NE0

Anything must be worth a try if you've tried everything. But i just did a quick google search for timing with unleaded vs leaded, and apparently you retard the timing slightly. One guy writes that for his classic bike engine he normally sets the timing where it "should" be and, providing he gets it going, goes for a ride up a hill, if it pinks he retards it a bit, if it doesn't he advances it until it does pink then winds it back very slightly. Remember it only takes a fraction of adjustment to alter the effect it has. Like I said it might be worth altering it, you can always set it back to 'normal', if it doesn't work. it's only a screw adjustment afterall, but having said that, I'm not so sure I altered my DT for unleaded! .....I may have done , it's so long ago though!.......good luck Phil keep us posted.

if its only done so few miles it won't be a choked exhaust then. However, you say the timing is 1.9. is that for leaded 4 star fuel? which it was designed to run on back in 1984. Now your using unleaded. I'm wondering if you need to advance the ignition timing slightly to compensate.

Okay, heres another thing to rule out..... What about the exhaust? if its choked it won't run. Try disconnecting the silencer, it will be noisy....but unchoked. 2 strokes rely on a bit of back pressure to run successfully, so you won't be able to ride without it, but it would give you an idea if it's choked up too much. if it doesn't start you know its not that.

Might be worth ruling out the petrol cap.... they are vented to stop a vacuum forming inside the tank, which can cause the engine to stop with fuel starvation. Had this happen on my 400/4. Sometimes the seal sticks or the outlet tube on a trial is blocked.... Rule it out by seeing if it will run with the cap unscrewed/off. ....if it still won't start/run you know its not the cap. it's a simple enough test with no cost involved....unless it's the cause!! The seal acts like a one way valve, it lets air in but stops petrol leaking out.

and oddly enough...I answered a similiar post a few years ago on the very same subject. I also gave the process of how I got mine reclassed as HISTORIC. Have a gander, it may help.... P.s. I have a multibike insurance with CIA for my two classic bikes DT and Honda, limited to 2000k miles per year. It was £120 fully comp this year for both.

Oddly enough I answered a similiar post recently, when another member asked about the two spark plug holes on the top of his DT head. He posted a photo of a cylinder head with two plugs screwed into the head. Here's the text from my post... I know the early 70s AT2's (125 &175) had dual spark plug holes. As far as i know they weren't designed to run with twin plugs as one thread hole was often plugged. However, my mate had one and had both holes with spark plugs,(like your photo) when one became fouled up he'd pull the HT lead and put it on the 'spare'. It doesn't look like an AT2 though as the plug holes were either side of a central fin. Your photo shows one directly in the centre and one offset which means its from a dual shock DT175. Here's a brand new one on ebay. https://www.ebay.co.uk/itm/334146920758?hash=item4dccb56136:g:3E0AAOSw6fBhPx2o I appreciate that my "answer" doesn't answer your decompressor question but it may be of some help.

Hi there Johnners, As a big fan of all 70's/80s bikes and having restored mine several times, the biggest problem you will have will be that old fuel thats been sitting there especially inside the carb. Old fuel not only makes rubber seals turn solid it also leaves behind a dark brown shellac like coating which is just as hard and is difficult to remove! The fuel evaporates and leaves everything stained brown, at best it can be wiped off like a dust, but it does tend to block up the carb jets and float pivots. (unless of course you drained the carbs before putting it away?) If you didn't you're going to have to strip the carbs and clean them out, which means you're going to need carb rebuid kits to replace all those dry seals. It's false economy to trust them (after so many years) to be honest, even if they do come apart and look intact. Is it worth trying to start it without stripping the carbs? I don't think so!! I did it once, fuel everywhere!! floats jammed up, fuel coming out the overflows, a right mess. Then you have to strip the carbs anyway!! and everything is wet with petrol! You'll also need to put in new plugs.! keep us up to date, wish i was there to see to it spring into life,....its very satisfying when that happens.

hi there, welcome back, although i didn't know you'd gone. As regards to posting photos, it seems the best way is to open a free Flickr account, upload your photos to it, then it gives you a code to paste in your text message, which then shows up in your message. I did it, and it seems pretty easy to do, I just followed the instructions here https://yamahaclub.com/forums/topic/45201-uploading-photos/ it's a little out of date as the sequence has changed slightly but its pretty straight forward and i was up and running quite quickly. Once uploaded, you preview your photo and the code is automatically produced so you can copy and paste it. hope that helps. and once again...welcome back.

Trouble is he states:- the only thing working is cut off switch, start button and the bike still starts and runs fine. Which implies the battery is working if the start button turns the engine over. Hence why I thought it might be Earth failure as he's lost everything else.

