Symptoms of bad crank seals GT380?

[hahahaurugly]

I am bringing a 72ish GT380 back from the dead. It runs but doesn't like to fire on the center cylinder consistently and has a huge dead spot midrange in the power delivery. For the engine mechanical I have 100 psi compression on the 3rd kick and 135-140 psi by the 5th kick. My ignition is a new PowerDynamo set up. Exhaust are stock mouse nest filled oil gooped junk and about to be replaced with Jemco's. I am wondering if there is a way to diaginose the condition of the crank seals without tearing the motor apart needlessly. My compression makes me think the top end/engine is ok and my carbs are what need to be sorted still. Any input is appreciated.

[tz375]

Leak down test is probably the easiest and quickest way. It requires intakes and exhausts to be removed and plugged but the motor stays in one lump.

A four stroke is leak down tested at 100psi and a 2 stroke at 6-10psi. Do not pressurize the bottom end more than about 10psi or is will turn the seals inside out.

A cheap way to leak down test a motor is to blank off intake and exhaust and manually pressurize the cylinders one at a time and listen for leaks. You can usually hear hissing inside the crankcase if the seals are shot. A more accurate way is to have a pressure gauge and see if it will hold 6 psis for 6 minutes.

[hahahaurugly]

So my dumb question here is; is the piston at btc if you're plugging the intake and exhaust? (If so would the air just escape the oil port for scavenging the sumps into the next cylinder anyway?)

[tz375]

The SRIS and CCI injection system both have valves and theoretically won't leak, but you can always just pinch off the SRIS lines. My bikes don't have SRIS connected so I tend to forget about it. I blank them off.