Yamaha DT125RE restoration derestriction catastrophe!

[Consmos]

Hello Everybody

First time posting anything so bear with me, but I can't think of a better place to start - I need to

crowd-source some wisdom, and hopefully lots of help

Long story short - I have no experience whatsoever in fixing bikes. Or maintaining them. I've never even

changed the oil in my car. But after I finally got round to getting my bike license (after a number of

unfortunate setbacks) I decided to whip the dust sheet off my pristene DT 125 RE. It may be eight years

old, but it only had 4000 miles on the clock and was in brilliant nick.

So I thought. Turned out a friend of the family had 'borrowed' the bike as part of some half-hearted

crisis and wanted to learn to ride. This, he did not. He took it for a couple of spins and left it to

rot in the sun and the rain outside his house for months. I hadn't even known it was gone.

So imagine my horror when I saw this:

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The photos don't do it justice, but in brief: A mirror missing, the other so badly corroded that it

was hanging off. Indicator torn off. A large hole in the left side of the seat. All the pastics were had

faded and warped from exposure. The wheels were orange with rust. Rust eaten through the paint

and into the frame, joints, radiator, tank. Gearbox oil seal rotted and oil was leaking out. Crankcase

badly scored. And so on... I don't even know how he managed to be so careless with such little use,

and he didn't offer to fix it.

Determined to bring it back to life, I decided to throw myself in the deep end and learn by doing!

One Haynes manual, a load of new plastics, nuts, bolts, mirrors, indicators, clutch levers, elbow

grease, chrome polish, wire pads and partridges in pear trees later the bike was as you see below:

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Not a bad job for a complete novice there was still the matter of the rusted patches on the frame and

tank to take care of, I know they will be the end of the bike if I don't sort them, but after pricing a

new tank and failing to find a second hand one, I've decided on stripping the paint back, priming it,

painting it again and lacquering it.

Does anyone know how best to take the rust and paint off in the first place? Any ideas would be great!

Now here comes the really serious problem!!!!

I decided that I'd now derestrict the DT and see what it was really made to do. Spent a very long time

digging about on the net until I had figured out some tricks - power valve is the correct way round

and it lets the bike reach 74mph, although wheelies were just never going to happen - I've wheelied on a

75cc kawasaki when I was a kid so I knew a 125 could do it! I grounded the CDI and got a little bit of

extra pep off the starting line and it was more responsive at low speeds, but I didn't get the major

power gains that so many people hd harked about online :/ I knew the major restriction lay in the

exhaust and the expansion tank, so I eventually bought the bullet and ordered an Athena expansion tank

and exhaust can. My Dad and I had some father-son bonding time putting it on when I should really have

been studying for an exam

All seemed well, the wee bike sounded a it louder, but when I took it out there didn't seem to be much

difference. I kept her at tame speeds about a tiny minor road, then after about six minutes got out on

to a major straight and pinned the throttle to see how fast she could go.

The needle hit 74 and just kept climbing, and i was about to break into a smile at 80mph when there was

a series of bangs and rattles and the bike rolled to a stop. Non-starter, trailer pickup required. The

starter motor turned over but engine didn't start, putting the choke on would give the occasional

backfire. The gearbox seemed fine. I later stripped the bike down and saw the damage:

WTF! I had completely cooked the piston. The spark plug element was completely missing, looked like it had

been totally burned off. And now the interior of the cylinder seems slightly scored, although I know that

even slightly scored can be a big problem.

Can anybody shed any light on this?! Can anyone help? I'm going to take the cylinder to a mechanic to see

if he can hone the cylinder out and fix it for me, he was recommended by my local Yamaha dealer parts

dept so hopefully he'll be able to sort it for me as the cost of a new cylinder (£435!) would just mean

the end of this bike and a deposit for a new one

I poked about with the remains in an attempt to find out what had happened, clearly the engine had been

running too hot, but I couldn't figure out why. I hadn't altered the air intake at all. The website

selling the Athena kit recommended standard carb jet settings for the exhaust so standard is what I

kept, particularly since I don't have a bloody clue how to change that! I also asked the Yamaha dealer

who said that they used to sell aftermarket exhausts and never recommended any change in jetting, and

that he didn't think a new exhaust alone could have been the cause of the damage.

