking the crank,spacing the barrel 9mm and boring it out and welding up the crankcases to strenghen them,a wee bit too serious for me.I have got a couple of motors and could make 1 good one out of the 2.Any info would be much appreciated.
   cheers the noo Brian #88

[cloggy]

Ah you bought that recently advertised bike?
Mead and Tompkins in Hereford built an SP up for the world endurance series, then the rules were changed. They used  an Amal 32 mark 2. I found the 34 had a flat spot straight off idle whatever parts I used.
Dirt Bike reported that quite a few tuners had attempted to tune the DR400 only to have problems if they went much above a high comp 420cc. White Bros did a Wiseco high comp 420 kit but that's long gone {standard liner]. They also had a 36mm round slide carb kit which may be better for a revver. I've got the body but I've lost the carb specs. It's possible if you contacted Tom White directly he might still have the jet specs [he sold the firm].
I've got a bunch of clutch plates with bugger all wear that started slipping. It's possible that all I ever needed were new springs. The standard ones sack pretty quickly with a lightly tuned motor.
 The XR 400 has a slightly weak clutch and thus it's advisable NOT to use synthetic as apparently this causes clutch slip, something to think about.
I'd go to Megacycle for the cam, springs and keepers. They do a high reving cam profile.
All my tuned 400 motors have the intake weir ground off. It doesn't seem to have much of a detrimental effect straight of idle, doesn't bugger the jetting and breathes better. I don't know if the weir was on the 370.
 In the USA the DR was only around for a year so they are rare as hens teeth and there seems to be no tuning info available. You could approach Wiseco directly and ask if they have any old stock.
The 400 motor was beefed up, I think in the gears, kick start and crankpin; whether the cases are the same I don't know.
Interestingly the 370 seemed peppier than the 400.
 I've wondered if overbored XT500 pistons might fit, but not found any info.
Good luck!

[cloggy]

PS I've got a feeling M and T were going for a capacity of 350cc
The motor mught be better suited for that class if you had it relined and matched up a smaller piston.

[#88]

Hi there
 the class that I run in is up to 500cc(Junior Forgotten Era),our class goes out with Classics and earlystocks so I get a race with somebody.Over the winter I have changed the front end for an Aprilia RS 125 with 17" wheel and put a NSR125 rear wheel with disc,its a wavey disc from a pitbike bought on Ebay along with a 2 pot caliper/hose and master cylinder for £35 all brand new.As far as I know the motor was tuned by Stan Stevens?? its got one of his stickers on the old fairing,an Amal carb and a reverse cone mega.Like yourself Cloggy I have been looking at XT500 pistons think Wossner do one,had a couple of cams done in the past by David Newman he does a good job at a resonable price and can repair worn rockers.I have a heavy duty clutch/springs to go in,whats the best oil to put in a quality 10/40 or a semi 10/40?
    cheers the noo Brian #88