DT125 / DT175MX

[PICTURE_MAN36]

I bought a cheap DT125MX from EBAY 2 years ago, with the intention of refurbishing it, and putting it back on the road. I've had a few yamaha's, DTR125, RD125LC, TZR250, and my current ride which is a '93 FZR600. I always wanted a DT125MX when I was a teenager, but went down the path of driving lessons, car etc, and whilst learning to drive I ended up with loads of 100's....

I am now at the stage where the DT is stripped, and my first project is to rebuild the top end. I've checked the con rod, and the bottom end, and it is sound, and I am therefore not looking to split the crankcases. I am however going to explore the age old myth of swapping the 125 barrel for the 175 barrel, as I've passed my test, and I am reliably informed that the 175 'went' better. Not having experienced the 175's performance I am not sure, but I have the following questions for anyone who may have attempted this modification;

a) The 125 will be restricted somewhere? I guess that its in the front pipe, or was it restricted in the carb?

I want to use the stock 125 carb, so I guess that the main jet will need to be changed, assuming that the 125 & 175 shared the same carburettor?

I will be using the stock air box & air filter, and I am looking to put it back to as near standard as possible, albeit, being a 'big fella' I have fitted a jack up kit to the end of the monoshock to the rear to raise the suspension (my kit modifies the length of the shocker, and not the backet on the swinging arm), and I have a similar kit for the front forks.

I've read quotes from lots of people who 'knew' someone who did this, but I've never spoken to someone who has actually done it.

I had considered grafting a DT125 LC mark 1 front end onto my frame, as I beleive that the LC mark 1 had a disc brake front end?

I would appreciate any comments from anyone who owns an MX, or owned one, and carried out the 125 / 175 barrel swap..

Cheers

[PICTURE_MAN36]

Forgot to mention... I have a 'Y' plate / 83' DT125MX and its the last model they did with the mono shock rear end, before Yamaha went over to the DT125LC mark 1.

Cheers

[Cynic]

Alright, 2 questions, the 175 top end goes straight on, same exhaust and in some cases same carb (with jetting changes) there is one thing to be aware of and that is that the clutch has an extra friction and plain plate in it but the basket is the same i believe. If in good nick the 125 clutch should handle the extra.

As far as performance goes its the torque that increaces. You have to rally spank the 125 to get anywhere wheras the 175 will run as fast with less rpm. Not actually a great deal quicker but a nicer engine to use with more stomp.

The 125lc front end. Dont bother, been there and done it. Its an easy swap you only have to swap out the head bearings but the steering lock is crap. The bike sits high at the front, the handling goes off and the real sincher. That disk brake is no better than the drum unless it is in perfect condition.

That help?

[PICTURE_MAN36]

Alright, 2 questions, the 175 top end goes straight on, same exhaust and in some cases same carb (with jetting changes) there is one thing to be aware of and that is that the clutch has an extra friction and plain plate in it but the basket is the same i believe. If in good nick the 125 clutch should handle the extra.

As far as performance goes its the torque that increaces. You have to rally spank the 125 to get anywhere wheras the 175 will run as fast with less rpm. Not actually a great deal quicker but a nicer engine to use with more stomp.

The 125lc front end. Dont bother, been there and done it. Its an easy swap you only have to swap out the head bearings but the steering lock is crap. The bike sits high at the front, the handling goes off and the real sincher. That disk brake is no better than the drum unless it is in perfect condition.

That help?

[PICTURE_MAN36]

Thank you. I'm not sure of the state of the clutch so may opt to fit new friction plates. I've ridden the thing once, and it was running etc, but tatty. The exhaust gasket was leaking, and the gasket that joined the front and tail pipes was missing so needless to say it rode like a bag of sh*te...

Looking forward to rebuilding the top end, and making the engine pretty. Thanks for the info with the front forks. It was something I was looking at in a perfect blue sky world. I planned to fit new shoes front and rear anyway, so it should stop, and to be fair it wont be thrashed anyway. I'm keeping the bike for me to re-live my youth on. It'll blast around our village with the odd run to town.

Will let you know how I get on...

Many thanks for your response.

Cheers

[JacobHendry]

Hi

I recently did this the other way round, DT 175 to 125. As Cynic says, full top end change, barrel, piston and head and I changed The Main Jet and the Pilot Jet in the Carb.

Paul

[PICTURE_MAN36]

I guess that you've done it the other way to make it learner legal? I am umming and rrring as to whether I want to re-register my 125 as a 175, or just leave it as it is on the V5.

In the '80's when we were 16 we all had FS1E's RD50's Suzuki ER50's etc which had 60cc & 80cc kits on them, and I remember that some of the RD125MX's running about where in fact RD200MX's which were identical except for the electric start, which the 125 never had. I ran an RD200MX for a while with race pipes on, and I had a big job getting in MOT'ed as it was so loud..

