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Alex Dark

Engine Overhaul - DT 125 LC mk1

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Hi there, I'd like to start out by saying that I'm an apprentice engineer by trade, but am a lot more used to bikes without and engine! ( bike-trials )

Here goes, I recently purchased a Yamaha DT 125 LC mk1 bike in pretty horrid condition for the princely sum of £45. Im looking to get the bike up and running with the minimum of effort and cost, but still have a fairly reliable and powerful bike at the end of it. My budget is to bring the entire cost up to £100 ( so another £55 on parts ).

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The bike has been running around 3 years ago, and I did witness this. Since then the bike has been left outside with a tarp over it that seems to have kept at bay some of the corrosion.

BAD POINTS so far.... :



  • Broken clutch lever
  • No Exhaust
  • Flat tyres
  • Non working brakes
  • No mudguards
  • only one plastic left

The first time I had a look over it after buying it 2 days ago I found the engine was seized for some reason, after a load of internet research I heard that cheap coke can help to free it up ( the phosphoric acid it contains ). I dumped 200ml ish into the top of the bore and waited a day or so, i then came back and tried to kick the engine over.... no go. So I took the ( side plate? ) off the engine to expose the flywheel. I chucked a spanned on the protruding nut and to my relief the engine turned over smoothly!

After this I pulled the whole of the head off to expose the piston to check for scoring in the barrel ( i tried to pull the top portion of the head off but it wouldnt budge?!.

After some examination I reckon the bore should be fine, with no deep gouges or marks etc. I will upload some pictures for a more professional opinion from you guys though! The piston seems okay too, some black soiling on top from combustion but no holes or cracks etc. The small end bearing seems to have a bit of play in it, but im not sure how much is actually acceptable? Happily a replacement bearing scrapes in at just under £5 so its hardly going to break the bank.



  1. Is there any way to tell if new piston rings are needed without a compression test or feeler gauge? They feel a bit sloppy on the piston... :S
  2. Should I put a new set of gaskets on the bike? Is this a cost effective thing to do?
  3. IS it worth blowing a large portion of my budget on a second hand exhaust, or would it be possible to fabricate one?
  4. My boss recommended draining the engine out by turning it upside down to remove all the coke, then pouring 2 stroke oil in, turning the engine over then pouring it out to lube it for the first run in.
  5. Is silicone sealant okay to seal the casing where the flywheel is contained ( crank case? ) or should I spend some moneys on gaskets / gasket paper?

ANY help I could get would be massively appreciated ! Especially where I could source parts from.

Thanks in advance,

Alex

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£45!

I don't know anything about fabricating exhausts, but I know that particularly on 2-strokes the engine won't run properly without one. I would try to get the proper replacement or an aftermarket one made for this bike if cheaper?

I'd get a Haines service manual for the bike. It will tell you service limits etc. on things like the piston ring gaps, bore diameter, pretty much everything!

I probably wouldn't replace any gaskets unless the covers, head etc. have been off or there is a leak.

I'd tip the coke out before first run but the oil in the fuel will probably be ok to lube it? Pouring a bit of 2-stroke oil and turning it over a couple of times wouldn't hurt though. Just be prepared for a smoke screen when you go to start it!!

I used just gasket paste on an old lawnmower crank case and have had no problems as of yet! I don't know about silicone sealant though. Would it take the heat changes? I read somewhere about using cereal box card as gasket paper! Proper gasket or gasket paper would obviously be preferred though I guess!

Parts source? eBay!

Hope this helps!

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Another £55 on parts :lol:

Might as well forget it right now ;)

+1

No mudguards, no lights, no exhaust and mx knoblies.

You wont fab an exhaust thats an art that needs experience to learn. Modifying one is ok but from scratch :o .

Best of luck to you. The most important piece of advice i can give. Don't listen to gobshite like fill the motor with 2 stroke oil :blink: and then turn it upside down.

If you believe stuff like that i can sell you a bucket of optimism for 60quid, i'll post it tommorow. Honest......

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.......The most important piece of advice i can give. Don't listen to gobshite like fill the motor with 2 stroke oil :blink: and then turn it upside down.

.....

Ouch! Jason I think he might have meant 'turn it over' as in rotate the crank in the oil :unsure:

Still its a non starter unless he has a huuuge budget, and lots of time.

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[*]My boss recommended draining the engine out by turning it upside down to remove all the coke, then pouring 2 stroke oil in, turning the engine over then pouring it out to lube it for the first run in.

Ouch! Jason I think he might have meant 'turn it over' as in rotate the crank in the oil :unsure:

Still its a non starter unless he has a huuuge budget, and lots of time.

Paul, is that not the most uninformed guesswork you have ever heard, it has 4 stroke written all over it, and coaxing them back from the dead is very different.

He is a self confessed novice, big up to him for that, he is taking on a 'very' possible moneypit, so fair play. Half baked info like that will wipe out his cash faster than a celebrity drug habit. He needs to consider every penny. My first bike started out in much the same disasterous state. With time patience and solid infomation it can be made into a goer.

