Hi Fellas, can someone advise me if this oil seal is the right way round or not . its the main crank seal on the DT 175 1974 . thanks....the inner spring side is facing into the crank with the flat side visible next to the clutch ??

If you don't have a spring on both sides of the seal, someone has installed an SD "single lip" seal instead of the original SW "double lip" seal. 

Two strokes need to seal pressure and vacuum...  hence the "double lip". 

You might be okay...  For some reason Yamaha chose to use a single lip on the ignition side of the 125/175 Enduros.  There is a "dust lip" that will provide some sealing.  Probably won't last nearly as long as original though as Yamaha also used special materials on the crank seals.

I prefer to use the dual lip SW seals on both sides of the crank...  77-79 IT175 (and the YZ125) used the dual lip seals on both sides of the crank and are the same size as the 125/175 Enduro seals.

So just to recap is it the right orientation with spring facing the crank ?

To clarify, it sounds like you have the wrong seal, so technically not "correct". 

The way it's installed (flat side out) would tend to seal crankcase pressure better than vacuum...  so if you are or suspect you are drawing trans oil into the engine crankcase and burning it, that might be the cause.

Posting a picture using a 3rd party host and linking it here would be helpful.

Jez I would just get the correct seal and be done with it with my luck it would leak 

Hi Mark, thanks for your reply. for context i have restored this bike and fully rebuilt the engine using all new bearings seals etc . it did start and run for about 5 mins and tick over then stopped and i do suspect some oily deposits on the plug.i used an oil seal from a bearing company and not one specific for this engine  so that might be the reason it wont start now ? would you recomend one from yambits ? or try and find an original one .really helpful advice thanks .how do you send pictures to this forum ? best regards. Jez...

Yamaha still sells the original crank seals.  No need to go hunting for them, gamble with "aftermarket", or overpay for "NOS" when brand new is still readily available for the 125/175. 

(Wish I could say the same for most of the 250/360/400 models...  the crank seals for those can be hard to find.)

As I mentioned, I'd use the crank seals for the 77-79 IT175...  IT175 clutch side seal is identical to your DT175 but the IT175 ignition side seal is a better quality "dual lip" like the clutch side seal instead of a single lip seal.

You may have just fouled a plug or something...  it takes a decent leak for one of these engines not to run. It's impossible for me to know.  I would pressure and vacuum test to pinpoint any sealing issue rather than replacing potentially good parts but most people don't.

Posting pics directly is down right now but you can use Imgur, Google photos, or whatever photo hosting service you prefer and link here.


 

Hi Mark how would you prove the seal is good or bad is there a process for testing them ?

There are many posts here...  some may have been archived.  Here's an article on the topic and there are probably dozens of YouTube videos out there.  .  www.klemmvintage.com/airleaks.htm

I add the step of vacuum testing with a mityvac after any pressure leaks are fixed after having a seal that held pressure but not vacuum. 

Hi Guys, i found time to do some fettling this week . so i replaced the crank oil seals with the original yamaha ones from fowlers and replaced the spark plug and she fired up second kick. she is alive !!!! thanks for all your help. keeeeeep fettling. Jez..