It is almost new, used only a few times, then stored (with gas in it) for 1 1/2 years. Now got it going a little, but it won't stay running and is not going to full RPMs. Briggs&Stratton 3.5 engine on MTD mower.

Gasoline slowly polymerizes (forms varnish) when it sits around. That's why it's a bad idea to store gasoline in small engines for very long. Gasoline "stabilizers" help to inhibit varnish formation and a lot of people use stabilized gas so they can let their engines sit over the winter. I suspect the carburetor is plugged up with varnish. If it were my machine, I'd try cleaning it with a carburetor cleaner to try to dissolve the varnish. If that doesn't work, you'll have to pull it apart and clean out the small orifices.

It is almost new, used only a few times, then stored (with gas in it) for 1 1/2 years. Now got it going a little, but it won't stay running and is not going to full RPMs. Briggs&Stratton 3.5 engine on MTD mower.

Did you use fuel stabilizer before you put it in storage??

Hi,
I dare say you will need to replace the diaphragm between the carb and tank. Two bolts hold the unit to the engine and it slips onto the intake pipe. Then five screws hold the carb to the tank. Separate slowly observing the location of all pieces. Clean the intake screen on the pickup and install a new diaphragm and reassemble. Push the primer three times, or leave the air cleaner off at first but install the screw that would hold it on. This will permit you to see when gas squirts from the jet when priming. Once it squirts gas once then try to start.
Peace,
Clarke

At this point you know most of us think year and a half old gas must go so dump that tank and drain the carb bowl. No parts needed just yet but add fresh gas an 2 oz of SeaFoam or other good carb cleaner and try to keep it running for a slong as possible. If the Seafooam doesn't clean out the junk then disassembly and possible a carb kit may be in order.