Hi there, As Blackhat says "loose connection!".....in view that you've lost everything, then it's likely that the main earth connection is either loose or corroded. Somewhere on the frame will be a metal tag with the earth wire attached. It tends to be in safe place in the central part of the bike, under the seat.(or in the vicinity of it). The negative terminal on the battery is NOT the same as the main earth of the wiring harness. NE0

Maybe not, but YOU did!!!... You're top of the Leaderboard with a gold medal!

How many filaments are in the headlamp bulb, if you can't see them, how many solder tags are there on the base of the bulb and/or how many contacts are there in the bulb holder? If there are two brass contacts in the headlamp holder, then it is wired for both dip and beam.

Well, after a bit of detective work on the bay....I've found the cylinder head!! The pattern of the fins is identical and the central plug hole with offset hole position are a perfect match. It's definitely from a dual shock 1974 DT175. https://www.ebay.co.uk/itm/324876641280?_trkparms=amclksrc%3DITM%26aid%3D111001%26algo%3DREC.SEED%26ao%3D1%26asc%3D20160908105057%26meid%3Df980f97296284269b5abe196a98fd057%26pid%3D100675%26rk%3D1%26rkt%3D15%26sd%3D324876641280%26itm%3D324876641280%26pmt%3D0%26noa%3D1%26pg%3D2380057%26brand%3DYamaha&_trksid=p2380057.c100675.m4236&_trkparms=pageci%3A79750975-4221-11ec-a706-ae56a761f765|parentrq%3A09d0a33017d0ab846808caf5fff351d8|iid%3A1 and a brand new one! https://www.ebay.co.uk/itm/334146920758?_trkparms=amclksrc%3DITM%26aid%3D111001%26algo%3DREC.SEED%26ao%3D1%26asc%3D20160908105057%26meid%3Df980f97296284269b5abe196a98fd057%26pid%3D100675%26rk%3D2%26rkt%3D15%26sd%3D324876641280%26itm%3D334146920758%26pmt%3D0%26noa%3D1%26pg%3D2380057%26brand%3DYamaha&_trksid=p2380057.c100675.m4236&_trkparms=pageci%3A79750975-4221-11ec-a706-ae56a761f765|parentrq%3A09d0a33017d0ab846808caf5fff351d8|iid%3A I'll remove the ebay links once everyone has seen them.

I was a bit confused at first OT1 ? in your text...., on zooming in to the engine number it's a 'C' not a 'O'....it's a CT1. Which is definitely a DT175 1973 to 1975. (Page 124 Haynes manual 210 brown cover) I know the early 70s AT2's (125 &175) had dual spark plug holes. As far as i know they weren't designed to run with twin plugs as one thread hole was often plugged. However, my mate had one and had both holes with spark plugs,(like your photo) when one became fouled up he'd pull the HT lead and put it on the 'spare'. It doesn't look like an AT2 though as the plug holes were either side of a central fin. Your photo shows one directly in the centre and one offset. P.S. you might want to remove the serial number in your posting to CT1 XXXXX . instead of supplying the full number to any potential cloners.

hi welcome, you'll enjoy the restoration I'm sure, just be careful as they do become bottomless pits for money spent! (speaking from experience!) That said my advice is with digital cameras and phones:- take plenty of photos, especially close ups, before and after you dismantle things, they can become a lifeline when it comes to putting all back together in the months /years ahead, even more so when you've forgotten how it looked. It also helps to identify nearby parts and how they appear. Just download them to a folder and you'll be grateful later. Good luck and you'll learn loads. PS There's nothing like getting your hands dirty for the benefit of getting something back on the road, but.....Protect your hands, disposable gloves, barrier creams all help. Dirty oil under the nails and ingrained into your fingerprints can take its toll and you can seem to take forever standing at the sink to get them clean again. Enjoy.

Hi there, you might want to change your serial number to 23F-XXXXXX instead of supplying the full number to any potential cloners.

Faulty engine kill switch?, Presumably it has one on the handlebars? Also worth checking the wire that goes to it too, if it's grounding before it gets to the switch, it won't matter how good the switch is if the cable has lost its sheath and the wire is grounding enroute. (I had something similar happen to me with my 400/4 one Christmas Eve!, unbeknown at the time, the cold weather had made a cable brittle and had exposed the wire which was grounding next to the steering head, just wouldn't start.... long push home for something so simple!)

You say you wouldn't be able to test the Regulator, but the regulator might actually be ok!, ...it relies on the metal body being in contact with the frame to complete the circuit, so in the first instance clean up the 'Earth connection' and check that the wire to the regulator is intact from the Ignition/light switch. We already know the wire from the magneto to the light switch is intact. (because the excess volts is blowing your bulbs!) but within the ignition/lightswitch, there are internal contacts which diverts the excess voltage via the yellow/white wire to the regulator. Disconnect the regulator and test the yellow/white wire which goes to it. You should get tha same rising voltage you see from the magneto, BEFORE it gets clipped by the regulator. This test would prove that the voltage is at least getting to the regulator! If it is getting as far as the regulator, clean up the earth connection, by removing the body and taking the frame back to bright metal to make sure of a good earth connection. If it isn't, (no volts at the wire) you need to check the wire for breaks all the way back to the Ignition/light switch. If the wire is intact (and still no voltage) then check the ignition/light switch, the internal contacts might be preventing those volts going to the regulator!! It might turn out to be new ignition switch is needed! P.S, if you've not got a Test meter, use a 12volt bulb instead, at least it will give you a visual indicator that the volts are getting there! (just don't rev the bike to over 4000v otherwise that bulb will blow too!)

