Here is one of the earlier threads on this topic:

http://www.ktmduke390forum.com/foru...m.com/forum/engine-technical-discussion/2945-390-engine-into-125-chassis-4.html

 

Cheers thats the thread i read. No real info in here and im surprised theres nothing out there about converting the engines in these bikes? Is it because they are relatively new and parts are not readily available? As for being cost affective i did get my duke 125 pretty cheap so my calculations are coming in at £1000 cheaper than just binning the 125 and buying a 390. Perhaps as they age we will see some more conversions happening in the near future?

 

I always remember a school friend of mine back in the very early '80 once fitted a Zundapp 175cc engine in a Yamaha 50 cc trial bike . We were all very impressed till the day he got stopped with most of us near our school :The police was then clamping down on all the suped up 50cc that were supposed only to do 40 km/h max speed .They had portable rolling benches to check the speed : Needless to say that he walked home that afternoon as the bike got confiscated . I shall admit that my Kreidler RS clocked 105 km/h on such bench and had to present it a week later " limited " to 40/h again .. The only way was to fit a 12mm Bing carb instead of the 21 mm , blocking up the exhaust with steel wool and limiting the throttle travel to 50% .

Just to say , these type of conversions maybe challenging projects but are economically unjustifiable , not to mention the illegality and total absence of insurance coverage on such vehicles in most countries .

 

Costing is actually coming it at over £1000 cheaper than just selling the 125 and buying a 390. As for the legality of it. Its as easy as filling in the back of the log book with the new engine cc and away you go. All the DVLA are worried about is that your paying the right tax for the bike. As for insurance, motorcycle insurance are far more accomadating of modifactions than car insurance so dont forsee any problems there either. Just to clarify this conversion is not going to be slap a 390 engine in and ride it round illegaly. As soon as its running its going to be reclassified and done properly. I do like to tinker with things and this has my attention at the moment and to cap it all off when its finished i should be the owner of a one off 390 that will be staying with me for along time.?

 

Hope it all works out fine for you , I ignored the simplicity of the legal / techical procedure for this in the UK . It somehow surprises me as I have close friends in the classic car trade in the UK , who in the early '90 entered classic Jag XK,s , Austin Healey's and Porsche 356 from California trough Belgium just because it gave them less complications for importing /MOT's . Once they had Belgian documents they took them to the UK .

 

Thank you. I do believe importing is a whole new ball game. I was very surprised at the DVLA also i was expecting to have to take the bike to a vic test centre to check everything. But from information provided by .gov website it is just the tax class they are concerned about. Dont get me wrong things like this usually fall flat on thier face ? but i hoping i can do this in reasonable time? And im still expecting problems with converting and re class of the bike. Im one of those people who just gets something into my head and want to do it, with the lack of info out there on this im hoping if nothing else i can provide some accurate info on it, succeed or fail. Worst case i dosnt work and i change it all back and buy a 390 outright, ill then have a nice garage full of spares???

 

Go for it ! Physically it should be not that difficult ; after all the it was KTM who did it first by placing the 390 engine in the 125/200 frame . I just hope that it is not the wire harnesses and other electronic components that are going to complicate things for you .

 

Good point about the harness - I would have thought he would be wanting the 390 one if he doesn't have it already - there's one on ebay now for £68

 

I hope this project works out successfully. The software conversion, I believe, will be the most difficult challenge. It would be very useful to have a documented procedure for this conversion, which appears to stir a lot of interest.

 

Hopefully no major problems will crop up but i doubt im that lucky??

 

Some info on the wiring harness, both the 125 and 390 share the same harness according to the KTM parts catalogue (90111275000). So fingers cross the existing harness should be fine?

 

May have jumped the gun on the last post. I have one of the early Duke 125s and it doesn't have ABS. Harnesses are now slightly different im hoping it is just the ABS system??


First image is the Duke 390 second image is 2011 Duke 125 harness

 

This is the 125 harness

 

If it were too easy .. there be little merit and less fun>

 

Very true, dosent stop me hoping though ? just pooling the info at hopefully if anyone else does this all the info will be in one place. Ill get some pics up of all the parts shortly.

 

Bit more info on this, i have read somewhere in this forum that the 390 front sprocket has different splines to the 125? Can someone confirm this for me please?

Also the rear sprocket althought still 45T has a 4 bolt mount the 390 has a 6 bolt. Does anyone foresee and issues with this as it could be costly to convert it for what it is?

