77 dt250. Finally got around to getting a leak down rig together. Seems to hold 10 psi pressure or about 15 inches Hg vacuum with minor leaking. Minor means slow enough you can't really see the needle moving, but come back in ten minutes and it will have dropped significantly. So not sure here.... Good? Bad?

What confuses me more is while under pressure, I can slowly cycle with the kick start and the pressure doesn't change much. Maybe a couple psi bump when taking about 3 or 4 seconds to fully run the kick down.

Should I be more worried about rings? Alas I don't own a regular compression gage.

Bike runs. Symptoms are hard starting, likes to kick back. Have cleaned the carb several times, but the air bypass always seems to have little change on adjusting idle. Low to mid rpms always feel a little rough, like it gets into 4 stroking. Full throttle smooths out, but feels like it maybe is missing a little poop.

Soapy water in a spray bottle, hit your seals and mating surfaces to locate leak, not sure about your running problem, idle circit lean, kick back is usually timing related

I would research the leak down test requirements here. Believe it to only be 7 psi for 2-5 minutes or similar. Much more and seals can pop. Vacuum is not much either for very long. 1 minute maybe. So you are probably ok on both.

I only use 5 psi pressure. Yes, soapy water to find leaks under pressure.

Time the test... anything close to 1 psi/minute loss is going to definitely cause issues. Over 1 psi per minute loss is unacceptable.

It's not that hard to achieve zero psi loss or almost zero... even overnight. And worth the effort in my experience. (Make sure your test equipment isn't leaking)

After finding leaks with soapy water and fixing them... then vacuum test. If you have zero or near zero leaks under pressure but a worse leak under vacuum... it pretty much has to be the crank seals. Packing thick grease around them will seal it long enough so you can tell which one.

Timing is normally why it kicks back.

P.S. What you are describing when turning over the engine slowly is normal. This test does not test the rings... you're only making sure the engine is air tight which is critical on a two stroke. The gauge will move up or down slightly as you slowly turn the engine over but should quickly equalize back to the test pressure. If it loses pressure quickly when rotated slowly, very likely a crank seal. I don't normally rotate the engine at all... it really doesn't tell you anything.

Thanks for the info. I'll time the test more carefully and try the soap. I didn't expect to find anything by rotating other than the seals (they did not lose pressure with rotation) , but it did surprise me that the pressure didn't charge more. I guess with equal pressure on both sides of the rings they seal less than usual. If this is normal it makes me feel better. Off to mix some soap water...

Re your hard starting, does your float chamber resemble the one in the attached photo? If it does, the wide hole (not the brass tube), with a smaller hole at 45 degrees or so at the bottom, may have a jet at the bottom and this may be blocked still. Worth a quick check maybe.

I will definitely check. When I adjust the bypass screw, I can kill it if closing the screw completely. But then everywhere from about a 1/4 turn out to almost removed has no effect on idle, regardless of throttle screw position. So maybe something is still blocked.

Jet described above fixed in the float bowl is only choke related not idle mixture related. Idle mixture jet is up in the carb body & is accessed by a small [good fitting] screwdriver & should remove it to at least eyeball it in the light but not easy to see such a small hole. I poke something through it to be sure [even though others disagree]. If you screw the air mixture screw right in & it stops the motor [assuming its going rich & drowning it] then its doing its job. Although if set at about 1.5 out & sounds ok & you go to 2.5 out it should lean up a bit doing that if its jetted to spec.

also... Make sure your piston is BDC when performing Air leak down test.

Usual culprits are the Base Gasket front area and or Head gaskets.

5 to 6 psi is all you need as Mark says... need just enough pressure to make some bubbles with the soapy solution.

There are times I have had to apply a very, very thin coat of Yamabond 4 on the base area of the engine to solve air leaks... but try dry install on the base gasket first so see if additional step is even needed...

I always anneal my Copper head gasket each time I pull and spray with permatex copper spray.... never have an issue.

Yeah- I do keep a good inventory of base gaskets in my stock...

If you don't address/resolve any air leaks first... you will drive yourself mad as no matter what you do... carb cleaning- adjustment- jetting it will never run at its full potential.