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Catalina/Capri 25/250 Sailor's Forums

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unusual Honda OB problem

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JimB517
Past Commodore

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USA
3285 Posts

Initially Posted - 04/19/2004 : 15:54:45

My Honda 8 classic is running fine. It starts on the first pull and will motor the boat up to 5.5 knots all day without missing a beat, all below 1/3 throttle.

If I try to push the boat to 6 knots and above, the stern squats, the motor digs in, and the engine starts to work hard. I start seeing my knotmeter creep up. 5.5, 5.75, 5.8 ...

After a couple of minutes, suddenly the engine RPM will go skyhigh and the motor will shut down. If I hear the RPM increase, I can back down the throttle and the engine will keep running at 1/2 throttle or less, no problems. Speed back down to 5.5. Runs all day here.

OK, so I think it's running out of fuel, leaning out, and shutting down. I change fuel filters, clean the screen in the tank, and run the motor at the dock (firmly tied up) with the cowl cover off. I've got a large West Marine generic see through fuel filter between the fuel pump and the carb.

As the motor speeds up I see a steady stream of gas flowing through the filter. Above 1/2 throttle there is a steady stream of small air bubbles as well. Small, and at a rate of like 1 a second. When the RPM climbs and the engine starts to shut down, it is not out of gas.

Could this be prop cavitation? But when I first bought the boat I could motor at 6.3 knots and higher. The engine never shut down even at prolonged full throttle (I did this test during the sea trial).

Could the prop be spun (needing a new hub)? Never hit anything. Doesn't seem possible. Motor works fine at low speeds. Does not have any problems shifting forward to reverse, etc.

Why would it shutdown not just rev to the limits?

The obvious fix is to be happy motoring at 5.5 knots or less but I would like to find and fix the problem, especially before my summer cruise.

Here's another strange symptom. When hitting full throttle (even before the motor overrevs) the green oil pressure light dims and even seems to go off. The motor's not out of oil, its clean and the tank is full.

This has been going on for 3 or 4 months. Has no impact on normal day sailing.

Ideas?

Indiscipline 1978 FK SR #398

Edited by - on

bear
Admiral

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USA
909 Posts

Response Posted - 04/19/2004 : 18:51:34

Jim, Have you changed the "O" ring on the fuel hose coupling where it attaches to the fuel tank recently? Seems like when the engine demands more fuel its sucking air from some source ahead of the fuel filter. "Bear" C250 WB W/Honda 8 1995 vintage.

Edited by - on

ClamBeach
Master Marine Consultant

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3072 Posts

Response Posted - 04/19/2004 : 20:26:10

Check the venting on your tank... make sure it can pull plenty of air. Loosen the cap and see if the engine RPM stays up.

Also, any evidence of overheating at full load? Honda's used in saltwater can get 'clogged' up with salt deposits.

Edited by - on

dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts

Response Posted - 04/19/2004 : 21:14:33

If the engine rpm is going "sky high" while in gear, I would have to think that either the prop is slipping within the hub or the prop is getting ventilated causing it to lose its load and spin up.

I just don't think a lean fuel mixture is going to turn the prop at that high of an rpm with an 8 hp motor unless something else is going on to allow the prop to lose grip and spin..

Edited by - dlucier on 04/19/2004 21:27:26

Dave Bristle
Master Marine Consultant

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Djibouti
10005 Posts

Response Posted - 04/19/2004 : 22:23:01

I can't see how it would be cavitating now if it wasn't at 6 knots and above before this, unless you have loaded the boat much differently. The squatting generally puts the prop lower in the water. If you're cavitating, you should see clouds of small bubbles streaming back from the prop. If you don't see that, then something's slipping internally.

The bubbles could be air getting in somewhere, or they could be vaporizing gas from negative pressure leading to the fuel pump. That could be from lack of tank ventillation or some obstruction (including a kink) in the fuel line, or a clogged filter. (You do open the tank vent, don't you.

)

Now, I can see why you'd want to understand what's going on, but pushing her above 5.5 is just wasting gas, making noise, and stressing the motor (which is probably using a prop designed to plane a small boat, not push one that can't get over 6 knots). A 1.6 liter auto engine can push a compact car all day at 80 mph; but, using the same gearing, it probably wouldn't be able to get an 18-wheeler across an intersection. Low-pitch "high-thrust" props help deal with that issue on heavy displacement hulls, just as a 12-to-1 differential would help the little engine on the big rig.

Edited by - Dave Bristle on 04/19/2004 22:26:44

Arlyn Stewart
Master Marine Consultant

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USA
2984 Posts

Response Posted - 04/19/2004 : 23:00:35

I agree with both Don and Dave.

I doubt that a 4 stroke will lean up to the point of high revs like a 2 stroke will. A 4 stroke will rather gas starve and begin to miss and then die. Don is on track, look for a cause of ventilation when under heavy load.

The high revs on the other hand, could be producing the quiting problem. When rpms rocket, the fuel pump production increases which may be above some restriction threshold in the fuel line which then allows sucking air somewhere hence the visible bubbles. At the same time, the fuel restriction in relationship with the carb butterfly wide open, causes the fuel/air mixture to go bad and the motor quits which is what Dave said.

Edited by - on

JimB517
Past Commodore

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USA
3285 Posts

Response Posted - 04/19/2004 : 23:06:42

after running it hard in the slip tonight I am 99% sure its cavitation. The prop get surrounded by millions of small bubbles as it spins up to very high revs. If I grab it and throttle down all returns to normal.

I'm going to get a new fuel line just in case. The vent is open.

Either the bottom is more dirty or something else is changed (maybe the knotmeter is reading low).

Edited by - on

ClamBeach
Master Marine Consultant

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3072 Posts

Response Posted - 04/19/2004 : 23:35:17

I can understand the 'rev up' part... but I don't think cavitation will make the motor shut down... unless something else is happening as a side effect. I don't think the Honda rev-limiter works in that fashion???

Edited by - on

RichardG
Admiral

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USA
990 Posts

Response Posted - 04/20/2004 : 18:32:38

Jim:

It's a longshot...but you may want to take a quick look at the sheer pin. When my old Suzuki 6 was acting a little funny last year, I found that the sheer pin was broken on one side (where it sticks out) and significantly bent on the other. Replacing it eliminated the problems.

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