Also there is a difference in cooling pipe on the head of the 390, it has a rubber coolant hose that runs from the head just by the throttle body to the pladtic T piece just under the rad. The 125 has a reduced metal pipe that runs half way and then connects to a shorter rubber hose. Ive order the 390 hose and fingers crossed it looks to just replace the metal one.

 

Also the rear sprocket althought still 45T has a 4 bolt mount the 390 has a 6 bolt. Does anyone foresee and issues with this as it could be costly to convert it for what it is?

With the knowledge that KTM saw fit to increase the number of bolts to six, there is no question that if I were doing this conversion I would also want 6 bolts holding my sprocket on.

Motorcycle manufacturers normally have a good reason to spend money implementing such changes (after all it would have been easier and cheaper to just use the existing 125 4 bolt carrier).

In converting to a 390 engine you are looking to increase the forces acting on these sprocket bolts by a factor of roughly 3. Every time you open the throttle you apply a shear force to these bolts (literally the torque of the engine pulling on the chain and sprocket is trying to tear through the cross sectional area of the bolts).

In going from 4 bolts to 6, KTM saw fit to increase that C.S.A. by 50%, to greatly reduce the stress that is acting on each bolt.

My advice is you do likewise (i.e. use a 6 bolt sprocket carrier, I've no idea if that means you need to also change the back wheel)) otherwise, over time - and it's anyone's guess if and when issues would show up - you may be running the risk of sprocket bolts bending and ultimately failing - and you don't want to be riding a bike when that happens !!!!

 

And while you are changing the sprocket carrier, you might want to take the opportunity to upgrade the cush drive rubbers. Apparently the OEM cush drive doesn't stand up too well to the 390's torque.

KTM - MNNTHBX

 

Got some really nice bits of kit on there, dont know if they will ship to the uk though? My shopping list is just getting bigger and bigger ?

 

I think they routinely ship internationally.

 

Ill drop them an email see how much its likely to cost. Cherrs for the link ill break the credit card out??

 

Rule #1 of car / bike modifications:
NEVER BUILD WHAT YOU CAN BUY.

It's expensive, you never get anything like the money back, you'll never be able to insure / sell it. The absolute best you can hope for is that your 125/390 goes as well as a proper 390 and doesn't fall apart. Why do it when it's not a rare model?

Now, if you said you were going to put a V4 in a Suzuki RGV frame, that would be different.

 

Rule #1 of car / bike modifications:
NEVER BUILD WHAT YOU CAN BUY.

It's expensive, you never get anything like the money back, you'll never be able to insure / sell it. The absolute best you can hope for is that your 125/390 goes as well as a proper 390 and doesn't fall apart. Why do it when it's not a rare model?

Now, if you said you were going to put a V4 in a Suzuki RGV frame, that would be different.

WHY NOT?!


I Fail to see the logic in that statement? As already identified, cost at this moment in time is lower than buying a 390 outright, that's including serviced parts and upgrades.


Has this been done before?
Probably but not many and nothing that's been documented.


Can I sell/insure it?
That's easy yes to both, if I can convert a 1.2 car to a 2.0 and get insurance I'm sure that a 390cc engine into a 125cc frame (which is basically 90% the same bike) is not going to be an issue. Can I sell it? Not that I'm going to but yes the only one I have seen done sold for around £3k.


Why do it if its no a rare model?
As above WHY NOT if just the **** of it? Yes I could go and buy one, but where's the fun in that? The Knowledge alone that I've gained while doing this has been invaluable.


Hope that's answered a few things but most have already been address in the previous posts

 

Ive been quiet lately got a lot on, anyways going to start this tomorrow so im hoping to have the engine in and running by tomorrow evening then that leaves me with with Thursday to iron out and issues. I am still awaiting deliver of the cushdrive its taken an absolute age to come, hoping it will be here tomorrow and it can be fitted Thursday. Hopefully ill be posting up with pics tomorrow of progress and hopefully alittle vid of engine running would be nice

 

Well did go to pkan today. Only had 2 hours 9n the bike but it was basically a strip down. 2 engine bolts left and out comes the engine. It looks promising all connectors seem to be the same. So pkan for tomorrow is drop the engine. Refit the 390 and test fire. If all goes to plan re assemble. Problem thst i may have is still no cush drive.

 

Fingers crossed

 

Update on today didnt really get the engine in. All connection on throttle body are the same, so went straight in with the new air box as the old one was broken.