I ran a HONDA MT5 when I was 16, and a local kid had a MT5 which rang rings round mine. It wasn't until I spoke with him one night when he was parked up at the local Tates shop that he confessed that he'd actually got a big bore kit on it, and it was in fact 60 kit... His did close to 55 mph, and mine did nearly 40 mph dependant whether it has just been decoked. Thanks for the info..

Happy days eh?

[Cynic]

In the '80's when we were 16 we all had FS1E's RD50's Suzuki ER50's etc which had 60cc & 80cc kits on them, and I remember that some of the RD125MX's running about where in fact RD200MX's which were identical except for the electric start, which the 125 never had. I ran an RD200MX for a while with race pipes on, and I had a big job getting in MOT'ed as it was so loud..

Happy days eh?

Way back then there was a 200 with a 400 motor in it registerd as a 125. I never did prove it to myself. Was it possible do you think.

[PICTURE_MAN36]

Don't think that you'd quite get away with that one if plod stopped you... I know that a mate I did my YTS with ran a RD125DX with twin microns on, which did 85 mph ish.. I don't think my RD200DX did more than 90.. I think that a 125 doing the tonne might be a bit suspicious!! They were great bikes in their day, but were ultimately eclipsed by the 'LC's. I owned an '83 RD125LC which was good. Sadly I always wanted a 350 LC, but never had one, and don't think I could afford one now. I have a soft spot for 2 strokes...

Cheers

[PICTURE_MAN36]

Update..... 13/10/11

Got there in the end. Engine now back together and crated up in the stacker box. Next job frame!

The barrel does fit straight on to the 125 bottom end no problem. Note that the 175 barrel has 6 head studs not 4... thought I'd dropped a bollock but it works along as you have the 175 head for the barrel. Had the barrel and head vapour blasted, looks good. At the same time, I have reinstated the 2 stroke autolube pump. The bike had been run on premix and when I took the pump off it was easy to see why. The drive gear in the pump was totally wasted. The pump is lubricated by the 2 stroke oil, and I'm guessing that at some point the 2 stroke tank was ditched and the pump consequently seized. Bought a second hand pump from a DT175MX, thinking it was a straight swap, BUT this is not the case. PLEASE NOTE!

The 175 pump is geared differently to the 125 pump, and you need to swap the pump if you're swapping the barrel!. This is OK if you have a complete pump and shaft, but mine came as a pump only as the shaft had been sold seperately. I thought my 125 oil pump shaft would fit the pump, but it did not, and 48 hrs and a few calls to the local Yamaha Dealer, and it became apparent that the part numbers were different. I had a quick look on EBAY, and there were no shafts in the country. I ended up buying a new shaft through the local Yamaha Dealer, which came from Japan at a cost of £ 20! Bargain! Sadly during the rebuild, the pump shaft collar became lost, so another weeks wait, and finally shaft, collar, pump, and casing all came together followed by the engine cases which have been freshly painted and now I have a motor that looks like its just left the factory.

Next job Frame!

I am hoping that all the work done to convert the engine to a 175 will be all worth while. I don't expect to have the DT on the road until next spring, but I'm looking forward to running it in, and pottering around the village, and lanes surrounding where I live.

[YPVS TONE]

The DT125MX used a smaller carb a VM22SS compared to the 175 that used a VM24SS so it might be better to look for a s/h 175 carb. Certain that the DT125LC MK1 still only had drum front brake and monoshock rear suspension and it was only the later MK2 and onwards that had Front disc and rising rate rear suspension.

[Airhead]

The DT125MX used a smaller carb a VM22SS compared to the 175 that used a VM24SS so it might be better to look for a s/h 175 carb. Certain that the DT125LC MK1 still only had drum front brake and monoshock rear suspension and it was only the later MK2 and onwards that had Front disc and rising rate rear suspension.

Some used 24mm carb Tony, but it still might be an idea to source a carb, there were two versions so depending on the bike airbox arrangement the appropriate one could be sourced, it all comes down to the swing arm...square or round profile

[YPVS TONE]

I know the 1979 DT125MX I had had a smaller carb then my mates who had a 1980 DT175MX both had the silver box section swinging arms with both airboxes on the LH side.

[Airhead]

I know the 1979 DT125MX I had had a smaller carb then my mates who had a 1980 DT175MX both had the silver box section swinging arms with both airboxes on the LH side.

yeah I would hazard a guess it probably comes down to points ignition or CDI as to which carb was fitted.

[YPVS TONE]

I know the 125mx I had was defineately electronic ignition and the ignition timing was set at 1.45mm BTDC were the 175s were 1.8mm BTDC.

[Airhead]

must be just the airbox and square arm thing then eh?