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Author Topic:   Engine Speed When Running On Muffs
mark9199 posted 12-13-2005 08:48 AM ET (US)   Profile for mark9199   Send Email to mark9199  
I have a 2004 Mercury 60 HP EFI Bigfoot engine that I bought new last January with a 150 SPORT. When running this engine at home with water muffs, the engine speed is erratic. I can throttle up from idle up to about 1100-RPM and them it jumps to over 2000-RPM. I can not get the engine at a speed between. I have only had the boat in the water two times due to other problems, and I am still not sure that the engine speed is ok while in the water. But it does seem normal, or near normal.

I am curious if [problems maintaining engine speed are] normal when out of the water or not. One service department has told me that it is, but I would like to hear it here also.


peconicwaters posted 12-13-2005 09:07 AM ET (US)     Profile for peconicwaters    
Engines generally run at a higher speed out of the water and have different throttle response. When it is out [of the water and running on] muffs, the exhaust system has no back pressure. Some companies sell a product used as a muffler that you can clamp on propeller to solve this problem.
where2 posted 12-13-2005 12:25 PM ET (US)     Profile for where2  Send Email to where2     
To simulate back pressure like running in the water, get yourself a large trash barrel and fill it with water. Put the barrel around lower unit before filling with water. To drain the barrel, you may have to use your garden hose as a siphon, but it should accomplish your simulation just fine. You can adjust the height of the engine in the water using the tongue jack on the trailer.

Welcome to the backyard mechanic method.

TRAFFICLAWYER posted 12-13-2005 02:00 PM ET (US)     Profile for TRAFFICLAWYER    
I have a 15 SPORT with 60-HP Bigfoot. I don't worry about it unless it runs that way in the water. Like the others have said back pressure [problems].
mark9199 posted 12-13-2005 04:22 PM ET (US)     Profile for mark9199  Send Email to mark9199     
Thanks all for the replies.


kingfish posted 12-13-2005 04:35 PM ET (US)     Profile for kingfish  Send Email to kingfish     
In the event you consider the use of an exhaust hub "muffler", the gizmo that was described above that clamps on the propeller hub and develops back pressure, I have one to sell cheap. I won't use it anymore because I'm fairly certain the use of it on my 1992 Evinrude 225 was the reason the upper seal on my exhaust chimney blew.

If you want it let me know.


vink posted 12-13-2005 10:06 PM ET (US)     Profile for vink  Send Email to vink     
Why are you running this engine with muffs when it is designed to run on a garden hose connected to the tell tale outlet? Just wondering.
Moe posted 12-13-2005 10:41 PM ET (US)     Profile for Moe  Send Email to Moe     
Maybe that's why the bold print in the service manual says, "IMPORTANT: Do not run the engine above idle when flushing."


jimh posted 12-14-2005 09:10 PM ET (US)     Profile for jimh  Send Email to jimh     
It is not clear to me if we are talking about a two-stroke or a four-stroke engine.

When the exhaust outlet is not submerged there will be much less back pressure on the exhaust. This can lead to variations in the engine speed for a given throttle setting. I have heard of motors which have gotten into high speed run away situations aggravated by lack of back pressure on the exhaust. I believe this is one of the reasons that most owner's manuals caution against attempting high-speed operation of the motor when the exhaust is not submerged.

Two-stroke motors are somewhat fussy about their idle speed ranges, and they seem especially sensitive to exhaust back pressure. I don't know if four-stroke motors are affected in the same way.

Jkcam posted 12-20-2005 05:38 PM ET (US)     Profile for Jkcam  Send Email to Jkcam     
I believe that the 60hp EFI Bigfoot is a 4 stroke.

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