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Old 11-03-2008, 02:09 PM
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Join Date: Nov 2008
Last Online: 11-24-2008 03:57 PM
Location: Cedar Rapids IA
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Default 1974 Yamaha DT125 Rebuild Thread

Brand new guy here ...
I wanted - and bought - a project bike for my 13 year old son and I to get running together. He's never ridden, and I've never ridden off-road. He's excited about the project, and I'm wondering if I'm in just a bit over my head.

I would say I'm capable of basic mechanics, and I can read a manual and follow directions. I have a fair set of standard tools but no specialized tools.

Here's what we bought -
The engine number/steering head number are the same: 444-120579, which my local old-bike expert tell me is a 1974 Yamaha DT125 - the PO said it was a '71 AT125, so that's one strike. BTW - the "local old-bike guy" has a NOS exhaust and muffler hanging for this bike on the rafter of his shop from when they sold the bike new. He comes very highly recommended.

The bike ran - roughly but at idle - when I picked it up, as long as it was connected to a charger.

Stuff I know about it (in no particular order):
  • The gas tank is totally gunked and rusty.
  • The oil tank is slightly rusty.
  • It idled in neutral, but when I tried to get it into gear, it acted like the clutch wasn't engaging - it jumped and died.
  • The front forks were replaced with a longer fork and bigger front wheel, but I got the original.
  • The headlight is held on with baling wire, as is the exhaust system.
  • The PO apparently cut the muffler off with a sawzall.
  • He also put an oversized rear tire on with tread so aggressive it chewed itself off on the hacked muffler with every turn.
  • No taillights, but the wiring harness appears intact.
  • It has some kind of aftermarket handlebars (with a renthal pad) but I got the originals.
  • No title, but I got a notarized bill of sale.
  • The few nuts and bolts I've taken off have varied widely - it looks like whatever any of the POs had on hand was used.
  • The most-recent PO said he rode it last year, but I'm dubious.

The guy at the local old-bike shop said I should sell this one and buy a slightly newer 4-stroke - that he thought this one was probably too big a project for a beginner like me, and that the old 2-strokes could die suddenly without warning even if I did get it running.

I got a 1990 XR200r, the DT125, and an old homemade trailer for $700, so I don't have a whole pile of money in the Yamaha yet. (The XR200 runs pretty well, and will be better once I get the carbs cleaned up.)

So ...
Restore or re-sell?

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Old 11-03-2008, 05:26 PM
2strokesurvivor's Avatar
Novice Class
Join Date: Aug 2008
Last Online: 11-30-2011 09:28 PM
Location: Dallas,Texas
Posts: 72

I would want the bike that I plan on riding on a regular basis with my son to be a reliable ride with parts that were easy to find. It seems to me with the time, money, and many potential headaches that can come with any restore, that you would be better served by selling this bike. I would focus on getting the '99 in top form and keep an eye out for a good deal on a something in the same year range for yourself. IMHO riding is not much fun when you spend most of your time wrenching.
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Old 11-06-2008, 09:25 AM
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Join Date: Nov 2008
Last Online: 11-24-2008 03:57 PM
Location: Cedar Rapids IA
Posts: 5
Default Update 11-6

Yesterday I took the rear wheel off to get a new tire put on. If you're in the Cedar Rapids area, I recommend Pazour's Motorsports very highly - a couple of guys I know who ride said I should go there, and the owners are extremely knowledgeable and friendly. OK, kind of crusty, but they know their stuff.

The tire had to be replaced - the PO had put a 4.00 X 18 tire on with aggressive nobbies, and every time the tire went around, the cutoff muffler chewed a piece of the tread off.

There was one spoke missing - how does THAT happen?

The right side axle adjuster didn't have a bolt in it - probably the reason the wheel wobbled when it spun.

The brakes look good - dirty, but OK, with some usable shoe left and no gouges or significant scratches.

Bob at Pazours ordered me a 3.5X18 tire, sold me a replacement bolt for the axle adjuster, and after checking out the sprocket told me that the odometer on the bike (2816 miles - yeah, right) was probably wrong. The sprocket is still OK, and the chain should be fine once I get it cleaned and lubed up.

The axle's in good shape, though there's a bit of rust on it.

Next up: clean the carb and take the clutch cover off to see why it shifts while it's running but won't disengage (or engage?) enough to get into first from a standstill.
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Old 11-06-2008, 09:39 PM
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Last Online: 11-24-2008 03:57 PM
Location: Cedar Rapids IA
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Son Tanner changed the order of work for the night - we pulled the front forks off, and were all set to put the original forks on when we discovered the cast piece where the brake cable attaches to the brake cover was (is) broken. Because it's a different size than the wheel that's on the bike, we can't just swap it out.

So we put the front forks and wheel back on. Total time working: just over an hour and a half. Total work accomplished: Zero.

