Re: 28 Johnson
Compression should be 100+ psi and even on both cylinders. Is it?
With the spark plugs removed, at cranking speed, the spark should jump a 1/4" gap with a strong blue lightning like flame.... a real SNAP! Does it?
Spark plugs should be either Champion J4C or J6C plugs gapped at .030 . The J6C is slightly hotter and may result in improved performance.
If you've had the flywheel off, the flywheel nut should have been torqued down to 105 foot pounds unpon reinstallation. Was it?
The flywheel key must be aligned with the fiber rubbing portion of the points when setting the points. Set them so that a .020 gauge will pass thru but a .022 will not.
Anything under 105 foot pounds will result in a sheared flywheel key (out of time).
If the above three items are okay, and there are no fuel/air leaks anywhere, that leaves the carburtetor of which it is easy to overlook something.
Some boaters have a habit of switching parts around from other engines so I don't know if your carburetor has one adjustable (slow) needle valve or two (slow/high) so I'll include the adjustments for both below.
If the carb has one adjustable needle valve, there will be a fixed brass high speed jet located in the bottom center of the float chamber. If you haven't cleaned that jet out manually with a piece of single strand wire, do so to make sure that it is absolutely clean with no restriction absolutely.
Did you install a new small round cork gasket on the vertical venturi nozzle? It's got to be there.
Carburetor Adjustment - Single S/S Adjustable Needle Valve)
(J. Reeves)
Initial setting is: Slow speed = seat gently, then open 1-1/2 turns.
Start engine and set the rpms to where it just stays running. In segments of 1/8 turns, start to turn the S/S 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 (sounds like a mild backfire). At that point, back out the valve 1/4 turn. Within that 1/4 turn, you'll find the smoothest slow speed setting.
Note: As a final double check setting of the slow speed valve(s), if the engine has more than one carburetor, do not attempt to gradually adjust all of the valves/carburetors at the same time. Do one at a time until you hit the above response (die out or spit back), then go on to the next valve/carburetor. It may be necessary to back out "all" of the slow speed adjustable needle valves 1/8 turn before doing this final adjustment due to the fact that one of the valves might be initially set ever so slightly lean.
When you have finished the above adjustment, 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.
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(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.
