Our engine starts and runs fine for about an hour. Then suddenly stops. It can be re-started immediately and runs fine for another hour or so. The only thing we have done mechanically is add an electric fuel pump.
My first three guesses are all the same; i.e., the fuel pump you installed probably has too low a pressure rating.
The normal fuel pump pressure for an Atomic 4 powered boat is a nominal 3 psi. Boats (like Catalina 30’s), with fuel tanks lower than the engine and 6 to 8 feet away, will need a nominal 4 psi pump. If your tank is high and close, there may be a partial restriction somewhere in the system which your old pump was handling, but the new electric pump cannot. Updated: May 15, 2009
There is a tube going from the intake manifold to the bottom of the carb...what is its function?
This is a scavenge tube. Its function is to draw any fuel that may have passed through a malfunctioning float valve and collected in the bottom of the throat of the carburetor and deliver it directly into the intake manifold for burning. – Updated: February 11,
On my atomic 4 motor starts up ok, but then dies. Does this indicate water in the gas? What would you recommend?
I would need a more definitive description of your symptoms than “dying” to be sure, but my guess is that you have an electric fuel pump that is not working. In these cases, a small amount of fuel gravity feeds into the carburetor for a few seconds of running, after which time the engine stops from fuel starvation. You might find that the fuse to your pump is blown.
I do not believe that your problem is caused by water in the fuel. – Updated: February 11, 2004
Is it OK to mix other additives to the fuel (like stabilizers and moisture inhibitors)?
We get this question a lot, and I don’t really have enough data to support a strong position one way or the other. In responding to the question, I usually offer the following rationale:
1) We have overwhelming evidence in the form of customer feedback to the effect that Marvel Mystery Oil (used by itself) provides all of the great benefits that we talk about so much, including many of the claimed benefits of other additives, like corrosion protection during winter lay-ups, cushioning deposits to soften valve closure, etc. In the face of this history, I’ve never had any inclination to try to alter what we know works.
2) I have heard of a couple of cases where mixing additives has indeed caused some ill effects in the form of precipitate falling out of the fuel. While these cases involved stabilizer and moisture inhibitor (I don’t recall the brand names), rather than Mystery Oil, it was nonetheless sort of a wake up call for me in terms of what can happen when mixing different “medicines” when we don’t have any easy way to predict the outcome.
3) Obviously, you can always try mixing different additives in small amounts, and if you get the benefit(s) that you’re looking for, without any ill effects, you can continue using the products. I don’t know of any case where the ill effects of mixing two additives were so bad that it ruined an engine. – Updated: February 11, 2004
My fuel tank is higher than my engine. Do I need an anti-siphon valve? There is a small round valve installed just above the tank where the fuel line leaves the tank. Could this be some sort of anti-siphon valve?
The valve to which you are referring is probably a small spring-loaded one-way check valve. The spring-loading on the valve is sufficient to prevent fuel from passing through it statically through a siphon (usually no more than 2 or 3 psi), but low enough so that the fuel pump can pull fuel through its mechanism while the engine is running.
These check valves will sometimes create a problem with electric pumps, since they (electric pumps) are less forgiving of even slight increases in suction head than are mechanical pumps.
If there is a wire connected to the valve on your boat, it is probably an electric solenoid valve that spring-loads closed whenever the ignition switch is turned off, and power is removed from the valve. These valves have the advantage of adding virtually no head loss to the suction side of the fuel pump, but they are vulnerable to malfunction, since they rely on electrical energy to hold them open.
It is not at all clear to us that either of these valves is actually required by the Coast Guard. In our experience, most people rely on a manual valve to prevent fuel from passing from their fuel tanks when the boat is left unattended. – Updated: January 5, 2004
What are the indications of a lean fuel mixture, and how does one correct it?
A lean fuel mixture is most often indicated by a hesitation during acceleration (in neutral or in forward). In more severe cases, the hesitation is sometimes accompanied by a “popping” sound from the intake throat of the carburetor; in more subtle cases, a lean condition may cause nothing more than a slight reduction in performance at high power settings.
If your engine has a late model (aluminum) carburetor, it will have a fixed main jet, and you won’t be able to adjust the fuel mixture at cruising RPM. In this case, you will have to take the carburetor apart for cleaning – the problem is most likely a small restriction in the main jet.
If your engine has an early model carburetor, it will have an adjustable main jet with a small “T” handle extending out of the bottom of the carburetor at about a 45 degree angle. To make the engine run richer, you would turn the “T” handle counter-clockwise, as you would be facing the handle. The adjustment on the main jet is quite slow, so it may take at least half a turn before noticing any substantial difference. – Updated: January 4, 2004
Why does it seem that adjustable main jets are more susceptible to becoming partially blocked than the fixed main jets of later model carburetors?