Hi Doug, Yes it certainly points to the regulator not working. If you disconnect the yellow wire from the magneto and put a Meter on the end of it you'll get results like this... With the engine running and the generator disconnected from the rest of the bike, the output is as follows: Read the scale of the meter for AC volts. Yellow wire (lighting circuit) 2000 revs = 6v 4000 revs = 11.5v 5000 revs = 14v 6000 revs= 16.5v (This is my DT175MX lighting coil your result may be different) but the principle is the same.....now you can see why your 6v bulbs blow!!! the purpose of the regulator is to 'clip' back the ever rising output to a steady 6volts to run the lights. The regulator will get hot as it 'soaks up' the excess voltage, which is why it has the cooling fins on it. As you can see at 6000 revs it's soaking up over 10 volts! just to keep the output at the steady 6volts. You'll need a 6 volt regulator, presumably your bike is a DT175 1G1 model?

hi there, the output of the flywheel/magneto is AC...but....if it's not running...then you will only be able to test the output when you kick it over, don't expect a constant reading as the Flywheel will only spin a few revolutions. What model is it DT125 ?E ?F ?1F9. You mention "compensator" . In the UK it's referred to as a condensor, so is this a US or UK or another Icelandic bike?

So over a month on, have you now got it working? what was the problem.?

Hey Doug, There's a new old stock 1974 magneto on ebay Uk, Not cheap, but.... https://www.ebay.co.uk/itm/324829629386?hash=item4ba15acbca:g:yikAAOSw~35hZsJ1 you could make him an offer? Check it's the same one as yours of course.

You're welcome Doug, Think you're going to struggle to get one other than ebay or similar. You might have to think laterally , and instead of searching for a magneto.....search for another engine......just for the spare parts it will have.... to keep yours going. Doesn't matter even if it's seized ...if its got a magneto it will be worth buying. You won't need reminding that you're searching for 47 year old parts!...in a few years time it will be 50 years old!! and the bad news is ....I can remember being at school in 1974....and it doesn't seem that long ago!......gee life's short!!

I'm now convinced the part you're describing is indeed the advance and retard arm as you mention there is a sleeve which spins on the rotor body. For those that might be interested, this is the advance and retard mechanism for this bike. Typical of early car engines the timing cam was directly driven from the crankshaft. which meant the faster the engine went, the faster the timing shaft would spin. If the contact breakers (points) were fixed in one position and opened & closed accordingly, as the engine speed increased, the engine would not run properly as it would'nt be able to 'keep up' with the speed of the shaft. It was discovered that if the points were 'moveable' then they could be adjusted to open at a different position relative to the position of the cam. On a points based ignition system, the timing of points opening relies on moving mechanical parts to contol the opening and closing of the points. In an early 1970's car with an early LUCAS distributor, the points are attached to a moveable plate which is moved by centrifugal forces and is also attached to the vacuum of the carb. The faster the engine went the forces would move the plate altering the position that the points open. With a motorbike from the same era, there is not enough room to fit all these moving parts, so they created a system which moved the position of the engine cam instead. Here on the Yamaha DT of 1974, the points are mounted in a fixed position on the backplate. Unlike a car the backplate is not adjustable with this setup. Yamaha subsequently came up with the idea to advance and retard the opening and closing of the points by altering the position of the engine cam. In this case a sleeve which can be moved on the magneto body. Centrifugal forces makes the free moving sleeve spin faster, the arm and spring hold it in check, but in essence the position of the cam, when spinning faster, will cause the points to open sooner or later (depending on engine speed) Which hopefully answers your concern from your first post Doug:- No it can't be in a 'fixed position' otherwise it wouldn't be able to alter the timing. It's designed to move... but not by much. Hope this helps. NE0

Or Doug.... Having just re-read your post... I think you're referring to the Advance and retard arm which is fitted INSIDE the magneto unit. I looked on ebay and found a 1974 Magneto unit and there was a close up photo of the inside. Unfortunately, the part in question is part of the rotor assembly and does not have its own part number. You would need to source a replacement rotor (magneto unit) to obtain the part you are seeking. NB. For many of you who might be reading this :- Generator, magneto, rotor, flywheel are all (rightly or wrongly) terms used by owners worldwide for the same thing!