Best Start For A Yamaha 360 MXer

[Tahitian_Red]

I've really been bitten by the Old Yamaha Bug.  Thanks a lot Firko!

Now I'm selling parts I've stashed for other future builds to start on a pre-75 Yamaha Big-Bore.  Would an RT-2MX or one of the 73-74 MX360's be the best place to start this project.  I'm leaning towards the RT-2MX for the cool factor, but have heard the motor had a peaky powerband.  Could this be fixed or should I just go with an MX360 a or b?  What are the pros and cons of each?

[vandy010]

my 2 bobs worth,
peaky motors can be changed with all the info we have these days.
the MX or even DT360 is a better bike that the RT2MX but not quite as much "cool Factor"
but really, i'd be happy with either.

[firko]

I might be a tad biased but I reckon the RT2MX is the winner if just for its rarity and coolness. The peaky power delivery can be mostly smoothes out by fitting a heavy outslde rotor ignition, say, from a '75 period DT400 or maybe an early IT 400.

Thanks a lot Firko! Grin

You're Welcome Red but you're making me feel a bit like Charlie Manson or the Rev. Jim Jones for drawing you into my evil cult.

[Rossvickicampbell]

go the YZB 360 

[Freakshow]

Depends on the track if your MXing maybe the Rt if you want to go the full retro original look, but not a bastardised one.

IF you were going dirt ciruit i would go the MX 360, pull heaps and easy on the parts and reliable as all fork. 

[firko]

go the YZB 360  Grin

That's OK for you rich Western Australian mining magnates but us poor folk in NSW and Northern California are stuck with working class tackle .
Another good choice is the humble MX360. Like the RT2MX they're a bit on the lumpy side but they handle a smidgey better than the earlier bike. The engine's are positively bulletproof as everyone who's had one can attest. My first choice though is still the RT2MX because I think the earlier motor is a lighter and better unit. These things really come alive with some sensible porting, upgraded ignition and a decent pipe. A downside that immediately becomes obvious is actually finding one. They're pretty rare, even in the US.

[Tahitian_Red]

Firko,

When I purchased my current RT-1 I had to pass on a $400 RT-2MX being sold locally.  It was mostly there and mostly together, so I had to let another one get away.

Wasn't the YZ360A motor based on the RT-2MX?  When were the balancers installed on the 360 motors?

[firko]

When were the balancers installed on the 360 motors?

With the MX360. I don't reckon they're needed though. I had my RT1 crank balanced and it's now as smooth as a sewing machine .

[Iain Cameron]

As an owner of a MX360 all I can say is they are nigh on indistructable . I race (wobble ) both the RT in pre 70 and the Mx in pre 75 both have there faults but the mx wins overall I feel . I do have a RT2MX but the owner won't sell it to me yet . Hi Chris big hint . Iain

[OverTheHill]

Borrowed the Boss's SC500 when it was only a few months old & disconnected the balancer, "never told him" [just took the chain out] & drove about 6 hours to a christmas mx event just to get vibrated stupid, was bloody terrible [another bad theory proved--which was probably less rotating mass = more power or more acceleration etc]lesson learned. Same basic motor as mx360, & a good strong motor compared to the more fragile RT2mx [in my opinion]. Also [i think] the sc500 & mx360 use the same stud pattern for the cylinder & did sort of look into the possibility of putting an sc500 cylinder onto a mx360 [was actually a dt360a but same] both have 70mm stroke but the 360 cases were cast right up to the sleeve so a bit of machining to get the sleeve down there & she might be all go--[bang!!].

[Tahitian_Red]

OTH,

What makes the RT2MX motor more fragile? 

So, a different pipe, heavy flywheel and a little port work should tame the RT-2MX?



Passed on an affordable MX360 recently, but I have access to an RT-2MX frame and motor. I also have YZ forks and wheels on the shelf, so..................................... 

