Try a Mitaka piston, they are far better than the Chinese made ones. New cylinders are available from eBay.

.  Err back to this thread.

The barrel is going to be a six week turnaround, so job stopped.  Oil injection line, fuel line and carb slide disconnected and carb put to one side.
Nothing to do but wait.
And wait.
And wait.

A month goes by and the devil bike slumbers.  The Sun shines, the skies are blue and the conservatory floor mirrors it nicely.
What! Why is the floor blue?

It seems that the devil bike got bored waiting and after a full month of only a couple of drips of oil in the aerosol lid to catch any oil dripping that may drip from the oil injection line, it suddenly and without warning decided to empty its full 2stroke oil tank all over the conservatory floor in less than 24 hours.


Thick end of £20 worth of BelRay SI7 busily trying to dye the floor tile grout blue.

Why?  I had no idea.  Ideas were suggested and checked.. Favourite being, was the tank breather hose blocked or kinked allowing the pressure to rise in the tank due to the heat in the conservatory?  Duly checked and nope.  Breather was in fine fettle.
Why after a full month with only a couple of drips getting past the check valve did it suddenly dump everything?

I doooooooont knowww Marcie.

Beelzibub appeared at my side, patted my shoulder knowingly and offered me a Valium.  He said they do help.

​​​​​​

​​​​

Oil pump seal kit and new check valve ball and spring arrived... Oil pump for the rebuilding of.

Found a bit of dirt stuck to the check valve seat which is the most likely culprit for the leak, but why?  Everything has been completely stationary for a month.  Why leak now?

Oil pump thoroughly cleaned out.  Oil tank thoroughly cleaned out (again) and oil supply line to the pump thoroughly cleaned out.
Pump refitted and waiting for the cylinder barrel to arrive to check max output.

I had a week off work and the day I broke up, I had a notification that a parcel had been delivered and was waiting in the shed.  Only one parcel being eagerly awaited and indeed it was the barrel.

Of course, being a driveline engineer with my particular specialty being the deep overhaul of medium and heavy duty diesel engines, I have fitted literally hundreds upon hundreds of cylinder liners of every type.  Wet, dry, prefinished, post finished, slip fit, interference fit, you name it.  So I was very interested in seeing what they had done.

Have to say the workmanship is superb. 
If you did not know it was there, you would not notice the join between the upper flange and the counterbore, such is the fit. 
Obviously been made, then fitted using liquid nitrogen and then bored to size afterwards, hence the very high accuracy of fitting.  Without a doubt, those join lines would be a hermetic seal.
Likewise at the bottom, you could hardly see the join.  
The ports were spot on, no mismatching or ridges between the barrel and cylinder.  If I handed the barrel to someone who did not know it had a new liner fitted, they would not know.

The final honed finish was superb.  In all very pleased.  I can recommend PJME.

 

Good to hear. PJME are currently reboring my 1975 DT400

Are PJME still doing cylinder re-plating I wonder?

"Are PJME still doing cylinder re-plating I wonder?"

I think that Paul from PJME is a member of this forum.

So we now have the bike back together.  I cast a gimlet eye at the oil pump cover and flip it the bird.
​​​​​
Screwed off the fuel cap and peered inside.  Roughly 3/4 of a tank.  Guess about a gallon of fuel.  Scratched my head a little and decided to add about 150 cc of two stroke to the tank.
Guestimate around 32:1 mix.

I do NOT trust that freaking oil pump even with new seals.

So we get the bike outside and ready to fire up.  Ignition on, second kick and she is away.  Sounds nice, but then after the previous soundtrack of a dumper truck falling over a cliff, anything will sound nice.

Ignition off and oil pump cover removed.  Pump bled from bleed screw and injection pipe pulled off carb.
Start her up again, push the pulley right back against the spring and up and away, oil and bubbles pushing up the line in one direction only, so it is pumping.

Give my measuring cylinder a good clean out, set the idle to 2000rpm and spin the pulley to the top of the ramp and hold.
Push the measuring cylinder under the oil line and start to count 200 pump strokes.
Is it pumping sufficient oil?

A quick check of the good book and my 5ml in the cylinder agrees with the book, so the pump is delivering correctly.

However I still do not trust it.  It gets another dirty look and two fingers before the cover is refitted.  I resolve to run at least two tanks of premix till the rings are bedded in and pump output rechecked at least twice over the period; however given that I am not allowed to put miles on the soddin thing, this may take years...

Time for a test run.  Surprisingly, with premix and oil pump, the exhaust was not smoky.  I was expecting to be leaving a thick fog behind me, but no, no drama.
Until I gradually built up speed to around 3/4 throttle and 5000 rpm in top.  Extreme vibration and very rough running.  What the feck was going wrong now?  I thought the big end had broken up.

Close throttle, vibration stops. Tentatively open throttle, normal running, close throttle still normal, open throttle fully..Horrible clatter and vibration.  Half throttle, perfectly normal.

