Humidity Problems

Humidity Problems May 11, 2015

By Stephen Horsfal
from issue 27-5
I have an interesting problem with a late 70‘s, approximately 24 year old, 9.9 horsepower outboard.  I have owned the motor since it was new.  It has been well maintained and still looks great, all original. The motor always starts second or third pull.  

My issue with the motor is that it will not develop full power under humid or damp conditions and the boat has a hard time getting on plane.  This is not fun on a dark night coming down the lake.  Clear, low humid days she runs strong.  I took it in for service and they rebuilt the carburetor.  No other apparent problems, but it did not help improve the situation.  Next step, I changed the spark plug wires, no improvement.  I completed a compression test and both cylinders read around 90 psi.  I was told to run the engine in the dark to see if it is shorting out, but did not notice anything unusual.

Just wondering if you have run in to this problem before and would be thankful for any help you can provide.

J. Smith, via email

There are not very many reasons for an engine to lose power just because of a change in the temperature or the humidity level to the extent you are experiencing.  Here are few thoughts and observations that may help you with your diagnosis.
The compression figures are on the low side of ideal, not bad considering the age of the engine, but low enough that they could be a factor.
The main metering jet in the carburetor (high speed jet) might need to be changed to compensate for altitude if the water you are running in is at a higher altitude than say 500 plus feet above sea level.  If you take a look at your old spark plugs, they should have a nice light brown color inside.  If they are black, the engine is running too rich, and if they are white to light gray, the jetting is too lean.  If there are small flecks of aluminum on them, there is too much timing advance.
If the problem only occurs after prolonged slow speed, such as trolling, the idle jet may be adjusted too rich, causing excess carbon build up, that may take some time to burn off at high speed.
Another more obvious observation, if the problem only occurs at night when returning across the lake, have you checked for weeds fowling the propeller? You might not see them in the dark, but you can usually clear them be shifting back and forward with the gear shift a few times.  Then try getting up on plane again.
When the problem occurs, you could shut down the engine and remove the cover and make sure the throttle is actually going to the full throttle position. With the engine off, you can move the control to WOT and look in the air opening of the carburetor and see that the lower butterfly valve is open all the way.  If not, maximum throttle stop may need to be adjusted.
Lastly, if the problem ends up boiling down to the general age of the engine and it’s compression issues, you could buy some time before an overhaul to the purchase of a new engine by having the propeller pitch reduced by an inch or two to allow for a bit more out of the hole performance.  Just make sure that the engine is checked with a tachometer to insure that it does not over rev at the top end.
I am a very new (2 weeks) owner of an older Doral Citation 25 ft cruiser.  We did a lot of research and determined this type of boat would be the perfect starter for our family and we really love it thus far.  The boat has had a lot of major elements either rebuilt or replaced in the past three years so what little work we have left to do is mostly cosmetic.
I am also in need of replacing the fridge, as this one no longer holds a charge, and don’t really plan to invest much money in a 20 yr old fridge.  That being said, I was recently speaking with a friend who raised the question of possibly using a standard mini fridge, instead of using one made for marine usage.  I know this would only run off of direct power and not be AC/DC like the marine fridges, but given that our usage will be mostly going for shorter cruises it wouldn’t be as important that it run off battery power anyways to meet our needs.  I also know from doing work on campers and other portable devices that the compressors in many of the household type mini fridges are much more durable than they used to be and would stand up much better to the change in position which was one of the reasons people could not use these in the past. 
So I guess my question is, knowing what I know (and not knowing what I don’t know) is there any valid reason for not using a standard mini fridge as opposed to using a marine specific fridge with the understanding that it would only have power when we were connected to shore power?
Thanks in advance for your help.
J. Hughes, via email
The problem with using a camper or a household mini-fridge in a boat is that they don’t usually meet the requirements for spark arresting electrical devices.  In the enclosed space of a boat where there is the possibility of fuel vapors in enclosed spaces, this can be a very real safety concern.  I don’t think it is worth the risk.
This advice goes for any electrical device on your boat even and including automotive parts used as a replacement on your marine engine.  These pieces may look the same and work the same but they are NOT THE SAME!
They may work fine when they are first installed and there are no fuel vapours on your boat, but all it takes is one careless restart without proper blower ventilation of the engine compartment to turn this into the trigger for a bomb.
If you check out the camper refrigerator for example, you will see that when it is installed in a travel trailer or RV, it is usually mounted with the back of the fridge towards an outside wall and a ventilated grill covers the space to allow air circulation.  This type of installation is not possible in most boats with the possible exception of some pontoon houseboats.
Automotive starter motors, generators, ignition components, etc, may work fine in a non-volatile situation, but one small gasoline leak in the enclosed space of the engine compartment can blow the hatches right off the deck!  Please do not take the risk.

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