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  #21  
Old 05-29-2011
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bubb2 has a spectacular aura about bubb2 has a spectacular aura about bubb2 has a spectacular aura about
if you had a restriction in the exhaust it would show up both in neutral at full throttle and in gear at full throttle.
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  #22  
Old 05-29-2011
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It would be more of an issue under load though than full throttle in neutral.
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  #23  
Old 05-29-2011
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Quote:
Originally Posted by bubb2 View Post
You haven't got on my bad side. I am just a cranky old man
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  #24  
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Quote:
Originally Posted by mitiempo View Post
It would be more of an issue under load though than full throttle in neutral.
agreed
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  #25  
Old 06-01-2011
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Hi All - First, thanks again for everyones input.

Two things to add to the thread.

First, I opened the boat to full throttle in idle to see what max RPM I could get to. The answer is about 3400 and the throttle control is hitting the transmission lever which in neutral is right behind the throttle control which in the real world wouldn't be a problem since it isn't necessary to go wide open in neutral. While trying to inch the transmission control down to find the end of the throttle travel, i accidentally (although I guess I should've seen it coming!) dropped the tranny into reverse at about 3500rpm - probably did more damage then the rest of this issue ever will (seriously, does anyone think that one time shift is something i need to worry about or do anything to check damage - I shifted in idle several times fwd and rev after and all seemed fine).

So, it appears in idle the engine can hit max rpm's or something every close to it.

I did get inky water in idle running high rpms the first time I opened it up wide. The 2nd time I did it againt o try and find the rpm where it starts and it didn't turn inky, but i would say the water still had a grey tint and there was greyish exhaust smoke.

To point out something else I've never said before, I normally get NO exhaust smoke, and even when i have inky water there isn't always any visible exhaust color.

The next thing I want to report is that the PO responded to an email from me asking him if he'd ever seen inky exhaust water, what his cruising rpm targets were, and if he purposefully overpitched the prop - response is below in a couple of threads:


From PO in reply to a voicemail I left him:

Anyway, black exhaust is usually caused by unburned fuel, in turn caused by engine overloading - for instance, the throttle is pushed all the way up calling for more engine speed, but the engine just can't get there because of big waves, heavy load, or the displacement hull just won't go any faster.
I think you mentioned something about max rated engine RPM. You are only going to get there if the prop is perfectly pitched for the particular boat and engine combo, which can be hard to achieve in the real world.
So, in reality, if you see black exhaust water throttle back just a little and it should go away. If you want to pull the exhaust elbow it can't hurt anything, but it might turn out to be a PITA job. Hope this helps.

My reply:

Basically since I had the boat I had been cruising around 2200rpm – I think maybe this is what you told me was your guidelines. Then I read things online that says the engine wants to be run at 80% of the rated continous rpm, which would mean a cruising rpm of something like 2800-3000. When I started doing that I got the inky black exhaust.

I’ve posted the issue on two message boards and gotten a lot of great advise. I didn’t think the issue could be pitch (what people call being overpropped I guess) since you had bought the prop special I figured you got the appropriate size one. Some people did say that some owners intentionally overprop the boat to reduce noise and fuel consumption by running lower rpm’s – did you do that on purpose?

His reply:


Yeah, the boat might be a little overpropped, but not on purpose. I installed the prop the dealer recommended, and it wasn't worth it to me to haul the boat back out, send the prop back, and install another one to get the pitch just perfect. I think 2400 RPM or so would be a good long-distance cruising speed and 2600-2700 if you were in a hurry and not too concerned about fuel consumption. It's not so much percent of rated RPM that you are looking for, as much as percent of rated power output. If the prop is perfectly pitched, the two go hand in hand, but if the pitch is a little high then you get higher power output at a lower RPM
.

I bought the boat largely based on my experiences with the PO and knowing that he is both more knowledgable on all systems than I am, and more careful/concientious I suspect than I am on taking care of things. So I am going to basically now use his RPM guidelines, maybe check the elbow when I get a chance, and forget about it! But, since everyone got behind me to try and figure this out, I wanted to give you more info, and if you want to keep discussing or have other thoughts, please share.

Disclosure, the PO is now probably reading this thread so be diplomatic.
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Old 06-01-2011
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Forgot to add - I used decompression levers to check both cylinders, and with either lever thrown the engine sounded awful, so I think that means by injectors/valves are OK right?
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Decompression levers are for starting, either by hand or to help spin it before electric starting. I've always heard that releasing the compression on a running engine is a good way to do some serious damage to the valves and wouldn't recommend it.
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Old 06-01-2011
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Uhoh - I hope what you've heard is one of those hyperbolic tales that is a warning of something that could but usually doesn't happen.

Can anyone else chime in on whether flipping the levers very briefly at idle speed in neutral may have damaged the engine, and what I can check to see if it did indeed?

Sandy
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Probably less risk in neutral than at higher speeds. As far as I know the decompression levers keep the exhaust valve open for easier rotation of the engine for hand starting. When the engine is running those valves open and close each time a given cylinder fires. These 2 actions are at odds with each other and ultimately something will break.
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Old 06-01-2011
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I believe the decompression lever on the 2GM20F just pushes the exhaust valve down some so it is open. I tried this once on my 2GM while idling... and it immediately snapped closed--I don't think it is meant to be messed with on a running engine. Then again, I doubt you hurt it.

No one else has mentioned this, but it can't hurt to check your valve clearance. You sound like someone not afraid to mess with stuff and even if it has nothing to do with your current issue, it would be good to know the clearances are correct.

BTW, I've read the advice to run the engine at 80% etc. Not a bad idea to do that now and then, but you should cruise at what's comfortable, I think. My boat has the original factory 2 blade prop, and under most conditions 2500 rpm gets us somewhere around 6 knots, IIRC. We rarely use more than 2500.
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