No RPM under load

[altair]

Hello all!
I have recieved great info off of this list while refurbishing my Atomic 4 and wanted to help a buddy out. He has a 72' model Atomic 4. It runs fine in neutral, but when he puts it in gear, it still runs, but he cannot get any high RPM out of it. I had trouble like this and it was all in the timing of the engine. So, I checked his timing and it was dead on. We loosened up the distributor and tried rotating it a little each way and he had that set dead on. So, the only thing I can think it is, is a fouled prop, too tight of stuffing box or is it a possibility that his transmission is set to "tight"? I tried moving his forward/reverse lever and it moves freely, so I don't think it is that. So, what do the masses say? Please help!

Thanks,
Brian

:eek:

[Don Moyer]

Brian,

In most cases, low power in forward turns out to be something that changed in the engine itself. We prepared the following technical note in response to the many requests for help in this area:

LOW POWER IN FORWARD: 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,

Don

[higgs]

Dare I post after the guru himself. I had similar problem last summer and it turned out to be a bad coil.

[Above & Beyond Yachts]

I had the same thing. Turned out a stuck exhaust valve in #4 cylinder, causing 0 psi compression. It ran great in nuetral, and reverse. Due to the load in forward, it really presented itself... By the way, a precisely shaped screwdriver can loosen a the stuck valve...