Engine slows greatly when put in forward

[sundowner]

Suggestions, please: Rebuild recently completed. Engine starts immediately and idles beautifully, but when shifted to "forward," boat (30' Irwin) engine seems to labor and my top speed is not what it used to be. (4.5 vs. 6 knots)At full throttle, if I shift out of gear, engine races--except of course that I slow it immediately.

Boat is firing on all cylinders.

I thought it might be the propellor covered with barnacles, so I dived under and found a few barnacles, and some on the shaft, which I scraped off, with no change. The bottom was recently repainted, so it is smooth.

Can the transmission be tightened too much? It takes a firm push to get into forward.

I'm reasonably sure the shaft alignment is good, but I will recheck that.

If you rotate the shaft at the coupling while the boat is in the water, how loose should I expect to find it? Can the stuffing box be too tight?

What else should I check?

Addendum, 12/08/04

As I said further on in the thread, I think Don's FAQ that follows is on target, with regard to part of my problem. That is, the carburetor needed cleaning. Doing so improved the engine speed. But the engine still made a noise, and so to discuss that, I would like to end the discussion of this problem, and I plan to start a new "thread" with my new problem.

[Administrator]

Here's a post from Don's FAQ section which may help:

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.

[rgoff]

You might check for a restriction in your exhaust system. I've had the problem where my engine started and idled fine, but died as I reved it in neutral because of this. Maybe yours has a slight restriction that still allows it to run, but at lower speed.

Good luck,

[Don Moyer]

You might also check your ignition timing under load. Loosen your distributor hold-down bracket slightly (so that the distributor doesn't move by itself), and then with the engine fully warmed up and at your favorite cruising power setting, slowly rotate the distributor each direction. If you can improve your RPM, tighten the hold-down bracket in that new location.

[merrifik]

Engine slows in forward

Sundowner,
I had the exact same problem with my Universal 4 in my 78 Catalina 27 and I tried all normal fixes, such as carb inspection and set up, new plugs, points etc, timing checks, flame arrestor blockage etc, to no avail. I also noticed that it was always smelled of unburnt gas coming from the exhaust and the engine oil smelled the same. If I reversed quickly a carbon slick would burp out the exhaust. Then I noticed the flexible exhaust line coming from the water muffler did not have the recomended rise above my exhaust outlet in the stern. (Catalina 27 have a particular problem of "squatting" at the stern especially under power which aggravates this situation) I repositioned this line thinking that i may have been getting too much back pressure from water in the exhaust and it seems to have worked as now the engine purrs with lots of power, doesnt miss, and has no unburnt gas smells or oil slicks. Good luck with your problem.

[sundowner]

Thank you

From the FAQ I concluded that the most likely problem is in the carburetor. What with the election and other activities, I have just gotten to the carb which I have removed. I suppose the unit has needed cleaning for a long while, because removal of main jet cover and float chamber cover indicate all sorts of bits of "dirt." I'm optimistic that cleaning will take care of my problem.

Thank you for your suggestions.

[ericson_35]

Carbo repair

I think you may be on the right track if all the above suggestions have been checked. I had a case of 'stumbling' at various RPMs and tracked it down to the carbo. I kid you not, it was a 60 minute job: remove, disassemble, carbo clean all the parts and re-install, and wa-la, problem dissappered. I did notice just a wee bit of particulate matter in the bowl and upon completely soaking and carbo cleaning everything, it idles and runs much better now.

Few last thoughts though, how old is your gas? I notice gasoline now-a-days seems to decay at a rapid pace (over a year?). Not sure if this just a hunch on my part or a fact, but upon a fresh load I get better idle, minimal smoke and engine just seems to run better. Also, keep an eye on your filters and fuel lines. A plugged filter will cause similar symptoms such as stumbling at higher throttles settings, loss of power, etc. as it is starving for fuel. Also, a kinked or loosley tightend hose clamp on a fuel line allowing air into the line under suction will casue similar results.

Cheers,

John M.

[sundowner]

Thank you

As I said in an addendum to the start of the thread, cleaning the carburetor, was of significant help. So I will not be checking this thread any more. I do have a new problem which I will present in a new thread.

[Above & Beyond Yachts]

Hello, just had the same problem on a customers boat. Although it was firing correctly on all four, a stuck exhaust valve on #4 cylinder caused 0 compression. It would rev great in nuetral, ok in reverse, very poorly in forward. Have you checked your compression?