You say that you replaced the head gasket and the manifold gasket... Why was this needed?
The manifold gasket... Exhaust baffle manifold or intake manifold?
The no faster than 5 mph mention..... At what throttle setting? If not full throttle, what happens when you advance the throttle? We need a better description of your problem.
Linkage & Synchronization on that model involves...........
Make sure the throttle roller is approximately 3/8" in diameter... if the outer clear cover (if applicable) has broken away, leaving the roller to be roughly 1/8" in diameter... replace the roller.
Back the throttle roller away from the metal cam. Now, adjust the throttle linkages between the carburetors, making sure that the throttle butterflies "all" open and close at the same time. You do not want one or two butterflies closed and one ever so slightly open.
Now, adjust the throttle roller so the throttle butterflies just start to open when the scribe mark of the cam aligns dead center with the throttle roller... not before or after.
At that above setting, check the idle timing (whatever your manual states it should be (2,3,4 degrees, whatever). If the timing is not as it should be, adjust the linkage between the vertical throttle arm and the metal cam.
That's it..... however it's unlikely that this will correct your problem unless your linkages are really off base. Be sure to answer the questions above I started this out with.
