Engine Compression Testing

[Mr.humbucker]

I am restoring an 82 GS650G and I have two engines on hand that I can use on the build. I need to assess the "health" of these two engines currently on engine stands (not in a bike frame) to see which is the better of the two to use in the build. What is the best way to test the compression etc when they're not currently hooked up to the ignition etc? Is it possible to place a compression gauge one cylinder at a time and hook a battery/off/on switch to the battery to turn the engine over sufficiently to get a reading? Any suggestions would be appreciated.

[93Bandit]

What you're suggesting really isn't much different than how you would do it with the engine in the bike frame. Wire up the starter to the battery using appropriate gauge wire and a switch. Or if you're like me, skip the switch and just touch and hold the + wire to the + battery post and let the sparks fly and starter spin.

Pull all the plugs and make sure the throttle is open all the way if the carbs are still installed.

[rphillips]

Yep, what he ^^^^ said. The pistons will be going up & down as the starter turns the engine weather in the bike or on the stand.

[tom203]

Sure ,check the compression …..but this is a small indication of engine health . This is a plain bearing motor requiring high oil pressure …..depending on how it’s been treated, you could have bearing issues. Any idea of mileage on these two candidates ? My 650 still running fine at 65k miles, but I work on good karma.

[93Bandit]

Good points Tom.

[Nessism]

Check the valve clearance. If there is no clearance, that engine may have burned valves.

[Mr.humbucker]

Great advice from everyone ... thanks a million (Canadian dollars ) I have two engines and they're both believed to have a round 35K KM on them according to the odometers they came with so I am "hopeful" they've not been driven hard or mistreated. I'll start with the valve clearances and then try the compression. The carbs are off so no worries on the throttle remaining wide open.

Thanks again!

[Mr.humbucker]

What you're suggesting really isn't much different than how you would do it with the engine in the bike frame. Wire up the starter to the battery using appropriate gauge wire and a switch. Or if you're like me, skip the switch and just touch and hold the + wire to the + battery post and let the sparks fly and starter spin.

Pull all the plugs and make sure the throttle is open all the way if the carbs are still installed.

What would you suggest on the gauge of wire to the starter?

[Grimly]

What would you suggest on the gauge of wire to the starter?

6sq mm will be adequate for a test setup. Convert that to your AWG online.

[93Bandit]

If you have a complete bike, just use the battery cables off the bike if they're easy to remove and long enough.

[Mr.humbucker]

6sq mm will be adequate for a test setup. Convert that to your AWG online.

So I "think" that's 10 AWG from my online search ... does this sound about right?

[Grimly]

I'd say so.
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[rodneya]

Just remember that compression will be a bit low as the motor will be cold