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Old 04-24-2005, 05:42 PM
Paul65 Paul65 is offline
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Motor bogs down under load. Suggestions?

I have an Atomic 4 in a 27" sailboat. The engine ran great last season and I winterized it after the boat was hauled. We launched this season 4/22. The motor runs great when in neutral; responds well to the throttle, etc. However, when I put the motor in gear and increase the throttle, the engine seems to bog down. It's as if the throttle has little affect; there seems to be no difference between a quarter-throttle and full.

Today I took the carburator apart and cleaned all of the jets, etc. It looked to be in good shape. I also cleaned the plugs, each of which was pretty black. I put the carb. back on and although there seem to be some improvement, it was short-lived.

The only change I made to the fuel system since last season was to install a water/fuel separator (the one recommended for the A4).

I took the boat out the day after we launched it. It seemed as if the "bogging down" grew worse with time. Once back in the slip, with the motor out of gear, it revved like nothing was wrong.

Any suggestions as to how to fix this problem will be greatly appreciated. Could it be that my prop shaft and motor need realigning? Please help...
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Old 04-25-2005, 08:57 AM
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Don Moyer Don Moyer is offline
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Since your engine performed well last season in both neutral and forward, it would appear that you would have to focus on something that could reasonably happen while the boat sat idle on land all winter. Here is a technical note we prepared on the subject which should get you started:

"This condition is frequently the result of something that was done incorrectly during recent engine maintenance. The reason the engine may seem to run better in reverse is because the reverse mode has a 1.7 to 1 gear reduction. Here are some examples of maintenance problems that we've heard about in the past:

Three to five times each year, spark plug leads were found to be installed in an improper sequence following a tune up! Cylinders 2 and 3 can apparently be reversed, and the engine will run fairly well in neutral, but will have practically no power in forward. According to one caller, his plug wires were reversed for an entire season before he found the cause of his power loss.

Another recent maintenance "oops" involved a minor repair of the exhaust system which required that the manifold be removed. On reinstallation, the old gasket was improperly installed, resulting in intake leaks and very poor power under load.

Assuming that there was absolutely nothing done to the engine since it was last running OK, check the following areas for potential problems:

1) Check for something hanging on to the prop. In a recent episode of this low power syndrome, the caller was absolutely certain that there was nothing on the prop or prop shaft. After almost three weeks of mutual pulling out of hair, a diver discovered a small strand of rope had wrapped around the prop shaft between the prop and prop strut. After the rope was removed, the engine ran fine.

NOTE: The reason that even small things hanging on to a prop (or prop
shaft) can make such a profound difference in the engine's ability to accelerate is that as long as it is held below 1200 RPM or so, it is at a very low point on its power curve (7 or 8 HP at best). In fact, the carburetor is still metering fuel through the idle system until approximately 1300 to 1500 RPM. Therefore, as long as a pesky piece of rope or a bunch of barnacles is imposing a 7 or 8 HP load, there simply isn't enough reserve power to accelerate out of that RPM range.

2) Check basic integrity of the engine by a quick compression check. For the purpose of diagnosing a major power loss, a simple check using your thumb over each spark plug hole is more than adequate. If your compression is sufficient so as to make it impossible for you to hold your thumb over each spark plug hole while cranking the engine with the starter, the cause of your power loss is not a major mechanical failure. If compression is weak in two adjacent cylinders, or water is observed in any of the combustion chambers, the head gasket has probably failed. If compression is "zero" in any of the cylinders, a valve has probably stuck open. If you are using a compression gauge, compression values over 80 psi are considered adequate.

3) Check for proper carburetor function. Normally, an Atomic 4 will accelerate almost "explosively" as you flick the throttle forward. If there is any hesitation or mild backfiring, a lean condition caused by dirt in the main jet is the most frequent cause. In most cases, a problem related to a fuel issue will manifest itself in neutral as well as in forward, but in mild cases, a lean condition might be missed in neutral, but will surely show up under load.

4) Check for proper ignition. As in the case of subtle lean fuel mixtures, there are subtle ignition problems that can show up under load but not necessarily in neutral. In this category, plug condition is the best indicator. If one plug is noticeably blacker (or oilier) than the rest, look for the relatively few things that can affect only one plug, such as the plug itself, the plug lead, or a cracked distributor cap. If all the plugs are badly fouled, check things that can affect all plugs, such as a loose condenser, badly worn points, shorting coil (or oil leaking from the coil), cracked distributor cap, or a bad coil lead."

Best regards,

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Old 04-29-2005, 12:23 PM
Paul_NY Paul_NY is offline
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A4 runs great with fuel/water separator bypassed


Thanks for all of your pointers. The most salient of your points was "... something could reasonably happen while the boat sat idle on land all winter". It turns out that 'something' was the 10-micron fuel filter/water separator I installed just prior to launching.

I bypassed the new filter (leaving only a simple inline filter in place) and the motor performs exceptionally well; reverse, neutral, forward - It doesn't matter.

My question now is, do I buy a 30- or 40-micron filter and see what happens or just leave the water separator out all together. The motor was completely rebuilt in the fall of 2003. Any suggestion?

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Old 04-29-2005, 04:40 PM
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Don Moyer Don Moyer is offline
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I wouldn't give up on the 10 micron mesh for your primary filter. That is more or less the standard size for a primary filter.

My suspicion is that the filter never completely filled with fuel and you developed a large air bubble which cavitated the fuel pump. One of our recent technical notes (Fuel related shutdowns) suggests installation of a rubber priming bulb between the fuel tank and the filter to prime the filter. I recommend that you reconnect the filter but leave the carburetor end of the line disconnected so you can pump fuel into a clean jar. Then find some way to pump enough fuel through the filter to fully charge the system. After the fuel flows with no air present, reconnect the end of the hose to the carburetor and try again.

Best regards,

Don Moyer
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