I recently bought a '74 DT250 (my first motorcycle ever, Honda mopeds non-withstanding) with ~7500 miles on it and have been learning how to ride over the past few weeks. Initially it was great, fired up first or second kick and ran just fine in my alley as I learned how to operate the clutch, brakes, etc. On my first extended ride however I was cruising along at 35mph when it started backfiring, slowly lost power and eventually died, then refused to start again. After sitting in the garage overnight with the plug removed it started, but ran choppy with a clunking/rattling sound coming from the engine. I decided to stop there before anything else broke.

I pulled the engine and have been slowly tearing it down, trying to learn how it works (no automotive background whatsoever) while looking for the root of the issue. So far the piston and clutch have raised a few concerns that I'm trying to dig in to - would love to hear some second opinions on my hypotheses and whether my planned courses of action seem reasonable.

Piston
Observations
There's bit of blow-by, light vertical scoring, and black residue on the piston interior, but my understanding is that these are not indications of major power loss. The main thing that concerns me is the rather large chip on the piston body directly beneath the lower locating pin. The lower piston ring was also fairly well stuck in its groove and not free moving, unlike the top ring that spun around freely. I also noticed that when inserting the ring into the cylinder to check the installed end gap it did not form a full seal against the cylinder wall. 

Hypothesis
My uneducated guess is that the lower ring was well beyond its life expectancy and experienced some sort of failure, potentially even breaking apart and causing damage to the piston as it was ground against it and the cylinder wall. The resulting gap between the ring and cylinder is leading to a loss in compression, but I don't yet see why this would cause a clunking sound. I took some measurements with a bore gauge and digital calipers and believe the cylinder is still on it's factory bore diameter of 70mm but worn beyond the clearance maintenance spec.

Remediation
From here I'm thinking it's time to rebore the cylinder and get a new OS piston and rings. I would have liked to go with an OEM +0.25mm piston kit but can't find one, so am leaning towards a Wossner +0.5mm kit which is readily available and seems highly regarded.

Clutch
Observations
The clutch plates have substantial rust/glaze/residue build up on them that I was able to knock down slightly but not all the way with some 400 grit wet sanding on a perfectly flat reference surface. More concerning to me is that the two pieces of the clutch rod are fused together into a single rod.

Hypothesis
The build up on the clutch plates seem like normal wear and tear, but I'm not sure if it's indicative of a deeper problem beyond long gaps in the bike's maintenance history. As for the fused push rods, I don't yet have a solid theory on what may have caused this. I did spend a few red lights sitting with the bike in gear and the clutch pulled in as I couldn't find neutral, which I've since learned is probably not a good idea and may have overheated the rod.

Remediation
I've read that in some models the push rod is actually a single unit, instead of being two independent components. At the moment I'm learning towards doing nothing, seeing as the clutch was working fine when the bike died.


Thanks for reading, any insight is greatly appreciated! I'm as green as they come and can use all the input I can get.

Looks like the locating peg came out of the piston allowing the ring to rotate and start to impinge on a port. You are lucky the ring did not shatter or wear away the land and bend down between the piston and cylinder.

That has the potential to write off the cylinder.

New piston and rings required. As for rebore, unable to tell from pics.

As for the clutch rod, it is very usual to have a ball bearing between the two pieces and someone may have put the rod in back to front.  But looking in a parts book, it does not seem so in your case, but I have no direct experience of this particular model.
further advice required.

Ring sure isn't good & gap on that second ring would sit 180 from where it is when running in the bore. Not that it matters & i realize pic is only to show gap & not touching so shutup 'me'.
& ring peg has gone as TINKICKER #2 says.
How did it feel for compression on the kickstart.
I only ask because it looks like its been running for a while like that with top ring doing the job of two.
So thinking it might've stopped for some other reason even though it looks bad & likely needs reboring to make it right. Piston kit at least & a hone--scratch.
First thought is how tight was the flywheel to get off.
Was the locating key in one piece.
Need the correct puller to remove it unless it's loose & falls off pretty much.
Re the clutch pushrod. That type needs the rounded end against the actuator on the left of the motor. But to run it like 'that' it needs a ball where its now fused.
Looking on Partzilla it doesn't show one so i don't know!!.
Either they forgot to draw it with ball or it has a double ended rounded off rod.
The only reason i can think that it would fuse together is--having it like that with the flat end against the actuator--the actuator would be less than happy with it spinning.
So if the cable clearance etc was about zero & stopping it revolving then the mushroom rod the other end would revolve [if as i said cable was up tight] so it fused.
That'd be a 'first' but there's lots of first on this forum lol, They keep coming.
Welcome from NZ too.
I'll post this then fix mistakes before i loose it.

