It's time to stop throwing parts at the engine and to do the proper trouble shooting. Remove the spark plugs... they should be Champion QL77JC4 plugs gaped at .040 . Are they?
Check the compression. What is the psi reading of each cylinder.
With the s/plugs still removed, rig a spark tester up, having the spark gap set to 7/16" for the spark to jump. The spark should jump that 7/16" gap with a strong blue lightning like flame... a real SNAP! Does it? That 7/16" gap is important for this test... nothing else will do!
Hopefully the large round linkage leading from the vertical throttle arm and the metal cam (connected by a threaded nylon yoke) has not been moved. If it has, then that will need to be adjusted with a timing light. With the cam scribe mark aligned dead center with the throttle roller, the timing needs to be set st 3 degrees plus or minus 1 degree.
Make sure that the throttle roller is not touching the cam on which it rolls against.
Loosen the linkage between the throttle butterflies of the carburetors so that both throttle butterflies slam shut (spring loaded). Now, carefully adjust, tighten that linkage so that both throttle butterflies open and shut at the same time. You do not want one closed and the other slightly open.
Make sure that the choke butterflies close at the same time.
Now, look at the carburetor throttle roller and the scribe mark you see on the metal cam. The cam should just touch and start to open the throttle butterflies when that scribe mark is dead center with the center of the roller... not before or after!
Now, if you have compression and spark, and the linkage has been adjusted as above, now you can adjust the carburetors as follows. Hopefully they are clean and assembled properly as they should be.
(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 1: 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.
Note 2: If the engine should be a three (3) cylinder engine with three (3) carburetors, start the adjustment sequence with the center carburetor.
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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