[OverTheHill]

maybe being a bit unfair on the RT2mx, had a DT2mx new & was bullet proof but most things new are good. RT was hard on kickstart gears but ok with heavy flywheel [no decompressor bleeding off into exhaust port to help kick starting that the MX360 has]. When they get older the gears seem to round off dogs etc & selector drum wears at the high point where selector stops when in gear. Also can be fragile around the bearing in the center case behind kickstart idler gear. Clutch lifter is still the old design & plastic worm splits then binds in operation. Ignore me--just have boxfulls of worn out bits related to that shape motor starting with the dear old DT1 [but i just keep fixing them don't i so must mean something!!]. Bit more length in mid section of pipe might help spread the power to lower down on RT2mx[guessing]. MX360 is the start of next generation of motors--heavier--longer--stronger--better gearbox selector gearchanging set up etc etc. Had a rough old DT360A that i rode to work, thrashed in the weekend & just never gave up. Sorry for the essay--bad habit of mine. Someone might have other opposing views so let's hear them. Cheers.

[firko]

The way I see it, the YZ360A/B was the hot ticket in Yamahaville prior to 1975 even though it was based on the humble DT1/RT1 core. It's not a major engineering mountain to climb to get the RT2MX up to YZ360 performance specs. Sure, the transmission ratio's are different and the YZ's trick clutch and primary drive are hard to locate but as far as the cylinder, head and cases go, the RT2MX can easily be made to achieve YZ type HP figures despite what the sceptics will tell you. The wheels are the same as the YZ and while the forks might look different externally, are as close to identical internally as you can get. I've currently got YZ250A, MX250, DT2MX and DT250 forks apart and they're all within a cooee of being identical internally. I'm getting Uncle Frank to machine the brackets off the DT2MX forks so they'll resemble the YZ forks externally. The DT250 forks came from America fitted with Race-Tech emulators and special springs and they've got the nod to be fitted to the Hindall RT2MX, I'll possibly get Frank to machine the brackets and a layer of superflous alloy from then as well. All of the 'problems' Over the Hill related are there but the kick starter gear and strain on the main bearing area are reduced by using a later CDI ignition....less effort to start the cranky buggers. With points ignition this is a major problem. The Achilles heal is the stupid plastic worm gear clutch actuator but I'm looking at trying to modify a steel XS650 worm gear which I've been told turns the clutch movement into a "hot knife into butter' scenario.

Over the Hill has also identified one of the problems in the RT2MX in the pipe needing some more centre section. The problem seems to be that the factory used the identical same pipe on both the 250 and 360, showing that they weren't quite on top of exhaust technology in 1972. I've got a period Torque Engineering cone pipe for my RT2MX and it has a fatter and longer centre section. I'm certain that some modern pipe technology by GMC or someone else could improve the exhaust function far better again.

The MX360 is a good solid engine that really isn't all that different from the earlier motor inside except for the horsepower draining balancer. I had an SC500 fitted with a 360 5 speed gearbox and no balancer and it didn't shake itself to bits. In fact Brian Clarkson had a very similar set up in his SC500 road racer and it worked a treat.

[Rossvickicampbell]

Firko - from the bit I have seen I think the big advantage for the YZ is the suspension - makes such a difference getting the power to the ground.  Motor wise I think they are all awesome.

I know it is well within your capabilities but a mate made the same mods to his YZB 250 you are talking baout for the clutch actuator - much better than stock but still a significant pull.  Can put you in contact if need be.

cheers

[OverTheHill]

wrote a story & lost it--i dunno!! Guts of it was--you've got me going now & have a YZ360B cylinder sitting here [yes dear i'll get that dirty bike thing off the table in a minute & no i'm not going to start porting it--compressor line won't reach!!]. Plan [one day] is to use it on my RT2 but rear studs are wider apart for the bigger reed block compared to the RT2mx but anyway, just have to come up with a plan to plug the stud holes with [i dunno] thread some ally & screw it in tight then start drilling maybe!!. YZ360A/B have a sleeved cylinder with 2 oversizes available & mines on 0.25 as opposed to the YZ250a's chrome bore. DT1 GYT chrome bore was good to use in the late "60's [before i stuffed it] power stayed sharp till the end & didn't loose the sharp edge of power when hot. Have a 'rat bike' that goes better than it looks which is YZ250 A frame & engine cases with straight cut primary gears but using a DT1GYT kit top end & DT1 crank, DT2 gearbox etc[best of the rest trick]. Looks 'rat' & my kids won't stand beside it in case someone takes a photo!!. Just thinking back the RT2Mx & RT2/3's use a reed block the same [size] that my kids YZ80 used in 1982--maybe back to 80-81, not sure what i'm getting at but progress i suppose.