Max RPMs at hull speed?

[High Hopes]

Hi folks,

I need some advise. My A4 starts as easily as my car, sounds smooth when it runs, idles at about 500 rpm with no problem.

The only hitch is that once I reach about 5.7 knots, opening the throttle further doesn't seem to produce and more engine speed. I seem to remember hitting over six knots before the engine was pulled and refurbished. I am thinking that hull speed was just over six knots on a calm day.

The engine RPM peaks out at about 1800 RPM. It sounds more content to run at 1600 rpm. I remember thinking two years ago that the prop would cavitate beyond a certain speed. But I am not sure it really ran differently then, than it does now. I didn't have a tachometer at the time, so I can't claim that I really knew what was happening.

One other note. . . With the engine in neutral, the engine gleefully revs up over 3,000 rpm (about as fast as I want to spin it).

Once the boat hits hull speed, drag increases enormously. My question is, should I still be able to spin the prop faster, even thought the boat has reached its maximum speed? Or put another way, is there a problem? or is this expected behavior?

One other note. . . I fiddled with the main fuel jet, which is adjustable. It seems that at cruising speed, opening the jet (backing it out) beyond a certain point has no effect on engine speed. I can slow the engine down if I turn the needle in. The needle is at the nominal position (about 1.5 turns as I remember). I was expecting to find a center position between too far out and too far in that would be optimum. Does anyone have experience with the adjustable fuel jet?

Steve

[smosher]

Could it be that the hull and or prop has picked up some growth..

The prop will spin faster, fuel useage will increase. The bow will raise up but it won't overcome the bow wave.

[High Hopes]

Hello Smasher,

There probably isn't any growth on the prop. The boat was splashed in early May. There is a new cutlass bearing. I need to re-check the shaft alignment.

Steve

[Baltimore Sailor]

Are you sure your tach is set on 4-cyl? I had that problem once, it was set on 6-cyl. Made a big difference in my rpms when I set it correctly.

[msmith10]

I'm not sure about the drop in top speed (6 knots to 5.7) but everything else you're describing sounds very normal, or at least is consistent with my experience, whether that's normal or not. I can do about 5.4 at 1400, 5.7 at 1600, 6 at 1800. She'll top out at about 2000 wide open throttle in gear, but speed doesn't seem to increase, and the throttle difference (and I assume the fuel usage) from 1800 to 2000 is huge. The most comfortable (by the sound of the engine) cruising range is between 1400-1600.

[roadnsky]

Timing?

Steve-
Have you done the "advanced timing" procedure?
(Turning the distributor while under cruising RPM)
I found thru experience that it helps to do this while actually out on the water under load as opposed to pulling dock lines.
That'll gain you about 150-200 RPM which could be up to a half knot.
It would certainly get you from 5.7k up to 6k.

Now of course, you probably know all of this.
I'm just trolling for all possibilities...

[High Hopes]

Thanks guys.

Baltimore, the tach is set correctly.

Smitty, The behavior you describe is like mine. There is a lot of throttle throw after the engine hits cruising speed. On the slightly slower hull speed, it may have been due to weather/wave conditions - or a paint fouled speed wheel on the hull. I will calibrate to GPS when I get the chance.

Roadnsky, Yes I have set the timing under power at sea. The best position ends up where I originally set the timing on the bench at 1600 rpm. But I may fiddle with it again. I appreciate your experience. I could use an extra 1/2 knot.

Has anyone had any experience with the adjustable needle valve kit? It seem I can’t over-enrich the fuel mixture.

Steve

[roadnsky]

Has anyone had any experience with the adjustable needle valve kit? It seem I can’t over-enrich the fuel mixture.

Steve-
I don't think this will help much, but it's my understanding that the adjustable main jet is designed so you can "fine tune" to a leaner mixture.
AND I think the fixed jet that comes with it is shaped differently and possibly smaller.
Anyway, my point is, maybe by design, you may NOT be able to over-enrich the mixture.
Make any sense?

