# Boat Problem Solved- 4000rpms



## fish2day (Apr 9, 2011)

Thanks to all who gave me advice on repairing my boat. It would not run over 4000 rpms. After several hours and 3 mechanics who could not find the problem. 
Shane Livingston of Livingston Marine Repair removed the Air Silencer and it ran like new. The Air Silencer was choking the carbs keeping it from getting enough fuel. 
I hope this will be helpful to others who may have the same or similar problem.

Thanks to PFF, A GREAT SITE !!!!!!!!!


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## Sequoiha (Sep 28, 2007)

Did he leave it off.


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## floater1 (Oct 31, 2007)

had the same problem on a 92 evinrude 200 me and shane pulled off the air box and it ran like a scalded dog so to fix it i used a 1 1/2 holw saw and cut six holes in front of the the carbs covered the inside with some small diameter stainless mesh and air breather foam bolted the air box back on.that was two years ago sold the boat to a fellow forum member no problems to date still running like a scalded dog


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## TheCaptKen (Mar 14, 2011)

Strange, there's nothing in the carb cover to restrict the air flow.


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## X-Shark (Oct 3, 2007)

TheCaptKen said:


> Strange, there's nothing in the carb cover to restrict the air flow.


That's what I was thinking.....Thought maybe it was broken and the vacuum from the throttle bore was sucking the flapping part down on it?


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## fish2day (Apr 9, 2011)

Sequoiha said:


> Did he leave it off.


No, He didn't leave it off. It has a 1/4 in gasket that gets compressed from being overtightened. The gasket will compress in partically covering the slow jet or idle jet. It also had it half choked. It was starving for fuel. My inline fuel filter was almost empty until he removed the cover, then it filled up.

You can replace the gasket or use flat washers as spacers to move it off the face of the carbs. I would not run it with the silencer cover off.


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## duckgrinder (Jul 14, 2009)

fish2day said:


> No, He didn't leave it off. It has a 1/4 in gasket that gets compressed from being overtightened. The gasket will compress in partically covering the slow jet or idle jet. It also had it half choked. It was starving for fuel. My inline fuel filter was almost empty until he removed the cover, then it filled up.
> 
> You can replace the gasket or use flat washers as spacers to move it off the face of the carbs. I would not run it with the silencer cover off.


Your backwards on your thoughts of being starved for fuel. If you have an air restriction you will be running very rich on fuel. The carb dosen't know how much air is coming in, the jets will pass X amount of fuel no matter how much air is coming through the butterflys. I call BS on this being the problem unless something is loose inside the air box. If the air was restricted that much it would never idle.


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## duckgrinder (Jul 14, 2009)

I bet your linkage was hitting your air box, with it "shimmed" out now your getting full throttle.


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## hogdogs (Apr 19, 2010)

> The carb dosen't know how much air is coming in, the jets will pass X amount of fuel no matter how much air is coming through the butterflys.


Negative ghostrider...
Jets will flow X amount of fuel for every cubic foot of air passed across the jet at sufficient velocity to create a vacuum in the throat to draw gas through the jet. 

And the blades do not only determine cubic footage of air flow but they can be used to prevent the air flow over the high speed jet located above the cenerline axle of the blade until the blade is opened enough to expose the jet to the air flow. In essence the high speed jets are blocked from the wind so as not to be running excessively rich until a point in throttle travel that would occur above RPMs that the lower speed jet circuit can keep up with fuel demand solely...

So reducing the cubic feet of air WILL, in fact, reduce fuel flow as well as air flow.

Exactly how a restrictor plate works in nascar engines. He only had enough air flow to support 4,000 RPMs and the fuel supplied was delivered by vacuum to support only the 4K worth of air flow.

