680 won't start, I'm baffled...

[Matt G.]

I'll have to check one of my crankshafts tomorrow, but I'm about 90% sure that the PTO end keyway facing up corresponds to TDC. If you removed the bolt and washer retaining the PTO clutch, you could determine TDC fairly accurately and then see if the mark is where it belongs. You can probably try to slip a wire in through the plug hole, but the plug is right over the exhaust valve and not very close to the bore.

[BradLehman]

Originally Posted by Matt G.

If that was my static timing instructions you downloaded (probably was), you'll notice that I mention how rare it is for a 0.020" point gap to equate to correct timing. The most recent engine I timed, the one for those instructions, was the first in about 20 tractors that was timed right with the 0.020" point gap. There is no guarantee that it'll run with a 0.020" point gap. If the "S" mark on yours is 30 degrees off when the points are gapped to 0.020", there's NO WAY that engine will start and run. It doesn't take much at all to make the engine start hard, particularly if the timing is advanced. Follow the instructions and time it right and see what happens. It's highly unlikely that the flywheel key sheared. B&S engines are notorious for shearing flywheel keys; the Kohlers, not so. I do not recall ever hearing about a sheared flywheel key in a K-series Kohler. If your point gap is far off, you might try replacing the points pushrod, as they wear and would make the point gap radically different than what it should need to be.

So I finally got some more time to mess around with this thing. I kinda went back to square one and checked things over. It seems like .015 is where the timing wants to be according to the static timing instructions, but it still won't run.

What I am noticing is that it gets really close to trying to run when the choke is almost fully closed. To me, that seems to say that it isn't getting enough fuel, but the plug is definitely getting wet when you crank it over?

I hate to say it, but I'm getting close to hoisting the white flag and taking it to a repair shop.

[Matt G.]

If the plug is really wet, you're flooding it. A lot of these old Kohlers flood really easily. Set the main needle on the carb at 2 1/4 turns out and the idle at 1 3/4. That should be close enough to get it started. Every engine starts a bit different. The ones that flood easily need more care with the choke. Set the throttle at about half, and start cranking with no choke. Pull the choke, and then shove it back in as soon as it pops. You might have to stop at 1/2 choke on the way back in if it doesn't catch. I've also had tractors that would only start at idle, and other weird things like that.

If it's timed properly and is getting gas, something else is wrong. It might be time to pull the head and see if you have a valve sealing issue or something. If you have spark and gas at the right time and it won't start, you might not have compression for some reason. A leakdown test would be best, as you could figure out what's going on without taking the head off.

[BradLehman]

Originally Posted by Matt G.

If the plug is really wet, you're flooding it. A lot of these old Kohlers flood really easily. Set the main needle on the carb at 2 1/4 turns out and the idle at 1 3/4. That should be close enough to get it started. Every engine starts a bit different. The ones that flood easily need more care with the choke. Set the throttle at about half, and start cranking with no choke. Pull the choke, and then shove it back in as soon as it pops. You might have to stop at 1/2 choke on the way back in if it doesn't catch. I've also had tractors that would only start at idle, and other weird things like that.

If it's timed properly and is getting gas, something else is wrong. It might be time to pull the head and see if you have a valve sealing issue or something. If you have spark and gas at the right time and it won't start, you might not have compression for some reason. A leakdown test would be best, as you could figure out what's going on without taking the head off.

Thanks for the input...

Which carb adjustment is which? I can try resetting them, but they're set the same now as they were when it was running previously. I don't think it's flooding, but I can notice a bit of moisture on the plug (not like sopping wet or anything though).

Compression (or lack there of) I could maybe buy... but I'll be darned if it doesn't seem to 'suck' if I put my hand over the carb and crank, and 'blow' if I put my hand over the exhaust and crank it.

[Matt G.]

The top big needle is the main, the one off to the side is the idle. I guess I'd leave them be for now if you haven't touched them since it ran.

The engine may have SOME compression, but not enough. I've had two engines give me fits and drive me nuts because they seemed to have compression with the 'thumb over the spark plug hole' test, but one wouldn't run and the other would start really hard and not have any power. The one that wouldn't run had a cracked piston, and the other had badly worn valves that were no longer seating. I doubt you have a cracked piston, but worn valves are a possibility.

Take off the breather cover and check the valve lash. It's unlikely, but possible that the valve lash on one of the valves closed up and it isn't closing all the way. At any rate, you can observe the movement of the lifters and valves and make sure that you don't have a sticky valve.

The best thing to do would be a leakdown test...that would eliminate the valves as a problem, or tell you which one isn't seating if there is a problem. In the absence of a leakdown tester, you can rig up something to put compressed air into the engine through the sparkplug hole (with the piston at TDC) and see if air is leaking past either valve.