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XS650 disks and carriers fit fine on a TZ250/350 wheel and the forks are perfect for a stock RD caliper which works just fine on a TZ.

And if you can't find a good TZ250/350 C/D/E front end or and F or G front end, try a pair of XS650 sliders on RD forks. The sliders are longer but take stock RD type calipers and work with 298mm disks.

Always more than one way to skin a feline.

Or get a complete RZ350 or FZR400 front end and upgrade wheels and brakes.

Economy caliper is super light - especially when fitted with alloy pistons.  On a race bike it would flex/expand the same way as the TZ alloy caliper did but on our drag race TZ/RD it works fine with a thin disk.

Just a thought, but could you weld up the inside of the barrels at the mating flange and weld it flat and then do the same on the heads and cut a new O ring groove for a slightly shorter O ring - basically move the ring groove inwards away for the face that was machined for clearance.

Had an RD350 on the dyno a while ago that was leaner one side than the other and the carbs were perfect, so we swapped out a pilot jet to one size larger on the lean side and everything was fine all the way through the rev range.

We have to remember that jets are sized with a number but they actually fall within a range of flows, so one might be at the fat end of the tolerance or may flow more fuel that it should. 

Are the jets new and are the pilot air screws absolutely identical? There may be slight differences.

Even better.  aluminum frame dates it anywhere after 89 IIRC.

Would be funny if it turned out to be our old bike.

Maybe off topic, but how will a 4 cyl RD type crank with RH end primary drive hold up at TZ750 power levels?

That's a good question.  TZ750 takes power from between the two cranks IIRC.  Eqach as a coupling gear that holds them in synch, but it's two short cranks, not one long one.

On 180HP triple sleds, power is taken off the end of the crank, but the PTO end is pretty chunky and typically has a double row bearing to reduce shaft whip.  The Mag end is also typically pretty beefy and again uses a double row bearing.

Those are viable options with say a bolt on end bearing holder. 

They are not exactly cheap though....

Then neither are projects like this one....

And where's the challenge in that?  Built not Bought, as they say.  Some of us like a challenge.

That set up will not hold water, so the choices are:

Start again and make the combined crankcases wider and source a new pair of barrels. OR

Turn the pair of barrels into one monoblock and do the same with the heads.  You will have to clamp the parts and pre-heat them to minimize distortion. It may not work, but I don't see a way to get individual heads to seal with no surface to seal against.

That issue is why Yamaha made the 750 barrels and heads narrower than 350 parts so they could make the motor a little less wide, but this one looks like it is too narrow.

It might be possible to machine either the barrels or heads so that the combustion chambers still seal with stock O rings and the water jacket is sealed with a copper gasket. 

Take that idea one step further.  Machine the heads down by slightly less than the thickness of the custom copper head gasket and then machine out the combustion chambers to take Banshee domes which will sit proud of the lower head surface. The copper had gasket would have to be custom cut with "bores" large enough to clear the outside edges of the domes.

I have a GT750 head that was modified like that.  The domes sit lower than the lower face of the head and protrude through the gasket to seal against the top flange of the bores and the copper gasket is just there to keep the water in place.

Even with that set up, you don't have a lot of surface left to seal against.

I tried something similar when I wanted to graft Polaris XCR800 barrels onto a GT750 crankcase but bore spacing is too tight and would have left paper thin (or non existent) transfer port outer walls, so that would have required a crank redesign and new crankcases, so at that point I decided it might be simpler to grab a complete XCR motor and mate it to the back half of a GSXR crankcase for the transmission.  That's how project insanity takes off. 

IIRC, the 400 motor sits a little further forward than earlier RDs and ss far as I know, the difference between a late model RD250 frame and early one is that the 400 has a removable front engine mount to allow the taller RD motor to be fitted with the heads on.

What I don't know is if the later 250 also changed to the 400 engine position or kept the earlier geometry. 

The 400 motor should fit without any issues and if you fit a longer swingarm, it should help to keep the front end down.

If you go with a Lithium battery of any design, you need a regulator that is suitable.  That probably means using a MOFSET type, but do your research and test the output of it after you fit it.  Excess voltage/current when charging is the main cause of fires with Lithium batteries.

Lithium batteries big advantages are weight, or lack thereof, and the fact that they can handle huge discharge current but on an RD that isn't necessary.

I am guessing 1990 or within a few years of that.  TZ125H (1980 was steel framed and for some reason I thought they stopped production and restarted with the beam frame in or around 1990.  But I cannot swear to that.  Try over on NATS where racers tends to hang out.

Try RSCycles for parts or ask on NATS

Both of those barrels ahve been modified/screwed up, which is probably why they were available.  I hope they were cheap.

The top (boost) port looks like it has been modified and needs to be squared up. 

The lower port used to improve crankcase filling is way too deep and has broken through at the crankcase mouth, so it's junk as it is is.  It can be welded up and remachined back to stock but that adds to the cost of re-plating.

BTW, what are the casting numbers on the barrels 40900 or 40970 or 40971 or something else?

409 00 are TZ750 a/b and are 64mm bore for a 700cc motor.,  409-70/71 are later 66mm bores for full 750cc motor.

I suspect that the battery manufacturers don't realize that teh RZ not only does not have an electric starter but it also has a self energizing ignition, so you can get away with a much smaller Lithium battery.

If the (RD air cooled) has  DYNA ignition or spends a lot of time crawling in traffic, you may want to go up a size.

In both cases you will need a Lithium compatible regulator.  Old designs for wet cells are not accurate enough and allow over and undercharging, both of which will kill a lithium based battery.

For 12v you need a motorcycle specific battery which IIRC uses A123 cells with different chemistry that say a 4 cell RC battery which runs closer to 14 or 15 volts.

The real question though is where is it rich i.e. at what throttle openings. 

If you post the needle number you now have, we can see what else might work but it all comes down to where it's rich.  Lower mid range it's easier to change the needle jet, mid to upper midrange the needle has more effect.  Post the needle number and we can put that into simple excel spreadsheet software and compare the annular area with different jets versus different needles. 

Not exactly.

At idle and just above idle the thick parallel part of the needle sits in the needle jet , so moving the needle up or down makes no difference at small openings, but increasing or decreasing the needle jet size will make a difference.

Except that at low throttle openings, almost zero fuel is being pulled up through the needle jet, so the actual effect will be three fifths or five eighths of sweet F all. 

Once you get up into the midrange, raising the needle or increasing the needle jet size will achieve a similar result. Not exactly the same on a dual taper needle but pretty close.

The short version: go ahead and try P-2s.  On your bike we have no way to determine how much it's too rich, so we would be guessing, but that really is the simplest option unless you look at needle profiles to see one that is thicker where you need it to be.