Brent


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## TheCaptKen (Mar 14, 2011)

Being that both high speed and low speed jets are located in the bowl of the carb, only the air metering jets are in the upper part of the carb. 
The cork gasket between the carb cover and the carb is maybe 1/32 thickness and cannot squeeze enough to block air flow enough to drop that kind of rpms. Washering out does nothing being I have seen many installed without the gasket. But without the gasket, the shield is now not sealed and any flooding will allow fuel to drip into the pan and not get drawn back into the intake like it was designed to do. 
aka fire hazard.


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## duckgrinder (Jul 14, 2009)

hogdogs said:


> Negative ghostrider...
> Jets will flow X amount of fuel for every cubic foot of air passed across the jet at sufficient velocity to create a vacuum in the throat to draw gas through the jet.
> 
> And the blades do not only determine cubic footage of air flow but they can be used to prevent the air flow over the high speed jet located above the cenerline axle of the blade until the blade is opened enough to expose the jet to the air flow. In essence the high speed jets are blocked from the wind so as not to be running excessively rich until a point in throttle travel that would occur above RPMs that the lower speed jet circuit can keep up with fuel demand solely...
> ...


The jets have zero to do with air flow. You adjust your fuel flow to air with jets. I have worked on carbs for many years running my own business. I know how they work. You are wrong on this trust me. If you have an EFI you would be correct. Your mass air flow sensor will determine how much fuel will flow by the ammount of air comming in. In a carb once you get off the idle circut you will flow as much as the engine can take, if you restrict air from the top you will run fat. With a restrictor plate you reduce the total intake charge to the manifold not the carb. The carb dosent know there is a plate under it, it will still think it's flowing 850 CFM (NASCAR). When you cut air from the air horn of a carb (I don't care if it's a weedeater, boat or car they are all the same) your fuel dosen't atomize correctly you get fuel dripping and puddling and your to rich. I used to travel the country fixing peoples carbs at racetracks and for everyday drivers. I've seen it a million times, if you think your carb can actively manage fuel delivery by the incoming air charge I don't know what to say. Go start your vehicle and put your hand over your carb and see how long it idles, now try to restrict air flow and drive around the block, see if your plugs are wet.


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## floater1 (Oct 31, 2007)

what ever the cause is i know it has to do with the evinrude cover on the late model evinrudes i can show u 4 that will not run with the covers on not just 200's my next door nieghbor has a 88 evinrude 35 same problem with his we fixed his just as i did mine runs and idles perfect so some where over time some thing in the plastic housing bends or warps and causes this.


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## hogdogs (Apr 19, 2010)

> The jets have zero to do with air flow.


You are ABSOLUTELY correct on the above... Because the air flow has everything to do with fuel flow... 
What do you think causes the gas to shoot out of a jet from the bowl? Surely you will not say the fuel pressure created by the fuel pump.



> You adjust your fuel flow to air with jets.


Not really... You create a balance of the correct amount of gas for the amount of air flowing by choosing from the various orifice sizes...



> I have worked on carbs for many years running my own business.


As have I from the smallest 2 stroke equipment to the 1,100 cfm modified race engine carbs including dual 4's and triple dueces. Not only do I "work" on them but I fully overhaul them as well as full performance mods for wild performance applications such as converting a 50:1 2 stroke weedeater to run on 40% nitromethane hobby engine fuel... Super fun project figuring that one out with out blowing it up!



> I know how they work.


If you do not realize the venturi affect is the only thing supplying fuel to the air charge, than you are missing some important info...


> The carburetor works on Bernoulli's principle: the faster air moves, the lower its static pressure, and the higher its dynamic pressure. The throttle (accelerator) linkage does not directly control the flow of liquid fuel. Instead, it actuates carburetor mechanisms which meter the flow of air being pulled into the engine. The speed of this flow, and therefore its pressure, determines the amount of fuel drawn into the airstream.





> You are wrong on this trust me. If you have an EFI you would be correct.


