My gc430 compact size rototiller will start but when I pull the throttle the engine bogs down and stalls. The tines do not turn at all. It worked fine one week ago.

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So you start the engine and it runs, the tines turn, and when you apply throttle and apply it to the soil, it stalls? That means the Carburetor needs setting up, or you have a fuel supply problem.

sounds like low fuel pressure enough for idling but to little during accelererarion. eg. failing fuel pump,blocked fuel filter or fuel pressure valve or sensor

Did you pull the spark plug and clean it, as well as the plug well and compression ring. Blow it out with canned air and dry before replacing the plug. Be certain the plug GAP is correct and it is free of carbon deposits. Rest the plug, tighten by hand and let her go. See if that fixes the problem.

The Mantis has a speed controlled automatic clutch.

Lower the idle speed by turning the idle screw counter clockwise. The idle screw is attached where the throttle cable meet the carb. The screw has a pointed end that resembles a pencil point.

The engine speed should be adjusted just above stalling.

If the tines turn even when the engine pull rope is pulled, the the clutch is defective and needs repairing.
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Is the gear box full of oil? If it is dry it will bog down the engine and stall it. Will the engine run at full speed without the tines engaged? If it doesn't maybe the full throttle mixture screw needs adjusted out CCW a little.

First thing to do, if you haven't already, change the oil, and remove all 'old' gas.
Then, after that's done, remove the plug, in order to de-pressurise the cylinder.
Then try to crank it again.
If it turns, then clearly, it's not ceased.
Remember, don't loose you're patience, just take your time.
Good luck with that:)

If you find one please let me know as I have the same tiller and no manual.

These machines have an arm to activate the tines independantly from the wheels, make sure that the tine arm is in the on position.
If not, seems like you have a transmission problem, you'll have to get dirty to fix this one and open up the transmission casing to see if its just the link between the tine arm and tine crankshaft that broke or if its more serious.

This erratic idling on my machine usually settles once I put it to work. With a load the engine finds it's air/fuel balance and should work better. hope this helps.