| Detonation is a secondary ignition of the mixture during the
compression stroke. It usually sounds like marbles rattling in a can.
It is usually caused by too high a compression, or too much advance.
Pre-ignition is a similar problem where the mixture is simply
detonating before the spark happens. Sometimes these are accompanied
by "run on" where the engine will keep running after you kill the
ignition. You still have not said exactly what ignition system you have. The
merc thunderbolt electronic ignitions do have the advance built in
electronically. As I mentioned before if you have set the timing with
a timing light you should be able to rev the engine and watch the mark
move away as the timing advances. There are other electronic ignitions
that still use a mechanical advance in the bottom of the distributor. ( In truth the merc systems starts out retarded and remove the retard
as the rpm increases. If you think about it you realize that the
ignition cannot fire the spark before the pickup gets triggered.) Prop pitch will not cause your problem. The wrong plug can cause
detonation but only after the engine has gotten hot. But it does not
really sound to me like you have detonation. Fuel pump problems at high rpm typically initially accelerate and run
normally for a half minute or so and then just die out or fall back as
the fuel in the carb is exhausted and there is not enough being
supplied by the pump. Since this is a rebuilt engine can we assume it has not run right yet? I would replace the cap, rotor, and wires. Recheck the wires to
confirm they are all going to the correct cylinders. Check the timing
again and watch for advance to occur as the engine revs. Run it at
night and look at the wires for arcing. It is much easier to see in
the dark. At this point if it's still acting up I would do a
compression test. A stuck intake valve could cause this problem. As
well as improperly adjusted valves. A compression test will find those
problems. I've never heard of a problem cause by overtightening the flanges on
the thunderbolt ignition module. Mine just look like plain old
flanges. The thunderbolt module is pretty reliable if that is what you
have. I think because it is a marine environment the engineers used
some pretty overkill ratings on the components. You can always try one
but once you get to swapping ignition moduels and carbs it starts
getting pricey. That loud a noise as you describe it sounds to me like you are heaing
the mixture fire off through the intake. Or possibly out the exhaust
since it is a lot louder on land. I would see if I could tell if the
noise is coming back through the intake or going out the exhaust. Paul wrote: |
