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Correct, 750 motors run backwards. 
I want to say that originally only the OW31 works motors had TZ350g (3g3 casting) 6 port but I read somewhere that they were standard on later monoshock production bikes. 409-70 and 409-71 share the same RD type 4 transfer ports plus boost port.  Not sure what the later 6 port barrel casting number is but they may have been used on E and F models, but I have zero experience or documentation on those.  If someone has a set of those or E/F parts book, that would be interesting.

According to the parts books 409-71 (4 ports) was introduced on the D model in production December 1976, so a 77 model as we would know them.

On a twin the intake port becomes imprinted on the piston skirt. OK for low mileages and race motors that are often apart, but not ideal for a daily driver.

Good idea.  I had a TD3 years ago and decided to reinforce the frame with a front cross tube.  The fab guy did an awesome job, but unfortunately the motor was now too wide for the frame.  Funny how that works...

So I took a saw to the cross tube and watched the frame tubes spring open leaving a 3mm gap in that tube. Not sure how the welder clamped it all up, but obviously he got it wrong.

Bolt the crankcases in and then cut and weld the new cross tube.

Mark,

Me overthinking things?  Puleeze..... OK. Guilty as charged.

So with absolutely no hard data to work from, I decided to browse the source of all knowledge - the might interweb and there are so many opinions and half baked theories that it's kinda hard to draw any useful conclusions.

Some believe that Iridium plugs have a different thermodynamic profile and that the same heat (number) is misleading and that while Iridium and standard are the same temp at full load they are different at light load conditions.  No data to support the theory but sounds interesting.

Others say that Iridiums improved power or smooth operating compared to Standard.

Odd incidences of Iridiums failing completely or misfiring.  Not well documented but could be there's some bad plugs out there.

Some suggestions that Iridium is mandatory in high power motors and others saying that standards must be used.  Two stroke sled manufacturers seem to like Iridiums and in some cases they want extended nose plugs and others say never use them.

The only good comment I found so far was that different applications need different plugs.

So if we discount all of the opinions, it comes down to why do some iridium plugs fail and are they actually any better in our low mileage applications, since none of us is expecting to get 100,000 miles between changes of plugs.

And in your case when the motor and jetting was set up running iridium plugs, did a simple change to standard plugs need the jetting to be revised?.   Looks like it needed a large jetting increase. When you rebuild it and set it up for standard plugs and jet it on a dyno or butt dyno, it will be interesting to see how much more main jet it likes and to see the difference if you run that setup with iridium plugs after setting it up.

Thoughtful response.  That misfire thing still doesn't sound right to me.  Doesn't mean I'm right, but let's walk through some of the possibilities.

If there's a misfire (electrical), some of the fuel and oxygen did not ignite.  That absolutely happens with most ignition systems but we mere humans can only detect that missed cycle if there are lots of them - relatively speaking.

So, on that cycle without ignition, fuel and air pass into the exhaust where they may, or may not burn.

In terms of gas analysis, we see that as a high HC level but the Air:fuel ratio which measures unburnt oxygen, doesn't really make a huge difference to the Lambda reading, and the unburnt fuel makes little to no difference to the plug color because nothing burned so no heat effect to see.  That's what we see with poor fuel atomization too.  If fuel droplets are too large, not all the fuel gets vaporised and burned.

If the spark is "weak" and lights the fire, the flame propagation rate arguably is slower because the flame kernel is smaller and less intense, so we might expect that to show up again as more unburnt fuel and oxygen passing down the pipe.

If that is the case, we should expect lower average combustion chamber temperatures and therefore lower plug tip temperatures which we read as "rich".  We know that when we read a plug, all we are really doing is to look at the temperature the plug reached, so rich jetting leads to lower combustion temps and sooty looking plugs.  "Hotter" plugs like say a B6 run higher temps and self clean at lower running loads, with the same Air:fuel ratio we read hot plugs as if they are lean and harder (e.g. B10EGV) will tend to look rich with the same A:F.

That said, if the fine wires are not generating a sufficiently strong spark, they will run cooler that they should.  We read that as rich and jet down.  If replacement plugs generate a stronger spark which creates a more complete burn, then you get the result you describe.

The problem that causes me is that fine wire plugs are supposedly deigned to fire more consistently with inconsistent combustion conditions.

Your situation suggests that Iridium plugs are not only not better, but are actually worse in a high performance two stroke.  I have not tested standard plugs back to back with say B*EGV or Iridium, but it's an interesting hypothesis.  Needs a multi gas analysis on the dyno which are not typically available on dynojet but are on eddy current dynos we were lucky to have access to a few years ago.

Left side looks much hotter than right side.  Looks like it's run lean but when it's detonating, the temperature rise tends to mask everything else.

The left appears to have eroded the front (exhaust side) and blown the particles down the pipe.  I prefer fine wire plugs on high performance motors and have never experienced that sort of meltdown with iridum or standard plugs caused by the plugs even with plugs of too low heat range - say B7s or 8s.

