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That looks on par with the set of US/UK spec repros I got.  They are a rougher casting than the India spec repros, but the ports are in the right spot, and the lining is thicker. 

Clean them up and run them!

Beautiful preserved MXer  are you aware of the issue with kick lever snapback breaking the clutch case? Was very common in some YZ465 and on into YZ490s. I once bought a 1987 that suffered the damage and learned the hard way about the history of the failure. It involves the short kick start shaft assembly with gear and spring. It occurs when the tab (pawl?) on the end of shaft that is supposed to stop return action of the shaft but sometimes breaks off and let's kick lever slam into clutch cover. I was addressed by Yamaha at some point, and I think there is a TSB on it. Fortunately, many of the years of 465 & 490 covers are interchangeable as well as center cases if it is real bad like mine was.

No, I was not - thanks for the heads up on that!

This bike will be laying waste to any sand berm in sight.  So yes, thoroughly flogged.

It kills me as I take it apart though, how everything is pristine inside and looks untouched from 1989.  Was just dirty on the surface.

Lifelong MX'er here, and I had never swung a leg over an open class machine until I just picked up this all original 89 YZ490.  And I mean ALL original, other than the bars & grips.

I was shocked when it started easily on the 3rd kick, and does not feel THAT heavy moving it around the garage.

Goes nicely with the 75 RD250/350 and 97 YZ250.

It's nice to be precise, but these old 2 strokes are very forgiving and will run under pretty bad conditions.  If the piston slides up and down through the bore, I'd run it and just break it in super carefully.

That said, I'd only run Pro-X pistons in my bike, after checking the bore. 

Someone, please make a convincing argument talking me OUT of buying one of these.  Please.

I can't...stop...looking

I run Kveldwulf jugs with Banshee Pro-X pistons on my 250 turned 350, and I use the 250 primary.

A tip with the Kveldwulfs - make SURE you are getting the US/UK spec.  I ordered what was listed as US/UK, and was sent India-HT spec.  The same seller on ebay sells both styles.  Measuring the port heights is an easy check.

[Concept:]

Thought I'd post my experience with the Zeeltronic programmable ignition on my 75 RD350 (with Vape, of course).  It wasn't hard to figure out, but I learned some things along the way I thought might help others.  Curious about anything others have to add or correct here:

Concept:
The Zeeltronic CDI intercepts the signal from the stator pickup and delays it before sending it out to the coil.  It can only provide as much advance as you set mechanically at the flywheel.  So you are setting your static advance to quite a high setting (as much or more than you anticipate ever needing), and then behind the scenes the Zeeltronic is retarding it to your curve. This makes total obvious sense, but I guess I never thought about it before.

Wiring:
It basically splices into the Vape Yellow wire, and gets switched power:  Red to switched power (I spliced into the Vape relay), Black to ground, Zeeltronic Brown in from stator Yellow, Zeeltronic Yellow out to coil Yellow.  There is also a resistor that gets spliced in.

Mapping:
My mildly modded '75 RD350 with Wicked chambers was kindof flat below 6,000 RPM and I was looking to improve that with some ignition advance.  The jetting was well sorted and as good as it could get.  I was running slightly retarded timing from stock at 1.8mm, which is 19* BTDC.  I made a curve maxing out at 24.5* from 5,000-6,000, 17* at idle, and down to 15* at 10,000.  Again, this is compared to a flat 19* (1.8mm) across the board.

How it worked:
It made a BIG difference.  Where before I might have to downshift, now I can roll on the throttle and it picks right up.  It made the bike totally fine for lower-rpm backroad cruising.  Before, I was running the stock pipes for casual rides but now I don't need to.  Most unexpectedly, my fuel mileage went up by 10 mpg with the Wicked pipes, to the same as I had been getting with the stockers (37 mpg on premix).

Other notes:
I purchased from Economy Cycle and it came with RD400 maps loaded. Not far from where I landed for my 350, but I did not use them.  Mapping is easy, and it's cool how you can run the bike with the programmer attached and see the timing advance in real-time.

(this post was 100% human-written - no AI)

I run Metzlers from Dennis Kirk.  "Perfect ME" 3.00 front for $68 and 3.25 "Block C" rear for $125.  On stock width rims.

I tried BT46's and could feel the extra weight versus the Metzlers with no performance gain.

Here's the YZ

I street-legaled a YZ250.  Did not go with SM wheels, but was still pretty fun. It's what gave me the bug to get an RD.

Looking for opinions/advice on adding boost ports on my '75 RD350.  The cutouts are already there on the liner, and I have the jugs off anyway.

I'm capable of basic backyard porting, but don't have a way to measure the angle of a boost port and am worried about making it worse than better.

I like your method of holding the exhaust plugs in for the leakdown - very clever.

Yes, I have bare pipes (not stainless) and all it takes is 5 minutes with WD-40 and a scotch brite pad every couple weeks.

Are they US/UK spec cylinders? I fought all sorts of jetting issues until I realized I had been sent India-HT spec cylinders.