Most of us have assorted tat in our garages that may help, old tyres, an exhaust that needs welding etc. If he's going to get this bike as legal transport then cheap second hand stuff is all he can afford. Cardboard cereal box gaskets and hylomar etc. I made a set of Mk1 cortina rings fit an old (shed) mk2 escort i had as they were going begging and mine were had it. That is the kind of stuff he is going to be looking at.

Crappy advice is the last thing he needs.

Be usefull to know where he is though eh.

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£55,,,,,,,,ya'd be aswell ta let it blow away in the wind,,,,,,it would be cheeper on ya.

Bike needs a lot of work,,,,,,,,,, is it just for a field bike ya want it for??

Is there a spark ?

You would buy a fine bike for the money it would take to put this one right,,,,unless of course ya only want it for the field.

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Bloody right Paul

Alex Dark,,,,,,,away ya go and buy that :thumb:

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After quite a break I thought id come back and update this topic now I've made some progress

After being completely stripped down and refurbished to the best of my fairly limited ability she's all back in one piece at long last!

p4pb6772073.jpg

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The list is as follows:

Aesthetics



  • Wheels have been refurb'd ; hubs and spokes have been wire brushed then primed and sprayed matt black. Rims have been cleaned up best as possible and the whole kaboodle lacquered.

    p4pb6772101.jpg

    p4pb6772062.jpg



  • Front hub had its brake cable guide snapped off, so I fabricated a new one, ground the old one off and then tig'd that back on. Once smoothed over it was passable as being forged as one so I was quite happy with that!

    p4pb6772039.jpg

    p4pb6772046.jpg

  • Rear swingarm was removed and inspected, on the right have side it had rotted / rusted through so I patched it with some 4mm plate. Mig'd this on and ground her back. This sub frame was then painted with white hammerite, along with the rear brake arm.



  • The tank had one massive dent near the filler cap that had originally been filled, but the rust had pushed itself underneath and started to displace the filler. There was another serious dent on the corner near the rad, probably 3/4" deep. Bit of a learning curve here, I decided to fill it all in with Isopon P38, my first experience with filler. I was really pleased with the final result.
  • The crowns were removed and sprayed black, along with the bars and other smaller parts. Also bought some new A2 stainless 6mm m8 hex bolts to remove the rusty old 13mm sockets originally used.

    p4pb6772082.jpg

  • I also took the forks out, wire wool'd the lower legs back to a nice polished alloy finish and lacquered these. The stanchions were cleaned with autosol and aluminium foil.
    p4pb6772051.jpg

Mechanical



  • Mechanically the bike was actually fairly sound, all that was really needed was the unseizing of the bottom end and a good clean of the plug, piston and top end.
  • The rear brake pads literally dropped off their backings, probably due to the water exposure. Because of this I ended up buying some more pads, put these new ones in the front hub and the older front pads back into the rear. Of course, the drums were sanded out prior to this.
  • Happily I managed to beg some cables off my boss and one of the students at my work, new clutch and front brake cables certainly helped with the feel of the bike.
  • The throttle cable was actually knotted instead of having a barrel end to attach to the throttle itself. I ended up making a new end out of some 3mm bar stock, drilling a 1.5mm hold and soldering it to the end. Felt a fair bit safer.
  • Front and rear axle spacers were really badly rusted, so I just made some more on the lathe at work out of some ally lying around
  • The exhaust guard was a home-made affair as im sure you guessed ^_^ It certainly works well, even if it isn't the prettiest thing around.

    p4pb6772021.jpg

    The rear exhaust was an old dep sport can that I scrounged off a mate, I then bent some 30mm 14 gauge ally tubing to match the DT frame. This was then TIG'd to the dep endpipe, I ended up packing the exhaust with washed loft insulation so we'll see how well that holds out. It certainly quitened the bike down massively

    p4pb6772057.jpg

    To summarise, the bike actually runs really quite well now and ive had a fair bit of fun on it. I've definitely learnt a hell of a lot already.

    I could do with some help tuning her up to get the most out of it! The engine will only start on choke, and when revved will stammer and bog down. If you carry on blipping the revs it will rev right up on the third go? Quite strange, obviously something to do with the mixture screw. Id appreciate any advise on this matter!

Another £55 on parts

Might as well forget it right now

Total so far is £12 on paint, and £26ish on parts

£45!

I don't know anything about fabricating exhausts, but I know that particularly on 2-strokes the engine won't run properly without one. I would try to get the proper replacement or an aftermarket one made for this bike if cheaper?

I'd get a Haines service manual for the bike. It will tell you service limits etc. on things like the piston ring gaps, bore diameter, pretty much everything!

I probably wouldn't replace any gaskets unless the covers, head etc. have been off or there is a leak.

I'd tip the coke out before first run but the oil in the fuel will probably be ok to lube it? Pouring a bit of 2-stroke oil and turning it over a couple of times wouldn't hurt though. Just be prepared for a smoke screen when you go to start it!!

