Leak down or tear down???

[Guest]

Gents,
I'm starting my first cafe project...and my first motor rebuild for that matter. I bought a '79 gs750 for a bargain but it has around 75 psi in #3. I plan to tear the whole thing apart (as a learning experience), possibly bore it out, and rebuild it no matter what is causing the low compression. My question is, if it is a valve issue, will that be something I will be able to figure out once it is apart, or should I really have a leak down before I break it open? I figure I will be able to tell if the rings are the culprit...but I'm a greenhorn in this world and wasn't sure if my assumptions were correct. Also, could it be something I haven't considered? Head gasket? Any advice is appreciated.

[TheDuke]

The bike at the moment.

[rustybronco]

Gents,
I bought a '79 gs750 for a bargain but it has around 75 psi in #3. I plan to tear the whole thing apart (as a learning experience), possibly bore it out, and rebuild it no matter what is causing the low compression. My question is, if it is a valve issue, will that be something I will be able to figure out once it is apart, or should I really have a leak down before I break it open? I figure I will be able to tell if the rings are the culprit...but I'm a greenhorn in this world and wasn't sure if my assumptions were correct. Also, could it be something I haven't considered? Head gasket? Any advice is appreciated.

Personally I would do a leakdown test before opening the engine. That, and checking the valve clearances as well. There maybe no logical reason for having to rebuild it unless you want to, or prefer to expend the money. Opening it up to renew the gaskets and vale stem seals sure, but boring it or replacing the rings, especially if it has low miles on it, is not what I would do.

http://thegsresources.com/_forum/showthread.php?p=1559962#post1559962

[Nessism]

I agree with Rusty (Dale). And to reiterate what he said, you need to adjust the valves before performing a compression or leak down test otherwise you are just spinning your wheels.

[TheDuke]

The bike currently has around 30k on the odometer. I planned to rip it all apart and put it all back together (boring it out only if the rings had fragmented and damaged the cylinder walls). This was mostly so I could learn how to do it and see how it all works. My goal isn't necessarily to get it back on the road ASAP. But, I do not know exactly what I'm getting into, if I need a ton of specialty tools to do the job, or if the expense of doing it (new gaskets/seals and a thorough cleaning) is higher than I'm expecting. I think a valve adjustment may be a good place to start though.

[Flyboy]

^^^ What they said.
If you want to tear it down for sh1ts and giggles to educate yourself on the inner workings, that is up to you.
To answer your question, yes you will be able to tell if it is valves or rings leaking, if it is a valve, you will clearly see a patch of dark black colouring on the valve seat where the leakage is taking place.
If it is piston ring blow by, you will see dark marking down the sides of the piston skirt where it is leaking.
However, if you are pulling it apart, it is of no matter, as you will be lapping the valves in anyhow.........right????
check the valve clearances and give it a compression test, or better blow by if you have the equipment, if the reading stays low, drop a teaspoon of oil down the plug hole, redo test.
If the compression comes up, you know your rings are bad, if, however the compression stays low, valves are the culprit.

[1_v8_merc]

If you teardown, it's going to be a few hundred bucks for piston rings and head gasket kit, plus any other gaskets you need. Plus you need special motorcycle piston-ring tools, and Two people, to make installation a breeze. Engine timing is easy to deal with, just Don't touch the Timing plate on the Right side of the engine, and try not to drop the timing chain down into the engine belly!

[cowboyup3371]

If you teardown, it's going to be a few hundred bucks for piston rings and head gasket kit, plus any other gaskets you need. Plus you need special motorcycle piston-ring tools, and Two people, to make installation a breeze.

What special tools? Fingers work great if you pay attention to what the rings are doing...at least on my 550

[TheDuke]

Perfect, this is exactly what I was looking for. I didn't want to rip it to pieces just to find out I needed to have it tested to ID the problem.

So yes, a complete tear down is on the horizon. I have the service manual and you all, so I should be able to work through any issues I find along the way. Fingers crossed.

To answer your question, yes you will be able to tell if it is valves or rings leaking, if it is a valve, you will clearly see a patch of dark black colouring on the valve seat where the leakage is taking place.
If it is piston ring blow by, you will see dark marking down the sides of the piston skirt where it is leaking.

[tkent02]

You just like tearing stuff apart? Go for it.

If you just want to fix the bike, do that instead.
Either way, read this first, especially the part about rescuing old engines.

http://members.dslextreme.com/users/bikecliff/