The opposite... On EFI the sensors send signal to the computer which in turn sends signal to injectors... With failure of sensors or outside manipulation, you could cause the fuel to flow with out the engine running at the RPM the injectors are trying to supply fuel for unlike the carb engine...



> now try to restrict air flow and drive around the block


You mean like a slightly clogged air filter? NO WET PLUGS!!! Now if the carb has a plunger actuated accelerator pump it will wet the plugs because the accelerator pump is a rudimentary pump that squirts a shot with no air flw required. If you clog air flow to the point it flows less than allowed parameters in the venturi affect it will surely wet the plugs. On an engine without this pump, not so much... But where you are partially accurate. When you clog the intake horn on a running engine, the suction of the pistons will momentarily suck extra gas in through the jets via trying to suck the CFM of air they were before you clogged the venturi... "seek path of least resistance" comes to mind.

Brent


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## fish2day (Apr 9, 2011)

TheCaptKen said:


> Being that both high speed and low speed jets are located in the bowl of the carb, only the air metering jets are in the upper part of the carb.
> The cork gasket between the carb cover and the carb is maybe 1/32 thickness and cannot squeeze enough to block air flow enough to drop that kind of rpms. Washering out does nothing being I have seen many installed without the gasket. But without the gasket, the shield is now not sealed and any flooding will allow fuel to drip into the pan and not get drawn back into the intake like it was designed to do.
> aka fire hazard.


You may have seen many installed, but you are mistaken on this one. The gasket is much thicker than 1/32 on this silencer. And , only the high speed jet is located in the bowl, the two low speed jets are located in the front of the carb exactly next to where the the gasket seats. 
We ran it with the gasket off and the silencer on and it ran great,so I know that was the problem. The washers will give it more air if it is still restricted any. You are right about the fuel running put, but if that is happening you have carb problems. 
We can all learn from troubled outboards.


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## duckgrinder (Jul 14, 2009)

All I got to say is you know everything about everything, and no one esle has a clue. I understand all the principles, trust me. what causes the fuel to flow is the vaccum from the piston going down in the bore, the venturi causes the air flow to speed up atimotizing the fuel. If you take the air away, piston goes down and wants fuel and it will flow. What causes the fuel to come out of the jets is the high speed air bleed, and vaccum. If the throttle is open far enough to get into the primary circut the air bleed will let fuel flow, (just like a straw full of water with your finger over the end take your finger off and water flows either wide open or a dribble). No Air No ATOMIZATION = FAT. But you will always be right. Whatever I'm done.


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## fish2day (Apr 9, 2011)

duckgrinder said:


> Your backwards on your thoughts of being starved for fuel. If you have an air restriction you will be running very rich on fuel. The carb dosen't know how much air is coming in, the jets will pass X amount of fuel no matter how much air is coming through the butterflys. I call BS on this being the problem unless something is loose inside the air box. If the air was restricted that much it would never idle.


I'm afraid you missed it on this one Duckgrinder. We ran it with the gasket off and the silencer on and it ran normal. The throttle was opening all the way but it was restricting the fueland air flow keeping the engine from opening up to full throttle. No BS here...


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## lastcast (Oct 12, 2007)

Thanks for updating your problem for us. I'm sure someone will have the same problem.


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## TheCaptKen (Mar 14, 2011)

Ok, OMC Carb class 101
The low speed is metered through the metering tube that you can see in the throat of the Carb. Air is controlled with the air jets in the front of the Carb. There is three holes in the top of the throat. One behind the butterfly and two in front. The rear hole is for dead idle and the other two come into play as you open the throttle to richen the idle mixture. Once the butterfly passes the third hole, the idle circuit is dead and the jet located at the top front quit sucking air. Then all air is sucked pass the venturi tube which meters the high speed fuel via the high speed jet in the bowl. Fuel richness is also governed by the float level. Higher the float, richer the mixture. If you restrict airflow in any way, you will cause more fuel to be sucked through the venturi due to increased vacuum in the Carb throat.


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