Obviously a real rough misfire, I have never felt the like before.  It felt mechanical, not combustion related.
I surmised that the CDI was going waaay over advanced for some reason.  But why is it throttle position related, not RPM related?

Only thing I could think of that would be throttle position related would be a peak voltage bleed, either a bad plug, or bad ignition coil...

Put the soddin thing away in a great huff, gave it the finger with both hands because one would not sufficiently convey the extent of my loathing and stalked off in disgust.
I pondered over a beer or six that evening and remembered that it had developed a slight misfire at the end of the previous season.
Now suppose that was the start of the peak volts bleed.  It occurred with a cylinder that was toast. I had a new cylinder and higher compression now.  That would make the misfire worse.

​​​​​​Not messing, new plug and new coil ordered.  I do not care which it was, I just want it gone.  The coil resistance was double checked during the build and the plug was a brand new one for the full throttle carburation test.  It had less than 25 miles on it.
In the meantime I will go over the entire ignition system with a fine tooth comb.

The paranoia quotient was off the scale....

I could hear the old git who sold me it chuckling away to himself.  Good thing is that when the old git and the chuckling are in your head, you can mentally set a pack of dogs on him.

I feel much better now.

​​​​​


 

So with a new plug and new ignition coil on order, I set about checking the system out.
Unplugged the stator harness wires dealing with the CDI and the 7 wires from the CDI at the other end and checked them for a high resistance. 
Nothing.  They were good.
Paranoia kicks in with its typical lack of patience.  Is the NoS CDI I bought actually faulty?  It was cheap, very much so compared to others appearing nowadays.  At the time, it was the only one available.
Was it cheap because some cycle shop in the US had changed one under warranty 40 years ago, and the old unit found itself back on the shelf? 
Was it cheap because it was "open box" and the seller could not determine if it was in fact brand new, or a very lightly used old one?

I found myself looking on Rex's website and about to buy a replacement CDI.
Had to take a hold of my left ear with my left hand and pull me into a corner to have a word with myself.
Stop letting your paranoia about the devil bike make you impulsive and take a shotgun approach.
Cool down and work the problem as you are trained to do.  Be patient.

So deep intake of breath and start again...

Most likely point of failure given the circumstances is the high speed coil..

High speed coil checked and everything seems fine.  Within tolerance.


Next likely culprit is the trigger coil.  Checked and within spec.



Source coil.  Checked and in spec.


At least it confirmed what I already knew, the fault was on the HT side.  I can never recall coming across a LT fault that caused a misfire at a specific throttle opening.  Yes, loose connections getting jolted by bumps in the road causing misfires, but generally a bad LT side either works or it doesn't.

Checked the primary and secondary on the ignition coil and everything, as before during the build was fine.  However there was a tiny line of rust where the core enters the windings.  Had this rust penetrated inside and was allowing a path for the PV to leak to ground under high load?

Coil and spark plug went in the bin.  Stringent isolation procedures were in operation.

Finally I removed the rotor and stator to replace the new left hand crank seal with a better IT175 example and before the nut was put back, I checked the  rotor and stator timing marks again to ensure everything was peachy.
My luck would be to tighten the nut, then find I had a senior moment and the marks were out, and having to pull the rotor again, so checking first meant they would be fine.

I did have a moment of consternation when setting up the gauge and checking the marks.  They were miles out.  What the fecking great halls of hell was going on....
Then it came to me.  A senior moment where experience can work against you.  Timing a motor from the front usually means it is turning clockwise.
The DT is turning anticlockwise.   Doh!

Timing marks just fine.




But as I was looking I was wondering why the rotor was so rusty.  I recalled the bike history and came to another potentially horrifying prospect..... Was the crank bent?

 

As you can recall, the bike had been in a significant incident way back in 1979, involving the left hand side.

Gear selector mechanism bent internally due to a heavy blow on the gearlever.
Incorrect switch assembly on left handle bar.
Handlebar bent back slightly on left side.
Forks out of true.
Wrong indicators ( were they replaced with ultra cheapo ones because the left ones were broken?).
Tailight assembly twisted to the right.

And we have a rusty generator rotor.  Never saw one rusted before.  Was the generator cover, being plastic, actually broken in the incident, allowing moisture into the chamber and rust the rotor over the 40 years it stood?

You would not believe how that rusty rotor offends my sensibilities, even though it is behind a cover.  I have a constant urge to source a replacement.  However I have to draw the line somewhere.  The generator appears to work just fine and only the outer surface is involved.

However the prospect of the cover getting broken and subsequently replaced by the seller has sparked off another thing to worry over.  A very common theme in motorcycle accidents is the crank end taking a hit from hitting the road and getting bent.  I myself have seen this state of affairs several times including my 1981 Honda CB900F that a Mitsubishi Shogun 4x4, fitted with huge bullbars, thought would make for a good parking spot.  This was June 1st 1997.

