Rebuilt and still no Forward Thrust.

[tdorr]

I just completed a rebuild and reinstall of my Atomic 4. The mechanic said that it is working perfectly and it does start and run smoothly. That said, when I engage forward and throttle up I get no change in the RPMs. In reverse I do get a normal increase in RPMs and thrust when I throttle up.

My mechanic believes the lack of power in forward has to do with a fouled prop/shaft. As I switched from a two blade to three last year he feels that this could also be the problem. The three blade was spec'd for the boat. I didn't use the boat other than to go into the slip after completing the bottom job so I have no way of knowing if in fact the prop is the issue.

Has anyone experienced this situation where advancing the throttle in forward has no effect on the RPMs?

Thanks!

Dr. Tim

[joe_db]

First things first - what prop exactly and how clean is it?

[joe_db]

Edit - put the engine in neutral. How hard is it to turn the prop by hand?
If the stuffing box and/or cutless* bearing is binding, the 2:1 reduction in reverse is more effective at overcoming the resistance.

* boats have cutless bearings, pirates wield a cutlass

[tdorr]

Thanks guys.

I am getting the prop and shaft cleaned tomorrow as the mechanic feels that is the problem. The stuffing box and cutless bearing seem fine as the shaft turns.

[roadnsky]

Tim-
First, welcome to the forum!

Here is a copy of one of Don's FAQ answers...

What would cause my engine to run with almost no power in forward, while it runs OK in neutral? It also seems to have more power in reverse than in forward.

This condition is usually the result of low power caused by something that was done incorrectly during recent engine maintenance.
The reason the engine seems 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:


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.


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.

This will give you some other possibilities if the prop isn't fouled.

[tdorr]

Thanks for the info.

Based the comments here and some research on my part it's looking like a fouled prop is the culprit.

[Tommy jones]

Thanks for the info.

Based the comments here and some research on my part it's looking like a fouled prop is the culprit.

that doesn't sound right to me. If you can spin the shaft when its in neutral your shaft is free. we had a prop foul this weekend and found that the engine would die in forward becasue the shaft wouldn't spin. I tried to turn it by hand and couldn't budge it. prior to the fouling i could spin it with moderate force by hand when in neutral. It wasn't easy to get going, but once you got it moving it spun ok.

My question is, if the shaft turns by hand how is it able to bind the motor enough to bog down the RPMs?


It sounds more like the swapped plug wire issue that was described in the FAQ above.

[JOHN COOKSON]

That said, when I engage forward and throttle up I get no change in the RPMs. In reverse I do get a normal increase in RPMs and thrust when I throttle up.
Has anyone experienced this situation where advancing the throttle in forward has no effect on the RPMs
Dr. Tim

Reverse seems normal, forward doesn't seem to have full thrust.
The first thing I'd do is check the linkage to be sure the trany is in full forward. If the linkage is OK an adjustment inside the trany may be necessary.
That said a dirty prop is also a high on the list suspects also.

TRUE GRIT