if it is a 250,I bet each cylinder bore and stroke has around 125 CCs. this thing will be a screamer with high RPM as it's forte. it will love high revs.

a 350 would have 2 175 CC cylinders.

As Bigdog stated ...125cc each cylinder...

In stock trim the Ascot is rated at 35 hp @9500 rpm.... which is the same for the early TD1A roadracers.

Here's a start up video of a TD1-C slightly different that the TD1-A
but you get the idea...

Dave- Do you have the Honda starter motor option?

Let's hope I don't need anything like that !

Next I decided to pull out the stock YDS2 Iron jugs and compare them to the Ascot's aluminum TD1 barrels


On the right are the YDS2 barrels


The YDS appear to be in good condition and unmolested ...

the base of the transfers appear to be similar in size to the Ascot's.....but the similarities end there !



Here you can see the YDS's smaller bridged intake ports.....

Versus the Ascots much larger unbridged intake ports



Besides the intake ....the exhaust ports on the Ascots are a full 5mm taller.....
this explains the YDS2's peak hp at 7500 rpm's versus the Ascot's 9500 rpm

Also the YDS has a different intake manifold arrangement seen here.



I believe the screws located at the base of the YDS intakes are for the autolube connections ?

Answered my own ? I wonder why most m/c motors don't equal there stated size like my RT is a 351cc and most 125 = 123cc or 250 = 246cc ect. ? I looked up and most cars like 302,350,426 are those sizes.

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At 12-1 this oil will send ya to the Moon

The hp rating I have seen are .....25 hp @7500 rpm for the YDS2 street going and early scramblers
as per the ad below


And the Cycle World article rated the Ascot @35 @9500...

some performance detail from the article below....based on the depth of the article I believe the data is accurate.
235 lbs curb weight and 9.1 compression.



what I have found is that manufactures purposely undersize their displacements usually by a .50 mm so you can do stay under the max
stated displacement. For example a 175cc CT at stock 67mm is 171 cc 67.5mm is 174cc and 67.75 = 176cc.
So you get a couple of bore jobs before you exceed the stated displacement.

Yamaha making its name in racing here in the US. with the YDS2 based TD1....circa 1964



The 250cc YDS twin is good for over 120 mph in the right hands!

By 1964 some very capable riders were sporting Yamaha's....

the results speak for themselves.

Win on Sunday buy on Monday....these early Yamaha's could fly!

Al Gunter and Neil Keen were big time racers from back then.