The tiller has sat idle for several years

Transmission/differential.
Perhaps the unit sat for 4 years because the tines would not turn and your replacement engine has not solved the real problem.
I'd say disassemble the chain case to discover what's locking the tine drive.

Your car is 18 years old. It's likely history. Sell it as is for what you can get.

Hello,
My name is Dane and I am going to assist you in solving your problem.
You should have an idle screw adjustment on the carb to adjust the idle RPM. As you pull the trigger, watch on the carburetor for a small moving plate. The idle screw should touch that plate. Be very careful, and have someone hold the tiller and start it. Back off the screw just enough until the tines stop turning. I hope I've helped. If you found this reply useful, please leave me some THUMBS UP, if you feel I've deserved them. Thank you! Dane

If it has been sitting for a while empty fuel tank. Prepare fresh fuel/oil mixture to fuel tank. Add two ounces of 100% acetone (available at Walmart) and attempt to start several times. Let sit for several minutes. Repeat until it acts like it wants to start.

Then watch this video and do it:

http://www.alamia.us/mantis_parts_s/148.htm

Never let tiller sit with fuel for any prolonged period of time. Run it out of fuel before storing or sitting. Acteone will remove varnish from inside.

Gas came out because it is flooded. Check the spark plug for damage.

If it has been sitting for a while empty fuel tank. Prepare fresh fuel/oil mixture to fuel tank. Add two ounces of 100% acetone (available at Walmart) and attempt to start several times. Let sit for several minutes. Repeat until it acts like it wants to start.

Then watch this video and do it:

http://www.alamia.us/mantis_parts_s/148.htm

Never let tiller sit with fuel for any prolonged period of time. Run it out of fuel before storing or sitting. Acteone will remove varnish from inside.

Hello. you have some gunk in the carburator. Get a can of starter fluid or carburator cleaner aerosol and a helper. Remove the air filter and start it up. Spray a little starter fluid into the air cleaner to keep it running for three or four minutes and the problem should clear. If it does not clear the carburator will have to be disassembled, cleaned and rebuilt. j.

Mantis 2-cycle tillers use Echo engines in several variations. A list of your local Servicing Echo dealers can be found HERE.

Hope this helps,
Dolf-

i'm having the same problem. cleaned carb but didn't help. I found that if I spray starting fluid on the gasket between the carb and the motor it speeds up. I'm going to try to find a new gasket, I can't tighten it anymore.
Good luck

Improper adjustment is a possible problem. However could also be trash in carburetor. Adjustments can be tricky. Recommend a tech.

I recently had the same issue with a tiller that hadn't been started in over 8 years. Turns out that old gas had left a varnish coating in the carburator keeping the gas from feeding. Rebuilt the carburator with new float, needle valves and bowl, new Welsh plug. Fresh gas and two pulls later it went to churning. Took a short time to adjust the two needle valves for idle and full throttle and we went a-tilling.