While the total open area around the needle and seat of adjustable main jets is approximately the same as the open area of the orifice in fixed main jets, the cross-section of the annular space between the needle and seat of adjustable main jets is very thin, and therefore very vulnerable to trapping tiny particles of dirt – particles which will in many cases would move right through the orifice of fixed main jets.
In many cases, backing out the needle of an adjustable main jet a couple turns will allow these tiny particulates to move through the main jet and out of harm’s way. Once the tiny particulate moves through, the needle valve will need to be readjusted to the proper setting. In cases where the pesky particulate is too large to pass through, the carburetor will have to be removed for more serious cleaning. – Updated: November 4, 2003
What are the pros and cons of removing the fixed main jet found in late model carburetors and installing an adjustable main jet?
An adjustable main jet is very convenient if you are planning to do a lot of long-distance motoring. Our intracoastal cruising customers like them because they can fine-tune their mixtures to a fuel consumption in the range of 1/2 to 3/4 gallons per hour.
An adjustable main jet can also cure the tendency that many earlier late model carburetors had to run a bit rich. In many cases, an adjustable main jet will enable you to lean out the fuel mixture enough to clean up sooty spark plugs, particularly in engines that have good compression (90 psi or above) and are otherwise performing well.
For many weekend sailors (particularly those with very poor access to the carburetor), the adjustable jet represents additional hassle they don’t feel they need. – Updated: November 4, 2003
Why does dirt get trapped in the float valves of carburetors, rather than move on through as the floats raise and lower the needle against the seat?
The needle of the float valve is actually always being pressed up against the seat by the buoyancy of the floats. As the fuel level lowers, the pressure against the seat is somewhat less, and the pump is able to force some more fuel into the float chamber. As the fuel level rises, the buoyancy once again is sufficient to close off the fuel flow, in opposition to the pressure from the pump.
The fact that the needle is always being pressed against the seat explains why a piece of dirt will remain held between the needle and the seat once it works its way in.
It’s sometimes possible to get rid of such a piece of trapped dirt by draining all the fuel from the carburetor and then running the pump for a few seconds. The incoming fuel will sometimes flush the dirt past the needle valve and out of harm’s way. If this quick fix doesn’t work, you’ll have to take the carburetor apart and remove the particle.
While the carburetor is still apart, it would be a good time to polish the seat by taking a short length of 3/16″ wooden dowel rod (pointed on the end) and pressing it against the seat while rotating it back and forth. This dressing action sometimes assures a better seating of the needle. – Updated: November 4, 2003
What is the proper float setting for Atomic 4 carburetors (early and late model)?
The float assembly should be adjusted so that it lays parallel with the underside of the upper half of the housing, holding the upper half of the housing upside down and sighting along the plate onto which the floats are attached.
We have gone to this “eyeball” method of float adjustment because it is so difficult to describe more precise methods of measurements. The problem is made worse, due to the fact that several different measurements appear in different manuals, as well as within the instruction sheets from various repair kits.
Float settings that result in the float assembly being slightly below parallel in the above check would be in the direction of richness, while settings in which the float assembly is slightly higher than parallel would tend to be leaner.
It’s important to note that fuel pump pressure also plays an important role in the ultimate level that exists within the float chamber. Technical folks from Zenith inform us that the recommended float setting (as described above) is based on a maximum of 4 psi from the fuel pump. Fuel inlet pressure to the needle and seat over 4 psi will result in slightly higher float levels (which leads to richness).
We never gotten any indication from Zenith that there is any concern over too little pressure, but in our experience, inlet pressure less than 1.5 to 2 psi will start to give lean indications such as hesitation during acceleration, and/or some power loss at full power. – Updated: November 4, 2003
What is a good initial setting for the idle mixture and the adjustable main jet?
Our recommendation for the initial setting of the idle mixture is to turn the needle in against the seat, and then back it out one turn from the seat. After the engine warms up, the adjustment should be fine-tuned to the point where the engine idles most smoothly.
For adjustable main jets, there is quite a comfortable range of initial settings, ranging from one to two complete turns off the seat. The adjustment should reset for the best RPM at your favorite cruising power setting with the engine fully warmed up.
Remember that in the case of the idle mixture you are controlling the air, so turning the valve in (clockwise) reduces the air and hence makes the mixture richer. In the case of adjustable main jets, you are controlling the fuel, so turning the “T” handle in (clockwise) reduces the fuel and makes the mixture more lean. – Updated: November 4, 2003
What size screws fasten the flame arrestor frame to the flame arrestor housing?
You will need (2) #10-32 x 3/8″ machine screws. If you can’t find the 3/8″ length, 1/2″ long machine screws will also work.