This type of puller. 27mm left hand thread. Lots out there & cheap too. Fit multiple bike brands.

yambits.co.uk/dt3-flywheel-puller-p-24058.html

Yamaha removed the ball on some models around the 1974 redesign and then usually (maybe always?) redesigned the pushrods to use a ball again... because of issues like the pushrod pieces welding themselves together without a ball. 

Mostly was a problem on the 100/125/175...  can't say I've ever seen it on a 250.

Thanks for all the input, I see what you guys mean about the missing locator pin now, and agree that the top ring was likely doing all the work.

> How did it feel for compression on the kickstart.

After the bike died it felt about the same kicking it over on the side of the road, but with my limited experience it's probably best to take that with a grain of salt.

> First thought is how tight was the flywheel to get off.
> Was the locating key in one piece.

I haven't gotten to that side of the case yet, just getting the rest of the clutch assembly removed today. Thanks for the tip about the pulling tool, I just got one ordered.

After removing the clutch basket today I saw this hole in the side of the crankcase and got a sinking feeling in my stomach.. I'm still new to engine work but I don't think this is supposed to be here. Comparing to a new crank case right side I don't see the hole there, it's closed off as I would expect.

After the bike died and I got it back to my garage I forgot to turn off the fuel petcock and the next day on the first kick it made a huge bang, not the normal turning over sound. I figured it was excess fuel that had slowly leaked into the carb and engine igniting but quickly forgot about it after I managed to get the bike running (choppily), now I'm wondering if that was the death of my crankcase. 

So, is it as bad as I think it is? If so, are these types of things repairable, or does it mean the case half it toast and needs to be replaced?

Umm, thinking. I don't think that's something that happened.
See randon pic link below i googled for, or search for other pics.
If there was a BANG i'm thinking it exploded fuel in the crankcase & pushed a crank seal out.
Looks like the right side is still in place so go for the left behind the mag stator plate.
Might be retained on those so not easy to blow out.
Other--other thing is huige bang can happen if the flywheel is way around [key broke] & can fire when the transfers are open so goes bang in the C/case especially if it has a load of fuel down there.
All a wild guess on my part, but gotta styart somewhere.
See if the flywheel nut is loose & if so then flywheel might come off easily. Of course guessing again.
photos.app.goo.gl/pDabsDcYsiNNwKPc9
Oil comes & goes between clutch & trans thrown by the clutch gear back through the gap.
But did think they had a tin plate baffle across there but maybe 360's do. Not sure. No screw holes visible to mount a baffle either.

Two different problems me thinks . . . I'm with RT325 on the flywheel being #1 suspect for power loss & backfiring.

I have a slightly different take on what may have happened to the piston, but I'd need to examine it in person.

So here is my proposal . . . . Send me the damaged piston and ring, and I'll take it as partial "trade-in" ($50 off) on a new Genuine Yamaha 1st oversize piston and ring set ... that you'll need for a new bore job.  These obsolete pistons typically sell in the neighborhood of $200+ dollars, if & when found. 

However, my price is still at $159.95, and with your trade-in (for my collection of destroyed pistons) that'd bring the price down to $110 + shipping.
If we can work this deal out, I will give my version of what happened to your piston after examination.

Let me know ... you can message me

That's a relief about the case hole, will see if the flywheel key broke/rotated this week.

Thanks Enduronut! That's a generous offer that I think I'll take you up on. Will send a text soon.

Got the flywheel puller in the mail yesterday but it turns out I didn't need one. As RT325 predicted the flywheel and nut were loose enough that I could easily remove them both by hand. The key was still intact and holding the flywheel in position on the crankshaft though.

With that and the magneto removed I took a look at the other main seal. I'm not sure how obvious a blown-out seal would be but it seemed relatively intact (pic attached along with right side seal now that the primary drive gear is off).

From here is it safe to just reinstall the flywheel and torque it to the correct spec?