When we removed the stock expansion tank, there was a hose coming from the top of the tank and leading

up behind the radiator to a plastic block and another hose. Apparently this is the YEIS (yamaha energy

induction system), which Yamaha explained was mostly for power regulation and cold starts, and wasn't

strictly required. The reason I had asked them is that the Athena expansion tank had nowhere to attach

this hose, so we just tucked it away. Could this be relevant? The Athena system definitely was the right

one, and I can't see how they would leave that out of the design if doing so would wreck the bike.

I'm looking forward to hearing some opinions, really want to get this machine back on the road!! Thanks!

[Airhead]

Thats a serious case of running lean isnt it!!! whats this Athena kit isnt it an oversized 170cc barrel and head?

you mention expansion tank several times...do you mean expansion pipe aka exhaust?

see my signature line

[Consmos]

Yep sorry meant expansion pipe! No didnt go for the big bore kit, just the exhaust can and pipe. How the hell could it suddenly run so lean?? The top of the piston was reduced to sand and there are tiny flecks of the remanents of molten spark plug on it!

[Airhead]

whats the athena connection? usually folks fit DEP exhausts, do athena make one then?

[Consmos]

Yep it's a little bit cheaper than the Dep equivalent, i just didn't want chrome as I read some reviews and heard that chrome exhausts rust quite easily after they take a few knocks. I reckoned a black one could be easily resprayed!

[Airhead]

yeah fair enough, It's just that I've never heard of the athena exhaust but I am familiar with their big bore kits,

There is no connection from the stock exhaust to the YEIS (Boost Bottle) is there

[Consmos]

Yeah there is a connector on the stock expansion pipe that was attached to what the Haynes manual said was the YEIS, although i have nonidea what that did or why. When i revved the engine with my finger over the now loose YEIS pipe I couldnt feel any positive or negative pressure, so no clues there either

[Airhead]

I think possibly the Haynes book is wrong there myself

what I think is that the pipe from the original exhaust connected to the airbox...you will see the boost bottle (YEIS) has only one connection and that is to the top of the rubber inlet manifold...confirm?

[Consmos]

Yep you're right there, the pipe from the original exhaust leads to a solid plastic joint which then feeds to a second hose leading to the airbix under the seat. The black boost bottle also connects only to the manifold via the rubber connector, no other inputs or outputs. What does the boost bottle do and how does it function? Could the hose from the airbox be relevant to the damage done to the cylinder? I don't see how it could if the aftermarket exhausts dont have any connector for it but i'm running out of ideas! Is it possible that something was wrong in the first place and the new exhaust just accelerated the damage?

[Consmos]

Allow me to correct myself - the pipe from the top end of the stock expansion pipe (and is no longer attached to an exhaust) is NOT the YEIS, it's the AIS - air induction system, which apparently "uses exhaust gases to pulse fresh air into the exhaust port, where it mixes with hot combustion gases. The additional oxygen provided allows combustion to continue for a longer time, reducing unburned hydrocarbons jn the exhaust gas content". So that additional pipe was part of the emissions reduction system. Again, out of ideas as to the cause of the problem!

[dt502001]

As the bike was left in the sun to rot that bad, I would be looking at the crank seals,me thinks they got dry and craked alowing air in causing the lean condition. I would also have a good look at the crank if the piston melted that bad chances are the crank is dammaged as well.

As well before installing the carb make sure it's 100% clean possibly the jets were slightly pluged up and not alowing 100% of the fuel.

If your going to scrap the bike a young lad here had his bike stolen and is looking for parts I'm sure he would be interested in whats left of your bike. I will have a look and find his user name. Adam Jones is his name

sorry about your luck that is one of the best/worse melt downs I have seen in a while.

[AndyDtr08]

Welcome to the forum.

To start with whoever borrowed your bike and turned it from pristine condition to that needs some serious seeing to!

Anyway to the issue at hand.

Yes the pipe coming from the exhaust is part of emissions regulations and would be needed for an MOT i believe but shouldnt have caused a problem like this. I dont know if you should block it and cant see that you should if it just goes into the airbox.

Many people fit after market/ performance exhausts to these models without a problem. It is always advisable to check the mixture after changing your exhaust as most aftermarket exhaust companies reccomend up jetting. If your mixture is too lean ( too much air not enough fuel) then you'll get something like this.

Could be lack of oil getting in so check your oil pump is supplying.

Don't worry, you wont have to buy a new cylinder. The worst case will be that have you to rebore the cylinder. PJME is an engineering company that will do that for about £85 plus delivery charges. That will include a brand new piston kit and small end bearing.

