"Barn Find" motor, How do you guys test one?

[Northern Mike]

Being that every neglected and abused bike for sale online is a barn find, how do you guys test or check a motor without knowing any history of it?

I recently picked up a 79 gs750 parts bike that for pocket change that was more complete than expected. I may actually have enough spare parts to get it on the road again (excluding the tank and seat).
The motor is complete and has what looks to be decent compression (did drop some oil in the cylinders as last run was unknown).

So short of pulling the motor completely apart to inspect, how would you folks test a "barn find" motor?

[salty_monk]

If you've already turned it over by hand & then on the starter I would say....

Do you have a good set of carbs? If not clean those ones change the oil, prime the oil circuit (i.e. turn it over on the starter with no spark) & then fire it up. It will probably have air leaks but it will show you if it's going to run or not.

You could remove the oil pan and check for anything in there & in the filter down there & clean it out if you want to be more dilligent.

[Northern Mike]

If you've already turned it over by hand & then on the starter I would say....

Do you have a good set of carbs? If not clean those ones change the oil, prime the oil circuit (i.e. turn it over on the starter with no spark) & then fire it up. It will probably have air leaks but it will show you if it's going to run or not.

You could remove the oil pan and check for anything in there & in the filter down there & clean it out if you want to be more dilligent.

So fresh oil, prime and fire, see what's broken (if anything)?
Bike came with carbs which I've already rebuilt on the bench.
Would love to see this motor run decent. I'm currently riding my wife's Honda as my daily rider (79 gs750) is down for the season until I can get into the soon to be finished garage and do the head gaskets.

[chuck hahn]

Is the engine in the frame? I got methods but i need to SEE whats what to accurately post. Throw up a picture. Got a key? igninion and harness intact? If carbs are rebuilt throw them on and fire it off outside so the oil smoke doesnt choke you out.

[Northern Mike]

Is the engine in the frame? I got methods but i need to SEE whats what to accurately post. Throw up a picture. Got a key? igninion and harness intact? If carbs are rebuilt throw them on and fire it off outside so the oil smoke doesnt choke you out.

Motor is out of frame right now. My daily rider (currently down with head gasket needed) I the exact same bike so might try pulling one out and put this motor in its frame. Everything would be there

[Northern Mike]

Is the engine in the frame? I got methods but i need to SEE whats what to accurately post. Throw up a picture. Got a key? igninion and harness intact? If carbs are rebuilt throw them on and fire it off outside so the oil smoke doesnt choke you out.

Here is the motor as it sits now.
The wife and I stripped the bike at the guy's house and loaded the entire thing in the back of the Honda pilot (save dragging a trailer).
Wiring harness looks intact. Have a spare ignition set in my box of parts and everything else short of a fuel tank and seat.

And yes, that is another gs750 motor in the background on the workbench.

[chuck hahn]

ok...as it sets pull the plugs and do a compression check. Then bring each jug to TDC on a compresson stroke and do a leak down test. These will give a decent enough report on compression and condition of rings and valves. hook jumper cables to the car and negative to the engine block. tap starter motor cable stud with the positive to crank it over.

[GregT]

As it's sitting on the bench, I'd drop the sump. When they sit for long periods, everything drops out of the oil and sets as a thick layer on the sump floor.

Always nice to be sure the oil pump pickup is clean.

[bwringer]

As it's sitting on the bench, I'd drop the sump. When they sit for long periods, everything drops out of the oil and sets as a thick layer on the sump floor.

Always nice to be sure the oil pump pickup is clean.

Agreed. This will also let you see if there are any metal chunks in there.

I'd also remove the valve cover and look at the general condition of things.

The compression test should tell you if there are any burned valves. That said, it's very, VERY common for bikes to be parked when the valve clearances close up and they start getting hard to start. People in the '80s and '90s were scared spitless of valve clearance checks for some reason, and that's how a lot of bikes with 12-20,000 miles ended up pushed to the back of the barn and forgotten. Check and correct this when you have the valve cover off.

Also, I'd turn the engine by hand with the plugs out and make sure the gears shift as expected. Don't expect it to feel all that smooth or anything if the engine isn't running; just make sure the gear change operates. You'll need to turn the engine so it can klunk into gear.

Another test is to make sure there's no rod knock; position each piston just past TDC on the downstroke, then insert a wood dowel through the spark plug hole and push down in the center of each piston to make sure there's no play.

If you can scare up a bore scope (a cheap USB version on Amazon will do) you might also inspect the bores; rust in the cylinders is the biggest danger with barn finds.