GT 750 crank seal leak question

[Titan SV]

Looks like the right/center crank seal is leaking on my 72 GT750. I cant get the bike to run on all three cylinders below about 4000 RPM, will only run on the right cylinder at idle. Have ruled out the coils and just about everything else and it's smoking pretty hard on the center cylinder. How do you guys confrim it's a seal without pulling the cases apart? If it is the seal any advise would be most welcome from those who have done them..

[tz375]

There may be other ways to test, but someone has to go first.

Remove all 3 carbs and all three pipes. Plug one exhaust and one intake (the matching one obviously) with expandable rubber plugs from your hardware store. It is indeed highly likely that one of the center seals has pushed out of place. It was a common problem until they changed the design. We need to test all 3 pots.

We now need a compression test hose to screw into one spark plug at a time. We also need one pressure gauge and way to pressurize the cylinders to around 6psi. 4 strokes ate tested to 100 psi but two strokes to less than 10. Any more and it blows the seals out.

Pump the pressure up to 6 psi and see if it holds pressure. The old rule of thumb is 6 for 6. that's 6 psi for 6 minutes. Repeat on the other two cylinders and listen for air escaping. If air escapes, first determine if it's a poor seal on the welch plug and if it's clearly escaping into the adjacent cylinder or into the transmission, we know that a seal is dead.

To replace seals:
Remove motor
Strip motor
Ship crank to Bill Bune in MN with a set of new brown seals

[elbert]

On mine I could see it with a vacum gauge connected to the intakes. On the left and centre cyllinder i had a very weak reading. The seal was completly destroyed. Not sure if you would see it with the vacum gauges if there is only a smal leak though.

[two-stroke-brit]

will the sris valves have to be blocked of as well.?
mark

[oldjapanesebikes]

will the sris valves have to be blocked of as well.?
mark

Yes they will - you'd also want to keep an eye on the oil injection lines as well just in case a check valves on one of the ends is passing back into the injector line. You should be able to see that though.