[Dave Neptune]

Jet size

Guy's, I remember measuring the adj. jet when I bought it and it was only slightly larger than the stock one so you will not be able to "richen" much. If memory serves me right it was .002" larger or just the next jet size up ( maybe 2 sizes as I was raised in the 60's) from Zenith.

What size prop are you using? The prop is what will absorb the power at a given RPM, if you are getting around 2000 rpm you're making between 15~18 HP if you are at the end of your "throttle position performance"~~IE when you give more throttle not much more happens RPM wise. A vacuum gage will give this info to you.

Dave Neptune

[High Hopes]

Hi Roadnsky,

This is what the literature in the catalog says.

The orifice in the "fixed" part of the adjustable main jet assembly is almost twice the diameter of the orifice in the current fixed (non-adjustable) main jet in standard late model carburetors. This gives you control over the richness of the mixture, from total shut-off (needle part of the assembly all the way in against the orifice of the fixed part of the assembly) to extremely rich, as the needle is withdrawn from the orifice of the fixed jet.

Maybe I need to keep opening it up all the way? Maybe the engine can only draw so much air?

Dave,

Can you tell me more about the vacuum gauge? What will I expect to see when I open up the throttle?

Steve

[roadnsky]

Steve-
Well, that certainly details it. How far have you opened the needle?
I'd say it's worth a try opening it all the way.
If you're still not getting a response, I'd call Don and see what he thinks...

And Dave has a good idea with the vacuum gauge.
Combined with the tach, you'd have a really good scale of what's going on during the power curve.

[msauntry]

Two points. Your conditions are normal for a boat that is overpropped. Extra throttle produces no extra speed. Reduce the pitch of the prop to allow it into the higher HP range and you can get into the 6 knot range. Second point... I don't know where you are located, but you said you splashed in May and probable don't have growth on the prop? That's plenty of time for an ecosystem to get started. It would also give the same symptoms you describe. I know when my prop is dirty because my max speed goes from 6.2 to 5.8.

It only takes a few barnacles to ruin the efficiency of a prop. I'd bet this is your problem, which means you're lucky! Its easily fixed!

[Marian Claire]

I adjusted my main jet during my air leak around the throttle shaft issue. When all was said and done the best setting was right back to the factory recommendation. Dan S/V Marian Claire

[sastanley]

need to eliminate all the variables

Good point Micah (#2) - I had overlooked this until you mentioned it again.

I splashed in the middle of May also, and have cleaned my prop twice..and wiped down the water line as well. Actually a buddy did the second scraping for me last week, as I recently had rotator cuff surgery.

In late June, I was able to scrape the prop clean with a plastic putty knife. This past weekend, the barnacles tore up the plastic knife and we had to resort to a metal scraping tool to remove them..it immediately made an impact. Even my friend, who's only been on the boat a few times noticed the difference in engine efficiency once the barnacles were scraped off, which produced an additional .5 knots at the same engine RPM.

We ended up motoring all weekend, I sure am glad I had a clean prop!

[Dave Neptune]

vacuum

Hi Roadnsky,

This is what the literature in the catalog says.



Maybe I need to keep opening it up all the way? Maybe the engine can only draw so much air?

Dave,

Can you tell me more about the vacuum gauge? What will I expect to see when I open up the throttle?

Steve

As you open the throttle allowing the engine to "suck" (vacuum) air moves into the intake raising the manifold preassure/decreasing the vacuum. As you approach full throttle the gage will drop to alomost zero which is ambient air preassure or all that is available for a naturally aspirated motor. This means that the engine is getting all the air it can manage at a given rpm.

Lets use an example, on my E-35 I cruise at 2000rpm's which gives me a reading of 8" of vacuum when all is clean this gives me about 5.8 kn's. Now lets say it takes me 5 or 6" of vac to get to the same rpm I know something is amiss as the engine is working harder to achieve the same rpm's. It could be that the prop and hull are a little dirty an easy check or if things are clean I know that something is going wrong with the tune of the engine. Let the trouble shooting begin.

Dave Neptune