Working back a step or two:  If the old iridium plugs misfired, the exhaust temperature would probably be higher.  If standard plugs improved the burn, the motor should have made more power and exhaust temps should have been lower because the burn would have been more complete.

What else changed? Was it perhaps a different fill of gas that wasn't as good as prior fills?  Could the ignition timing have changed electronically for some reason?  The answer should be No F'n way, but stuff does happen. Did it happen at high revs/load and were riding conditions any different.  I'm sure you already asked yourself those questions and more, but that left piston missing the whole exhaust side ring land suggests that the problem was exhaust temperature prior to detonation.  Was air density very high that day?

   

TZ barrels have a number cast into the rear that represents model # 409 and year/ variant

409-70 TZ750 A/B 64mm bore 700cc

409-71 is later 66.25mm full 750cc version and so on

Interesting ports.  Is that a 40971? or later?

So if the resistance is low with say an LED bulb, it would probably be on all the time?  Was that an optional extra or OEM?  I don't recall ever seeing one on a bike and not sure I see the need for one.  I always check all the lights before I ride and to be honest I don't recall the last time a bulb failed.

Page 88 of the manual figure 5-1-2 shows the resistance values for the stator and rotor.  It does have the regulator and rectifier mislabeled though.  They got them back to front.

It is pretty comprehensive though.

Stop lamp checker relay?  What is that? 

Normally the brake has a switch which has switched 12v on one side and then goes to the brake light bulb and from there to ground.  You don't need an idiot light to let you know that the brakes are on, unless you seriously distrust the wiring.

If you wire up a front and rear brake switches, they must both have 12v switched on one side and the output sides should be connected so there's one lead to the rear (brake) light.

Not sure why you would want an idiot light or why there's a relay in the circuit except to feed the ignition.  The rest of the circuits should not be on that same relay feed to the ignition. If you want to use a relay on the light circuits I would recommend its own relay. I often do that for headlamps.

The flasher can is a relay and flashers should have an idiot light in the headlamp.  Has something be wired incorrectly?

There are a number of possibilities. 

The alternator generates Alternating Current which rises with engine speed.
The Rectifier changes that A/C into DC which is what our bikes use.The Regulator is there to keep voltage down to say 14.5 volts max.  Any more than that will fry the battery and bulbs.

There are two types of alternators on bikes - Permanent magnet and electro-magnet.  The RD are the latter.  That means that battery voltage is used to turn the rotor into a magnet.  The regulator switches the power to the rotor as required.

So, you need to determine which of those is faulty.  With electromagnet systems the usual problems are the Regulator/rectifier unit or bad connectors.  Sometimes a rotor will be bad or there's a break in the stator wiring but those are less common.

I would start with the regulator/rectifier.  I am not a fan of the OEM points type design, but they do work- for a few decades.  I would start by removing the R/R and replace it with a modern solid state unit. 

If throwing parts at a problem is not your style, Start by checking the voltage from the regulator to the rotor when the engine is running with a fully charged battery. 

Then you can test the alternator output which should rise with engine speed starting around 12v or so at idle, rising to 20-30v at 3,000 RPM.

You can also remove the rectifier and test each of the 4 diodes to be sure that it works.

If that's all  a bit much of an ask, start with a new combined regulator rectifier and see if that fixes the problem, but before you do anything, check all the leads and connectors just in case one is broken or corroded.

Also:  https://hvccycle.net/yamaha-rd-charging-problems/

Exactly.

Let's try a theory here that the smaller electrode Iridium plugs don't retain as much heat so they run slightly cooler that stock fat wire plugs which get hotter at the tip and cause overheating.

If a fine wire burns more of the available fuel/air mixture, and fat wire standard plugs don't burn as effectively, why does that create an issue when it's not on the pipe. Unburned fuel should theoretically cool the combustion chamber and raise exhaust temperatures. At lower speeds that shouldn't hurt, it should help. Mabye at high revs that hotter exhaust is pushed back into the barrel by pipe resonance, causing a temperature rise in the exhaust port and combustion chamber at high revs.

No worries mate.  Is Ron still doing the Fahron thing?  He knows his stuff for sure.

But while I'm here I have to inform you that the correct color for tanks on a red framed bike is bright metallic silver like a decent Triton or a small Dream 50 sports things from Mr H. 

That's you told so I am sure it will be corrected immediately, if not sooner.

You could do the tins in white like http://pinkpossum.com/td3/index.htm and that would look appropriate for a fast two stroke twin.

Don't mess about lad.  Get on with it..... 

In theory, if the choke is only on one side and the other is linked to it, then mixtures "should" be the same but in reality, it might favor the choke side.

That said, when the choke is OFF, there should be no difference.

What I have found a few times on a variety of different bikes with VM Mikunis is that the enricher plunger goes hard and fails to seal properly. That allows it to run as if the choke were partly on.

Check the choke plunger - they are cheap to replace and if you have the air screws at 2.0 turns, try going down one size on pilot jets.  With reed valves there will be less reversion in the inlets, so the motor will need to be jetted slightly leaner than the same set up on a piston port number.