Colin:

I sorry you're having trouble. I know it can be very frustrating, but hang in there you'll figure it out.

The following is directly from Don Moyer's website www.moyermarine.com . If you don't have a copy of his Atomic4 Maintenance and Overhaul Manual I highly recommend that you invest the $50.00 and get one, you'll never regret it!

"Of the three things you need to start your engine, you are reporting having two (ignition and fuel). This leaves only compression. Since you just replaced your head gasket, you would have had an opportunity to check the valves for proper operation, so I'm going to assume that you have compression, at least adequate to start the engine.

Whenever we see an engine which has the three necessary ingredients to run, and it still won't start, we always make the assumption that the timing of the ignition might be off, or perhaps the plug leads have gotten installed out of sequence.

Here are some other items that might help from chapter 10 of our service and overhaul manual:

BASIC TROUBLESHOOTING

The most elemental issue in approaching any troubleshooting situation involves answering 3 basic questions: (1) Does the engine have compression?
(2) Does it have spark? and (3) Is it getting fuel? We know that if an engine has those 3 requirements met, it has to run! It might run poorly, depending on its condition, but that is a different matter.

COMPRESSION?

In a trouble shooting context, we're only interested in a functional answer as opposed to a quantitative one, so the easiest thing to do is to remove all 4 spark plugs and hold your thumb over each spark plug hole while you're cranking the starter. Functional compression means that you can't hold the compression back with your thumb. With experience, you can say that compression is poor, normal, or "pretty good". Here are a few things to consider regarding compression:

a. To avoid getting shocked by the spark plug leads, remove the high tension lead from the center of the coil.

b. If you're by yourself, you can crank the engine from the engine compartment by shorting across the "S" terminal and the large terminal where the battery cable connects to the starter solenoid using a common screwdriver.

c. Be sure and close the raw water thru-hull valve on the water intake so as not to flood the exhaust system during cranking and cause water to back up into the engine.

d. Zero compression on one or two non-adjacent cylinders usually means a stuck valve(s). Zero compression on 2 adjacent cylinders usually means a blown head gasket. Zero compression on all four cylinders can mean water has gotten into the inside of your exhaust manifold and has resulted in all four intake valves sticking open.

The presence of functional compression usually confirms the general integrity of the entire mechanical construction of the engine, to include open intake and exhaust passages.

IGNITION?

With all 4 plugs still removed, reconnect the coil lead to the coil. Hold the other end of the coil lead about 1/2" from the head or block and check for spark while having someone turn the engine over on the starter.

a. The spark may not be as strong as you'd like, and you can't tell (without further checks) how closely the timing is set to T.D.C. However, unless the distributor has become loose, it's not very likely that the timing has shifted enough to keep the engine from running.

b. If there is no spark at all, as in the first attempt to start in the Spring, the most likely cause is a thin layer of corrosion over the points which prevents them from making contact and functioning as a switch.

c. If you have any other electrical apparatus connected to the + terminal of the coil (or even the - terminal as in the case of an electrical tachometer), these things all can fail in such a way as to interrupt proper functioning of the coil and should be temporarily removed while you're doing serious troubleshooting.

FUEL?

Here again, we're not talking about how correct the mixture is, but simply whether or not any fuel is getting to the combustion chambers. The easiest thing to look for is a damp appearance and fuel smell on the spark plugs.

A second indication of fuel availability is to remove the flame arrestor and check for some presence of raw fuel in the intake throat of the carburetor. If you have been turning the engine over on the starter for some period of time (especially with the choke pulled) there should be some evidence of raw fuel in the carburetor intake.

NOTE: Some people assume that if there is raw fuel in the intake of the
carburetor that the engine is flooded. Because of the updraft design of the carburetor, it is virtually impossible to flood the Atomic 4. In fact, even if there is fuel puddled in the throat of the carburetor, it may still be necessary to choke the engine for a good start.

If a major problem is discovered while performing the above checks, refer to the specific chapter of the manual dealing with that particular system. "

Good Luck!
Steve Kaminsky