Howerver, as said before. With damage like this, parts of the piston or rings could easily have gone south and when you tried to start it again, have probably been rubbing against the bearings and seals in your crank ( not good).

You could just check it and flush it out with petrol and oil but it's a risk. A bottom end rebuild will be a couple of hundred pounds at least.

You need to work out why it did it in the first place though before you rebuild and run it again.

If you decide it isnt worth rebuilding the engine. One option would be to buy a 2nd hand running engine on ebay and fit that. If you decide that the bike isnt worth it, you'll get a fair amount of money selling it for parts.

[Consmos]

Thanks for the help so far, took the cylinder to a mechanic today, he's going to try to hone it and worst case, bore it by tomorrow. He's of the impression that bigger jets were absolutely required for the exhaust and he believed that was the entire of the problem. That's kind of what i had thought, has anyone any ideas what kind of jet sizes might be best? The crank looks fine but there's a bit of aluminium dust on it :/

[Airhead]

you could try the labour saving way to clean out the crankcase, repeated filling / shaking about and emptying of the case with petrol using a fresh charge each time, you'll get a feeling if the crank bearings are clean by rotating the crank by hand while holding the con-rod, It'll be an engine out job of course. Most of the damaged piston will have exited via the exhaust.

so what size jet is fitted right now? I'm thinking a 250 will be about right.

Take the reed assy aout and make sure thats clean too

[dt502001]

You need to get 100% of all the melted alum out of the bottom end before you put it back together or it will blow up again,best to pull the bottom end apart and fit new seals and have a realy good look at the bearings. No point in fitting a new top end on a weak crank,you might as well throw the money for new topend in the fire if you don't get it 100% clean.Not a fan of "the labour saving method" just cant be certian that its all out.And it's way more labour to start all over again if it does blow because of a piece left in.

As far as jetting normaly 1 size up on the main jet from stock is all thats needed but your going to have to break in a new motor so fitting the old pipe to break in the motor is the safe way to go.

Then after the motor is broken in then put the new pipe back on and start jetting up.It's going to involve doing alot of spark plug removal to get the jetting right,what you will be looking for is a nice brown plug there are so many ways to fine tune the jetting and as you self admit you don't know alot might be best left to a pro.

[Cynic]

Unfortunate, but you did all the derestricting stuff and changed the pipe then with a bit more go pinned the throttle.

BOOOM, first rule of a 2 stroke, never pin the throttle in top and max it, it will die.

Airhead is there with the 250 jets, back in the stone age when i was derestricting the RDZ's and the first TZR's and putting the motors in DT's we opened the valve, gutted the pipe and fitted a, 250 main.

[Consmos]

Thanks, I'll have a proper nosey at the crank tomorrow and try to clean it out properly. As for the jet size.... This is where I fall down, I have absolutely no idea. I know that the two most complex systems are the carb and the gearbox, so I haven't touched either! The jets should be completely standard for an '04 dt 125, whatever that is! How will I know what size they are, are there markings? What is required to change the jet? Is there anything in particular to beware of or can I just amateur it apart and back together like the rest of the bike (with great care of course)? The engineer honing my cylinder almost slapped me when I suggested a plug chop, and he justified himself well - he claimed it was best to buy a pile of different jets and start with the very biggest I could find, graduating down slowly until the bike was just running, as a plug chop could mean that by the time I inspected the plug for damage I could have already destroyed my newly-repaired cylinder and another piston. Any thoughts? I need a proper education regards changing jets, I'm sure I could figure it out mechanically but I realise that misjudging it could send me back to square one!

[Airhead]

Changing the jet is easy, just take off the float bowl from underneath the carb, you will see the jet there...a brass hexagonal thing with a hole in it and a number on it, i would say only buy genuine mikuni jets. just see what size is fitted and buy a couple larger ones....like I said earlier i'm thinking 250 will be about right depending on whats already in there.

Allens sell them cheap .........google allens mikuni

Take care undoing the float bowl screws, they are easily chewed if your driver is not a good fit!

[Consmos]

Cool. How do i know what's already in there? Having a hard time googling it. Does the number on the jet indicate the size? And do you think I need to change the air jet / pilot jet etc as well or should they be kept as they are?

[Cynic]

Start with the main jet. Be sure to monitor each change with a proper repeatable test. Ride a control route at legal speeds a couple of times then note stand out points like hills and the gear and speed on them. That way you will know if things are good.

A favorite with 2 strokes is thinking you have gained a nice powerband when all thats happened is the midrange has gone. You think is better but it is in fact slower point to point.