Hi Sapper,
Do you have a spare carburettor? For a whole unit replacement, you're only looking at about $60, P/N: CR03773A. With that, you'll get a couple of jets and springs which you can use to adjust the full RPM speed.
The only reason I mention this is it's often the fastest way to determine if the issue is the carburettor or not. Also, if you're going to do this professionally or as a hobby, it's a good idea to keep a "known good" carburettor around for testing purposes.
Moving on from replacing the whole unit, consider:
- Fuel blockages/restrictions - Over-revving may be a result of running lean at full RPM. Check the fuel tap filter, the needle, the main jet, and the poppet valve position
- Vacuum leaks - To determine if the issue may be vacuum related, there are 3 easy things to do. 1, spray around the engine with Nulon's SYB. If there's an air leak it will be plugged temporarily and the engine will return to full RPM. To that point, if you want to determine if that's the case under the engine, your best bet is to try to spray SYB through a hole in the chassis if you have one, DO NOT ATTEMPT TO SPRAY UNDER THE MOWER WHILE THE ENGINE IS RUNNING. Sorry, not assuming, but I had to say that. 2, check the vacuum from the carburettor to the body. Remove the hose connecting the top of the carburettor to the engine. Connect an extended piece of fuel hose to the carburettor and start the engine. You should notice the RPM's change as you block the hose, suck on the hose, etc. If you notice no change, the issue should be fuel or carburettor related. 3, test/check the decompressor valve. You can remove the decompressor and replace it with a spark plug, then block the decompressor hose connector on the carburettor. If the engine runs better, there could be an issue with the hose or decompressor.
- Configuration - If you have the wrong main jet or spring configuration in the carburettor, that could cause the issue. Consider changing the main jet and spring to see how the engine responds. You can also try servicing the carburettor by adding washers, P/N CRO3619A, before you put the cam back on. Remember though, this will reduce the RPM's when the engine is idling too, so I'm not sure if that's the solution for you.
I hope this helps.
Cheers,
pau13z
