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labels we used to use would be Frankenbike or a "bitsa" with bits of this and bits of that glommed together in a project.

I had to sell my 1975s years ago while thinning the herd for major relocation. I do however still have 2 1974s and a nice set of Orange bodywork I kept that will go on 1 of the 1974s once I get around to building it. I might have just stumbled into a best of both year's combos for that project 

np. I'm sure you've forgotten more than I ever knew on these bikes 

any possibility of difference in clutch in 1974? I think I remember (no guarantee there!) the guy told me that was beefier when I pushed him to detail reasons the 74 was "best".

I have a 1973 RD250 Blue w/ White tank stripes and badges that is totally stock that just might get pushed to the front of the project line when everything finally comes home from storage this Summer  I'm basically starting over on all this stuff after diving into VMX full time 20 years ago. The 250 is the simplest of the bikes I have and will be training project trying to re-acquire all the necessary skills before moving on into 1 of my 74 350s and possibly tackle the 1KT TZR250R- unless I decide to sell it.

looks like a bastardized version of the Street tracker idea Moto Carrera used to make.

I always assumed the 1975 RD350 was the most technologically advanced and peak of development before RD400 introduced. Maybe I was just biased by how much I like the Orange/Black color scheme I do know stock main jets are smaller on the 1975 and assumed it was forced to run leaner due to USA smog BS or something- but don't know if Yamaha did that with performance in mind?

Sometime ago I was told by a former local (in SoCal) Yamaha Service Manager I knew through VMX racing that the 1974 RD350 was actually the best of the 3 years performance wise stock. Any verifiable truth behind his opinion? If true what are the differences that make the 1974 superior?

J-B Weld Cold Weld formula?  Package says rated 5021 PSI, Fuel & Chemical resistant. I've had success with JB securing rear wheel bearings that fit too loose in the hub.

yes, that Black scheme is eye catching I only remember seeing Silver RD400E in USA. Did we have other colors available?

I bought a Texavina solo style seat years ago for a RD350 project I STILL haven't gotten around to starting yet. Never test mounted it and it looks good overall, but the pan looks like it was hand formed by monkeys with ball peen hammers.

is the zirk/grease fitting still functional with bush in place there?

Treating the unique 1978E like that is just wrong on so many levels IMO the 78 is best looking of USA 1st gen RD400 and I would love to have a completely stock one someday. Never was a fan of the Daytona styling and "improvements".

so, is there any consensus on the forks with top out springs being preferred over those without? If so, maybe an easy bolt on improvement?

I have had mixed success with this on my Vintage Yamaha dirt bikes. Have multiple 1975 MX400B and 1 that ran perfect in races but had a broken mount for countershaft cover from a previous chain hit. Swapped on a Left center case from a parts donor basket case motor and the bike developed an air leak I could never track down that causes unpredictable WOT- usually on race starts- FUN times! I'm swapping back on the case with broken mount tab when I get time.

Also had a project 1987 YZ490 that had notorious kick start backlash case damage. Swapped on a new right center case from a YZ465 bought on Ebay that fit/ran perfectly. I opt for complete set of matching whenever possible nowadays.

YMMV

is that a current fair market value for a bike in that condition?

[the ring should be able to move]

i mean.. it could be trans oil but? doubt it in both.

no, the ring should be able to move . say hi to chuck. 

slight side tangent on stuck rings: I haven't rebuilt an RD top end in many years but remember there used to be a lot of discussion around the ring expanders that came with some RD350 piston kits. Some people said use 'em and some said toss 'em. Was there ring expanders application for RD400 as well?