But together we learned about pinch bolts, and the mounting system for the tach and speedo and headlight ears.

Also discovered that either the headlight or the wire harness is bad; and I have none of the bolts that hold the headlight on.

And I have no idea what bike the forks came from, so I'm going to be guessing in the dark about how much oil to put in when I drain and fill the forks.

And I know for certain I'll need a set of fork seals for a 34mm fork tube.
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Old 11-10-2008, 09:52 PM
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Last Online: 11-24-2008 03:57 PM
Location: Cedar Rapids IA
Posts: 5

Got the rear wheel back from the shop - one spoke, one tire, one inner tube, one wheel strip, mount, balance, true and dispose of the old wheel: $100 even. He even threw in a bolt for the axel adjuster on the right side, which was missing.

Threw the wheel on after work and took it for a quick ride up and down the street. Rolls true, and the motor is clearly gunked.

I can start it easily in neutral, but the clutch won't engage/disengage enough to let me shift into first. I've adjusted it (under the magneto cover) as much as I can, and no dice.

Start it in neutral, start rolling down the driveway, and it'sll go into first or second, and I can run it up and down through the gear as well as can be expected, given the engine is doggy.

So I popped off the tank and pulled the carb. Broke the carb down and soaked the parts for 30 minutes or so in carb cleaner, then used a spray can of the stuff to blast even more gunk off. The petcock was effectively clogged with rust, so I took it apart and soaked it in cleaner too.

Made a speed run to pazour's - got there five minutes before closing, and got the o-rings and gaskets I need to put the carb back together.

After dinner, Tanner and I put a handful of nuts and bolts into the tank, taped the petcock port shut, and dumped in a bottle of naval jelly and a quarter-bottle of water. Tanned shook the gas tank while I blasted the carb bowl and parts with more cleaner.

Set the carb parts on paper towels to dry, and cleaned/rinsed/repeat the tank full of nuts and bolts.

While I had the tank in the sink, went ahead and scrubbed it down with a soft sponge, soap, and hot water. Looks pretty good!

Put the carb back together and put it back on the bike. Ordered a replacement air cleaner element off ebay - the PO had a nasty foam filter in the box (not the right one, of course) and it disintegrated when I took it out.

The tank interior still needs work - lots of rust remains. So I'll soak it Friday night with Kreem part A and finish the tank Saturday. I hope to be able to put gas in it and see if I screwed up the carb Saturday afternoon.

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Old 04-08-2009, 03:11 PM
Welcome To ATM
Join Date: Apr 2009
Last Online: 04-07-2012 10:28 PM
Location: vegas
Posts: 2
flasher Dt125

Good Afternoon,
I have a 74' dt125 that i am currently rebuilding. I jut replaced a piston, wrist pin bearing($30), rings, new carb($60), tires, seat, and grips.
I pulled the handle bars off and took the headlight off and hand to unplug some of the wires that were behind the headlight. unfortunately i didn't mark where which wires plugged into which connector.

Does anyone on this site have a wiring diagram for this bike?

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Old 04-09-2009, 02:55 PM
Welcome To ATM
Join Date: Apr 2009
Last Online: 04-07-2012 10:28 PM
Location: vegas
Posts: 2

i bought a new piston for my 74 dt125 and its 1.0mm oversized. I need the specs on the motor. Does anyone have a service manual??!!


I started this project in November id like to finish this damn bike!!
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Old 04-09-2009, 11:06 PM
Welcome To ATM
Join Date: Apr 2009
Location: Midwest
Posts: 1
Default Good buy

Wow, I was looking at that same bike. As far as identifying the front forks, I would start by checking them against 1971 yamaha mx models. I don't know what to tell you with the clutch, but my old '74 DT250 was close to that (the clutch lever had to be within a mm of the bars to disengage the tranny). I only ran a 4x18 on my bike, but the 2-piece exhaust was weird on the 125. If you need a cheap battery, I have one I just bought in August that I am not and never have used (its just sat on my shelf for the last 7 months).

good luck
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Old 04-14-2009, 09:04 AM
Spikester's Avatar
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Join Date: Oct 2008
Last Online: 06-24-2014 11:17 AM
Location: Killen Alabama
Posts: 96

Let the Kreem cure out a few days before dumping gas in it and you can let the tank etch with the acid 24 hours if you want to. You have to rig up a block off plate for the petcock, be sure the Kreem coats all the inside of the tank, especially up around the filler neck. Did you take out all the carb jets and blow thru them, the pilot has a tiny hole and rust will stop them up easily.Sounds like your clutch plates are stuck together, it may free up but you will probably have to take off the clutch cover and take the plates out and clean them up. You will plobably need a hand impact driver for those phillips head case screws, they only cost about $15 and are worth it.
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