The fact that I was sat on it and tootling innocently past at 15mph at the time just added to the fun.

The 900 had gone through the same process as the DT has about 10 years previously and was kept in showroom condition.
Once I was out of hospital, I visited the bike shop where the 900 had been recovered to on my crutches with a view to see if it was worth saving. 
I noted the smashed generator cover with the broken ignition reluctor  and advance unit poking out into space at an odd angle.  Was it just the unit and bolt that were bent? 
Key on and spin the motor over.  Damn, the crank itself was bent.  Must have been clobbered by the massive bullbar with two tons of 4x4 behind it.
That, combined with all the other damage meant it was not worth saving. 
I was gutted.

Not mine, but identical in every way including condition.  I loved that bike and had kept it 10 years.  Left side impact.  My left leg broke in several places and pushed the bank of carbs off the inlets and out the right side of the bike.  There endeth my hopes of becoming a commercial pilot.


Am I to be similarly confronted with bent crank now?  I needed to bring one of my gauges home from work to check.

Never has a motorcycle been so cleverly designed to resist a mag mount.  Not a single place anywhere near the engine that would offer a stable platform. 
Ended up tearing a piece of rubber off the stand to offer a clamping spot.
Any movement between bike and stand would throw out the reading, so I would have to be very careful.

Gauge zeroed and engine turned a full revolution.  Zero to 15 thou then zero, then 15 thou again, then zero.
 Holy crap it looks like the crank is bent 15 thou.
Retested it up close and personal and noticed the gauge was actually telling me that both readings were high spots.  Readings lower than zero are indicated by a minus sign.

Found a high spot and zeroed the gauge again. 90 degrees past and it was showing -15.  At 180 degrees it was showing zero, 270 degrees and it was back at -15.

The crank was not bent, but the rotor was very slightly oval shaped.  What does it all mean.

Don't ask me, I dunno.  Neither did the good book. It offered no advice or specifications on rotor runout or cencentricity.

Something else to keep me awake at night.  Just great.

Another heart stopping moment till I paid more attention.  I love electronic gauges because I can choose between metric or imperial readings at the press of a button, depending on whether I am measuring US made or Rest of the world made machinery.  However, they are not intuitive in operation like analogue gauges.  You have to pay close attention to what it is actually telling you.



The incessant rain (this summer has been a complete washout, rained heavily and cool just about every day since late June) finally stopped for an hour or two, so bike outside, start her up and put on my strobe to check the CDI.
Idle speed and bang on the mark.  Rev her up and ignition advances to max advance line.
Nothing wrong with the CDI.
Buttoned everything back up and now needed a spell of dry weather long enough to allow the roads to dry out.





 

So the rain stops, a strange golden glowing ball appears in the sky and the roads dry out briefly.

I take her out for a spin, about 15 miles of my usual run to bed in the rings a little more and with mag hanging out in the breeze to ensure max airflow around the coils to see if overheating coils are the problem.

No misfire.

Next test, fit the generator cover and try again.  No misfire.

Looks like as predicted, it was something bad in the HT system.  My gut feeling is coil, but either way, both coil and plug had already been ceremoniously disposed of by way of allowing them time to contemplate their fate.

Pedal bin lid open.  Plug held by tip between finger and thumb over bin.  Long drop there boyo.  Bye.
 Coil dangled by its ht lead.  Pause.  Goodbye.

After the test ride, the cylinder was lovely and quiet (as far as an air coolled two stroke goes) but I noticed on idle, a distinct rattle to the transmission.  Something I never heard before, probably because of the previous cylinder clatter. AAAAARGH damn this infernal machine, will it ever end?

I thought about calling in an exorcist or buying a large crystal of white quartz to draw out the evilness overnight.  But no , further investigation revealed that the clatter went away with clutch pulled in.

So likely some slop in the clutch basket, but it felt fine during the build.  Had the rivets come loose?

More paranoia.  Has it trashed the basket?  

Shoulders slumped, I rode it in the conservatory and noticed the headlight was not lit.  Tried the on off switch, yes it was fully forward and therefore on.  Bulb blown or wiring connection broken?

Trivial thing to fix,  but something snapped.

Just fecking great.  I hate that pile of ... Words fail me.  There is nothing contained in the human language that amply demonstrates my feelings of hatred for that bike, the guy from hong kong for starting the ball rolling on this endless round of mental torture and of the demon himself. 
I would so like to grab him by the scruff of his miserable neck and throw him headfirst into a dustbin.

I am now fixing things that was supposedly fixed when I bought it, things that I subsequently thought I had previously fixed to a high standard,  and now needed fixing again.  How far is this mad merry go round going to turn?  Absolutely everything has been attended to.  Only original bit of the original "restoration" was the powder coating on the frame.

I was seriously considering cutting my losses and selling it.  I was at an all time low.