I used just gasket paste on an old lawnmower crank case and have had no problems as of yet! I don't know about silicone sealant though. Would it take the heat changes? I read somewhere about using cereal box card as gasket paper! Proper gasket or gasket paper would obviously be preferred though I guess!

Parts source? eBay!

Hope this helps!

After a lot of attempting to start the Dt it became really apparent that it wasn't having any of it! Luckily after some trawling around on Ebay I managed to find a whole front exhaust / expansion chamber for £12 posted. Pretty much as soon as I put this on the bike came to life happily. Lots of patching needed for it though, probably 1.5 hours with the mig, burning away the old shitty metal until i reached the decent stuff. The exhaust had 4 large splits in it, 3 large holes and too many small holes to count!

As a temporary fix it's just been hammerited to give it some protection for the mean time. It will be resprayed with heat proof ( bbq i think ) if / when the hammerite has deteriorated.

A haynes manual is a definite necessity, and something i'm going to buy off the Bay as soon as I get round to it. I ended up using silicone sealant for the water pump sealing, and just used the original gaskets elsewhere.

+1

No mudguards, no lights, no exhaust and mx knoblies.

You wont fab an exhaust thats an art that needs experience to learn. Modifying one is ok but from scratch :o .

Best of luck to you. The most important piece of advice i can give. Don't listen to gobshite like fill the motor with 2 stroke oil :blink: and then turn it upside down.

If you believe stuff like that i can sell you a bucket of optimism for 60quid, i'll post it tommorow. Honest......

She was a bit of a state! I really should have clarified this is a field bike only, even i'm not as ambitious as to try and get this beast on the road :P .

Not trying to disagree with you or anything, but I did actually end up filling the lower end of the engine with old 2 stroke and left it for a day or so to let it soak in. After that I literally flipped the whole bike up on end, crowns and seat down, wheels up to let all the oil and coke drain into a container. Result: one free bottom end

Ouch! Jason I think he might have meant 'turn it over' as in rotate the crank in the oil :unsure:

Still its a non starter unless he has a huuuge budget, and lots of time.

I did do this also, to try and circulate the oil, In the end all it needed was the extra backpressure from the exhaust it seems.

Paul, is that not the most uninformed guesswork you have ever heard, it has 4 stroke written all over it, and coaxing them back from the dead is very different.

He is a self confessed novice, big up to him for that, he is taking on a 'very' possible moneypit, so fair play. Half baked info like that will wipe out his cash faster than a celebrity drug habit. He needs to consider every penny. My first bike started out in much the same disasterous state. With time patience and solid infomation it can be made into a goer.

Most of us have assorted tat in our garages that may help, old tyres, an exhaust that needs welding etc. If he's going to get this bike as legal transport then cheap second hand stuff is all he can afford. Cardboard cereal box gaskets and hylomar etc. I made a set of Mk1 cortina rings fit an old (shed) mk2 escort i had as they were going begging and mine were had it. That is the kind of stuff he is going to be looking at.

Crappy advice is the last thing he needs.

Be usefull to know where he is though eh.

Im the first to admit that most of my "knowledge" is made up of guesswork :P and i do appreciate everyones concern about this being a very probable money pit :) Ive really tried to be careful with everything ive spent on her, and I think ive done pretty well so far.

Im based in Cornwall, PL22 0LW. Anything that could help would be amazing, particularly looking for plastics now I think. Also a rear wheel, as the rear rim is fairly shot from corrosion.

£55,,,,,,,,ya'd be aswell ta let it blow away in the wind,,,,,,it would be cheeper on ya.

Bike needs a lot of work,,,,,,,,,, is it just for a field bike ya want it for??

Is there a spark ?

You would buy a fine bike for the money it would take to put this one right,,,,unless of course ya only want it for the field.

It certainly is for the field, and I did have a fairly strong spark from the off which certainly helped matters! Dt's do seem to be fetching a fair price on the Bay and in classifieds, so I think it was probably worth fixing her up :).

Anyway, I hope you guys like the pictures of her...hopefully I'll get some graphics made for it soon.

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I had the bike out this morning, and was a bit worried by how loud it is really..... theres a constant rattle? that sounds like its coming from the bottom end, consistent through all revs and doesnt seem to change.

It pretty much drowns out the exhaust noise! Am I right in thinking that this is bottom end bearings? Will this problem get much worse...and is it worth changing them out? The crank didnt seem to have any noticeable play in it when the bike had the head off. No up and down movement in the crank and only a bit of side to side.

The youtube video shows the extent of the noise...

Also, when I looked at the piston rings they are pretty noticably worn on the back side ( opposite the break ), probably only 70% of the thickness when viewed from the top of the rest of the ring. Is this a sure sign that they need replacing? I dont have access to a compression tester, which would make it a lot easier to work out if it needed doing.

Cheers, Alex

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Another update.... new crank bearings and seals on their way, along with new 1.00 piston rings to replace the fekked ones ( pics to follow ). Full gasket kit coming too.

Bit of a steep learning curve but should be splitting the casing and pulling and replacing the old rough crank bearings.

Is it worth replacing the small end bearing as a matter of course?

Alex

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