Hey guys, I have been restoring my dt175 for a little while now, with reasonable success, except i seem to be fighting to solve my air leak problem. here is a list of mods/ restorations I have done so far

rebored cylinder with new piston
New base and head gaskets and exhaust gasket
New reeds
Gasket sealant on older reed block and carb boot
New genuine carb with standard jetting plus been cleaned multiple times and properly adjusted
Heat gun on exhaust and even cut open and rewelded to inspect
New petcock
New oil lines all over
New crank seals on both sides plus small print on trans side

all these mods have made a decent difference to the bike and generally it drives really well now, but I still have a couple of issues. On decel, it feels like it fires a 'power stroke' every 10 cycles so makes the bike pulse forward. At idle, I can't get the idle relaible below 2000rpm, and turning the idle screw 1/8 turn makes a big difference. When I come to a stop, the idle fluctuates from 1700-2500 rpm. The bike won't start without throttle open, hot or cold. The choke doesn't help at all with starting and really makes it harder. I have tried wd40 around intake during running with no difference. It doesn't leak oil or fuel

I'm 90% sure I still have an air leak somewhere,what other suggestions can you guys give me? Or what other things should I consider
Its a super cool bike that I really enjoy riding, and I'm so close to getting perfect, thanks  

Also, new coil, and I've redone/sanded a whole bunch of earth points all over the bike

My bike (DT400) had an air leak where the two engine cases join together. Had to split the engine and re-seal.

Were you able to see the leak? I will take off the skid cover and have a look

Without being able to test, I'd suspect the ignition side crank seal is leaking under vacuum.  

I'd replace it with a 77-79 IT175 ignition side crank seal which is a true "dual lip" seal designed to seal both pressure and vacuum.

(Best method is pressure and vacuum testing crankcase though... takes away the guessing)

Ok I will look into the double lipped seal solution, it will be just like the transmission side, makes sense

Just found some info on pressure testing, looks interesting, will give it a go

Just FYI, I got a really nice low mile always garaged 1979 DT175 for cheap because it had a massive air leak even the local Yamaha dealer couldn't fix.

I didn't know what it was of course... and then my heart sank when I read through the massive folder of repair receipts the previous owner gave me with the bike... The Yamaha dealer did pressure test and it passed.

When I pressure tested it passed with flying colors... held 5 psi for several hours. Then I hooked up my MityVac and it wouldn't hold ANY vacuum at all. Removed the CDI Magneto and packed thick grease around the seal and it held vacuum. New IT175 seal installed and it ran like new.

It's rare for the original seal to hold pressure and not vacuum... but be aware it can happen. Always test pressure first and find pressure leaks with soapy water. Fix the leaks, then vacuum test. Only thing that can really leak only under vacuum if it holds pressure is the crank seals.

Ok thats good info, I have found a double lipped seal, so I will buy that, and also start taking the engine out of the frame and figuring out how to set it up for testing

Well I have just been testing the motor, and it holds 5psi perfectly for hours. Bit of a shame as I was hoping to find something obvious.

I might attempt a vacuum test. Not quite sure how I will do it, I have an outboard motor priming hand pump which might work.

Or maybe I should just chuck in the double lipped seal and put it back together

Put the bike back together. All works as before. I have now tried to put in a 17.5 and then 15 size pilot jet, with reasonable results, it seems that some of my bad idle issues were a rich pilot circuit. I have also made the main jet bigger to 170. Still playing around with it but pretty interesting results, although I am surprised I am having to change jet sizes from factory