I can vaguely remember reading a good while ago that the MX175 (twinshock), came about as the result of a Japanese rider, winning a prestigious event on a similar bike?

Wonder if that's credible at all, or just a good example of a not very good memory?  Asking as someone who owns a quite nice one, contacted me recently.  

There used to be very popular off-road 100cc and 175cc racing classes in the USA. Many manufacturers made models because they sold them here.

I believe it was Larry Shoemaker that won a Motocross Championship (in the 250 class!) on a modified Yamaha 175 (CT) in the early 1970's (1971?), he wasn't Japanese though.

When the AMA started a National Motocross Championship in about 1972, they chose to have only 125, 250, and Open (500) classes. Within a few years, the 175 and 100 classes sort of faded away and so did the bikes... Enduro and Desert might have lasted the longest.

Thats interesting. Think there were parts kits to fit to some models, to make them work better for racing?

I use a 74 MX175 top end on a AT2mx bottom end, 30mm mikuni , PVL ignition and a custom made Scott Clough pipe. AHRMA age group weapon.

Is the MX 175 porting very different to other 175s? If you have a twin spark plug head, you could probably run a dual spark ignition if you get a 2 lead coil.

I run a webco head.

If it has 2 spark plug holes, your PVL should fire a twin lead coil.

The WEBCO head is a CT1 head, 1 spark plug hole.
 

Capt.Dave - Do you use the WEBCO head modified to domed combustion chamber or the original stepped shape of the WEBCO?

Anyone tried firing 2 spark plugs, on bikes fitted with CDI ignitions? Its something that was used on factory air cooled YZ495s back in the day..........