It is hard to assess and compare subjective things like performance. I think you have the same bike I started with. Mine would top out at about 80 MPH stock but it lacked guts. It would get there but not in a particular hurry.

Maintaining speed in high gears with a headwind or uphill requires a reserve of power that this bike lacks. Top end power is okay but low and mid range power is sub par. This will make it difficult to maintain speed without downshifting. This is also unacceptable to me that a 351cc two stroke would have any difficulty with low and mid range power.

As far as jetting, stock is a good baseline. If the plug looks good and you are near sea level you're probably getting all the power you're supposed to be getting. No harm in trying a larger or smaller main, see what it likes.

Reading this thread makes my head hurt thinking about all the concepts

"Top end power is okay but low and mid range power is sub par. This will make it difficult to maintain speed without downshifting. This is also unacceptable to me that a 351cc two stroke would have any difficulty with low and mid range power."

Just a couple of thoughts. Don't shoot me; I'm just a 125/175 guy...
I had to learn two things when I bought and started riding Yamaha Enduros:
1) You can't ride one of these old bikes like you can a newer bike, nor can you expect the same performance out of the engines, regardless of their size. I had an 1997 XT350 for a while, and it pulled from idle all the way up, and you could just twist the throttle in 4th gear and get on it. (I am 5'7", 163lbs; that may have a little bit to do with it, too. Just sayin'.) Same engine size as yours, here, but 4-stroke, and 22-23 years newer in design.
2) A two-stroke is not coming apart if you are turning 6,000 RPM's. Took me a long time to get used to the power band starting as late as it does on my 125's. I'd be thinking I was just about to get hit in the nuts with a piston, but they're just starting to get warmed up at 5,500-6,000. I wouldn't cruise all day long right there, but on the way up to cruising, winding it out higher than a 4-stroke is not a problem but an expectation/necessity.
3) There are mods you can do if jets are not getting you where you want to go. Boysen reeds? Sleeper pipe? Different sprocket combinations? CaptDave (older member; not the new one) once said on a 125, you actually get a faster top end if you reduce the size of the front sprocker one tooth as it allows the engine to rev higher and make more power in each gear. Seems counter-intuitive, but you might do some calculations and see if that would get you the highway performance you want, as it certainly won't hurt your off-road performance to go down one tooth in front.
Hope this helps and does not offend.

Mothersbaugh - no offense taken, getting to 255 lbs took years of deliberate focus!

I think you are right on the vintage/powerband thoughts...I have no issue winding her up, but when cruising the massage makes certain parts of my anatomy numb from the buzz. I am going to take another ride soon, keeping things between 5 and 7k and will report back. Personally, I think the stock main and timing with a very clean stock pipe is about as good as it gets without stuffing this turkey

I was going to offer that 'spinnaker' was maybe the word you were searchin' for when talking about resembling a sail, but I wasn't sure my 125 would outrun your 360.
I might stand a better chance on foot, you say?

Only thing I would add to Mothersbaugh comments are that the stock bigger bores... especially the 400... do NOT like to rev. They are happier shifting 1000 or more RPM below redline.

That difference is one of the things I love about the 125/175... they like revving.

I don't think my expectations are unreasonable. Yamaha built some excellent 360 motors in the early 70's prior to this one. Something changed in the mid 70's and these big singles got much slower. I have never found good data on the 74 DT360A but here is the 75 DT400 from the archives on this site:



Almost 14 seconds 0-60. The old piston port RT1B 360 does it in about 5. Quarter mile for the 75 DT400 is 16.5 seconds at 74.5 MPH, the RT1B 360 is 15.39 at 82. The factory specification 1/4 mile time for my Bighorn 350 is 14.8, I also have a magazine test saying 15.8 at 80 MPH. They don't have to be slow, the older ones weren't.

This is a 1971 test of the RT1B if you are interested:
www.yeoldecycleshoppe.com/roadtestlibrar...maha-rt-1b-road-test

What that article describes is what I was hoping to find in the 74 DT360. I'm getting pretty close to it now, at this point, after several months of throwing everything but the kitchen sink at this thing. It's got a big carb, an MX360 pipe, modern reed block with carbon fiber reeds and a very fancy modern ignition system. Porting is not much different from stock, last thing I want to do is move the powerband up. I've been looking for guts all along. Lots and lots of torque. Fortunately the cure for the inconsistent carburetion and pinging also punched up the low-mid power. That was nice.

I'm actually geared a tooth larger on the countershaft sprocket. First gear is short with stock gearing. For road usage, longer legs are nice. That ties into the whole point of a big bore. Displacement makes torque, shouldn't need to wind these things out to get anywhere. A respectable 360/400 should be a short shifting machine, acceleration should be effortless by rolling the throttle on and wicking up some ft-lbs from the bottomless well of torque. Hills should be completely irrelevant. Head winds should be no problem at cruising speeds regardless of the rider's cross section.

Agreed

But I mistakenly try to cruise under a high load condition in midrange to avoid harsh vibes when I should be in powerband.

360+ a larger reciprocating mass that would massively benefit from a counter balance shaft. But that would be modern and there would be less to play with and chat about.

Interestingly the MX version of the later 360s of that era did have balance shafts.

Not being familiar with the 74 360 at the time that I acquired one, I was curious as to what was housed in the cylindrical casting protrusion below the reed cage. It turns out to be where the counter balance would be had it been an MX.

Back on topic.

Interesting read on the Cycle World article of the non-reed, 4 bolt engine performance.

Looks like there is opportunity to gain considerably more performance from the reed, 6 bolt. And do it without needing to turn higher than 7K rpm.

One of the things that struck me hard a few years ago was how SLOW these bikes were compared to my memories of them! They used to be quick compared to traffic... now you need to ride hard to avoid getting run over!

Today, the cheapest econo car is a rocket ship compared to what we had in the 70's... a VW beetle struggled to go 60 mph! An average sedan today would give a mid-70's muscle car strong competition.

I'm sure the 5 seconds 0-60 mentioned was meant to be an exaggeration because a stock RT1b would have been 12 to 13 seconds 0-60 at best. That's extremely slow by today's standards. But quick back in the day when a 1975 Nova with a V-8 did 0-60 in about 15 seconds.

By the mid 1970's all vehicles in the US were struggling with meeting emissions standards... exhaust and noise... Yamaha was no exception... but they fared pretty well in my opinion. But early 360 to later 400 are not fast by today's standards.