When I had similar troubles with my Kohler engine, a rocker arm retaining nut had backed off, allowing the rocker are to turn on its stand, which allowed a push rod to pop out from under it. It pushed the push rod through the valve cover. I was able to see it through the oil filler cap that's on one of my valve covers, so that's why it was easy to diagnose. I removed the valve cover (broke a screw, whole extraction process was well documented years ago) set the rocker back in position, tightened the nut to factory specs, checked all of the others, replaced valve cover (another saga regarding gaskets) and I was good to go.
The Kohler in mine has hydraulic valve lifters, so there is no valve adjustment.
I think I read somewhere that the Robin needs valve adjustment from time to time, but I may be indcorrect.
So, if it is valves, it may be as easy as removing a valve cover, removing the spark plug wires, crank the engine over and see if the rockers are moving the valves first. If not, then it's obvious. If yes, then they still might be out of adjustment.
If either one of those are true, they you'd need to go through the valve adjustment procedure for that engine.
Good luck. I'm not trying to get you to tear into your engine. Just going down the logical troubleshooting path.
Remove air cleaner.
- If good spark and engine pop with starting fluid, then check fuel
- If good fuel to the carb, then check needle and seat.
- If fuel injection, I can't help you.
- If needle and seat are working properly, check valves.
- You could run a compression test to see if it builds and holds pressure.
- Or, you could remove valve covers and check valve adjustment per engine manual.
- If none of that works, it might be timing issue.
One other thing you could try is removing both spark plugs, crank the engine over for 10 seconds to clear out the cylinders, replace spark plugs and try and start it with NO throttle and see what happens. Sometimes that can indicate carb issues if it starts to fire with no throttle. Could indicate needle and seat again.
Good luck. Hope it's something simple.