Found out the the starter motor is different, totally overlooked this so trying to get hold of one now before we go any further.

Lamda sense cable was to short also so had to cut into the harness and luckly the wires ran back towa5ds the front of the bike so just traced back and exited the lamda cable from the main harness clisser to the exhaust.

Still waiting on cush drive? Chesked tracking today and its apparently back in miami? Dont know wats gone on there but emailed them to find out.

Looking to finish off one evening next week, but who knows? I did take pics but they are on a mates phone so will get them emailed to me and posted up.

 

Update on today didnt really get the engine in. All connection on throttle body are the same, so went straight in with the new air box as the old one was broken. Found out the the starter motor is different, totally overlooked this so trying to get hold of one now before we go any further. Lamda sense cable was to short also so had to cut into the harness and luckly the wires ran back towa5ds the front of the bike so just traced back and exited the lamda cable from the main harness clisser to the exhaust. Still waiting on cush drive? Chesked tracking today and its apparently back in miami? Dont know wats gone on there but emailed them to find out. Looking to finish off one evening next week, but who knows? I did take pics but they are on a mates phone so will get them emailed to me and posted up.

Hello mate not sure if your still on this but I have a ktm rc 15 with a 390 conversion I’ve run into problems like every lights up and works apart from the ignition switch, I brought this bike with the conversion but was told there is a look or ecu problem I’ve ordered a 390 ecu is there anything else I should look at ?

 

will be interesting to see the pics.

good luck with the rest of it

 

Not the best pics will get some better ones when we finish the bike. also ill try and take afew specific pics to show differences with lamda cable starter motor, front and rear sprockets.

 

390 motor is up on the stand and 125 on the floor. 390 is ever so slightly wider.

 

Will the 390 motor fit into the frame without modification? What about the radiator - The 390 engine generates significantly more calories than the 200, is the radiator the same as the 390's, or smaller?

 

Part numbers for the rad and pipe work are different in the KTM catalogue, but the size of the rad seems fine to me to be honest, its not that small and its not as if its being used to run a 900cc motor. All exploded diagrams seem to be looking the same and the pipework from the 125 fits the 390cc motor. But comments noted Ill keep a eye on the temp and could be a further cost in the future.

 

Thought i would do a run down of parts changed to date.

Essential parts:
Engine and Starter motor
ECU
Throttle body
Header
Sprocket carrier and cush drive
Front and rear sprockets

Ive also been changing a few other bits as well while its apart.

Airbox (optional it is slightly different but a 125 one will work)
Chain (New vxr2 gold chain going on again optional)
Uprated cush drive (Could of gone with a cheaper standard one)
Full exhaust (unsure if 125 one would cause issues)
DNA Air Filter ( Again not really needed)

Cost wise it is now approaching cost of a secondhand Duke 390, its is still cheaper but not massively and that's with out factoring in the resale value of 125 parts, but we all knew that this wasn't going to be cost effective.

 

Hi all been alittle busy but the bike is up and running with the 390 motor.


It was alittle more troublesome than I expected. Having some issues with the fuelling at the moment, it seems very hesitant in the midrange, also the rear wheels are different so got some orange ones on order, luckily the RC125 wheels are the same as the Duke 390. also the coil pack water pipes and front engine brackets were different.


But this is achievable with a day or twos work. should look nice with the orange wheels and tank cover when the come.


I am drawing a blank at the moment on the fuelling I had DNA filter in there at first and the idle was very lumpy. I Removed if for a standard filter and this has sorted it to a degree, I was wondering does anyone know if the 390 runs a bigger fuel pump to the 125, all my research is showing it to be the same?

 

New parts list as to what you will require to do this conversion


Essential parts:
Engine and Starter motor
ECU
Throttle body
Header and lambda sensor
Front and rear sprockets
Water pipes
Radiator Brackets
Front Engine Brackets and bolts
Coil Park
Duke 390 or RC Rear wheel (including sprocket carrier)
Sprocket cover
Clutch cable and bracket
Airbox (if your Fussy)


I Think that's it

 

Well done on your work to date. Looking down the list of what you have changed already the fuel pump seems a likely culprit (assuming all the other parts fitted are ok). A quick look at the part finder at AMS motorcycles does list the fuel pump part numbers as different between the 125 and 390.

 

Cheers


On the rebuild kits the pumps are showing as a 30mm pump for both 390 and 125 which is why I questioned it??


I do have an after market exhaust on the bike and it is super loud so think of returning it back to standard? Would this have any affect?