Hi team, i tried to find the answer to leakdown but to no avail.  Im a new guy with new 1971 AT1C project.  See pic below.    Long story but have cleaned carb a couple of times and tried a new china made carb.  I have tested compression and im at 135 after new piston and new rings.  I was at 125 but i went ahead with the top end because old owner said it needed it because cylinder would fill with oil.  I think im pulling tranmission oil into cylinder.  I am running premix and i have tried disconnecting auto lube.  No changes with that and no changes with any use of choke and or carb adjustment.
How do i connect leakdown air to the intake cylinder as it a flush mount?  PVC wrapped tape perhaps?  I understand i have to use less than 10 lbs of PSI and i have to use rubber stopper at exhaust port.  I hear some folks stating use spark plug hole.  im not sure that would help as i have new rings and piston.   
Also, could i use this old air compressor tank for my guages (see pic below) although they go up to 100 psi or would i have to have a guage that reads like 15 psi.  Any help would be appreciated.  Just trying to figure out why my 1971 AT1C will only run at only 2000 rpms at wide open throttle.

This is a good read, which should help.
www.yamaha-enduros.com/index.php/forums/...ak-down-test?start=0

I recently did a leakdown test on my CT1C which is very similar to your AT1C.
I made up a test kit for minimal expense and used a bicycle pump to pressurise things.
Here's a link which shows my set-up, but there are plenty of other home made solutions that will work.
www.yamaha-enduros.com/index.php/forums/...restoration?start=70
I'm sure the experts will be along later to help.
All the best.
Pete.

Thanks and yes yours is a flush mount just like mine.  It appears you drilled corresponding holes to the mounting flange and then tapped hole in middle to add the air.  I will start looking for some scrap metal or plastic.  I have plenty of rubber i can use as a gasket.  If no one comes up with an eaiser suggestion, i will try that.  Thanks for your assistance.

I can easily test through the spark plug hole as i have compression tester, but does this test the engine seals? I thought this would just check head gasket and or ring leakage? Am i wrong?

First, piston and rings do not interfere with this type of pressure test at all.  I use expandable rubber plugs in both intake and exhaust and use an old modified compression tester hose to add the air.  Don't go over 5 psi, not 10.  The two stroke engine needs to be air tight... .

So what you do is pressurize the crankcase and cylinder with a small amount of pressure and see if it holds over time...  just like a tire but at 5 psi. 

ok great, i have an old compression tester hose as well. That will make it easy for me. I already have the freeze plugs. Thanks to all for the guidance. Hopefully it will identify why i cant get my AT1C to idle and or run correctly. It may direct me back to the Carb which would be great news, lol

You can test through the spark plug hole, there's an example on the link I sent.
If the piston is below the transfer ports it will pressurise the crankcase too.
Just to point out I only did a pressure test, I didn't do a vacuum test, but the pressure test will give you something.
My compression tester has a check valve, so wouldn't show any leakage and it doesn't incorporate a schrader valve to introduce any pressure.
I am far from being an expert with pressure/vacuum testing, but the folks on the forum will chime in soon for sure, probably to tell you I've got it all wrong

Even if piston is at TDC, it won't matter....  within moments (at most) the low pressure will equalize through the ring gaps.  Some prefer to have piston at BDC but it doesn't matter in real life.

And again, not sure how long it's been run, but it still could just be loaded up bad...  can take several minutes to clear out a crankcase full of oil that often happens when bike is parked for years and autolube seeps past the check ball as previously mentioned.

That model does have a habit of popping out the clutch side crank seal...  especially likely if crankcase did fill while sitting...  so removing clutch cover might reveal a problem without any testing...  but often will be hard to start if that happens because it creates a big air leak.

It for sure will not start without holding throttle wide open.  Clutch is stuck as well but i wanted to check the seals before i took it apart so that i would know whether to replace seal while i was there.  I guess i could still perform the test even with the cover off as that is where the air would be going into if its that seal.  Just trying to follow the great advice.  Compression and then leak down before starting to replace a lot of parts and or chasing the wrong root cause.  Thanks so much for the guidance. 

The fact that it starts only with throttle wide open does point to "loaded up" too...  might have a belly full of fuel and oil if someone left petcock on...  or is really, really rich on fuel like it would be if small float bowl o-ring on jet setter is missing/damaged.