"If it does have a primer bulb
"If it does have a primer bulb then do as Lee suggests.
I thought perhaps that 2.5 has an integral tank.
If so make sure you have an uninterrupted fuel flow to the carb. There may be a filter in the tank that's blocked.
Do not know where you got the 3/4 turn info from but general advice is that the high speed mixture needle is open 1 turn and the idle one 1.5 turns. (might be different I concede on such an old one) and start on full choke
Sure you got the float valve set up correctly when you reassembled the carb?
Did you poke a soft wire through the carb jets to get them properly clean. No good just waving a can of carb cleaner in their direction.
When you get it running set the mixture needles as described by Joe Reeves:
<blockquote> (Carburetor Adjustments - Two Adjustable N/Vs)
(J. Reeves)
Initial settings are: Bottom high speed = seat gently, then open 1 turn out. Top slow speed = seat gently, then open 1-1/2 turns.
Setting the high and low needle valves properly:
NOTE: For engines that DO NOT have a shift selection, obviously there is no NEUTRAL position. Simply lower the rpms to the lowest setting to obtain the low speed needle valve adjustment.
(High Speed) Start engine (it will run pretty rough), shift into forward gear, take up to full throttle. In segments of 1/8 turn, waiting for the engine to respond between turns, start turning in the bottom high speed needle valve. You'll reach a point whereas the engine will either start to die out or spit back (sounds like a mild backfire). At that point, back out the needle valve 1/4 turn. Within that 1/4 turn, you'll find the smoothest setting.
(Low Speed) Slow the engine down to where it just stays running. Shift into neutral. Again in segments of 1/8 turns, start to turn the top needle valve in. Wait a few seconds for the engine to respond. As you turn the valve in, the rpms will increase. Lower the rpms again to where the engine will just stay running. Eventually you'll hit the point where the engine wants to die out or it will spit back. Again, at that point, back out the valve 1/4 turn. Within that 1/4 turn, you'll find the smoothest slow speed setting.
When you have finished the above adjustments, you will have no reason to move them again unless the carburetor fouls/gums up from sitting, in which case you would be required to remove, clean, and rebuild the carburetor anyway.</blockquote>"
