[Public-List] A4 Question

[Michael Connolly]

Steve, 
Carburetor ice is a distinct possibility. Especially as you have described your conditions of loss of power. There is no carb heat on our boats as an aircraft engine would have. On my engine (Gray Marine) there is really only in place a back fire arrestor not truly an air filter. This is all metal and would do nothing to keep the carb inlet temp elevated.  High humidity with temps just above freezing is a perfect scenario for carburetor ice. 
Michael #133 

----- Original Message -----

From: "Stephen Gwyn" <stephen.gwyn at nrc-cnrc.gc.ca> 
To: public-list at lists.alberg30.org 
Sent: Monday, January 6, 2014 4:29:00 PM 
Subject: Re: [Public-List] A4 Question 


My A4 sometimes runs less than perfectly in winter. It's 
usually hard to start if the temperature is close to 
freezing. Then it will run fine for 10-15 minutes, before 
losing a lot of power in gear. I'll motor out the marina 
in gear at idle, then speed up when I get out. A minute 
or two after that, as I'm in the middle of the current 
(and the rocks), the engine starts to slow down. Opening the 
throttle brings it back up to normal power, and after 1-2 minutes 
the engine starts to over rev. I then close the throttle 
a bit and the engine runs fine for the rest of the day. 

The loss of revs only happens sometimes. It only happens 
before the engine reaches operating temperature. It never 
happens in summer. 

I have some theories: 

1) Ignition system is damp or otherwise compromised by cold or 
possibly age. Damp isn't good for the points in particular. 
I replace the points, condensor, cap and rotor 
in the spring, so in winter they are at the end of their service life. 
But I don't think this is the case, because then the engine would 
run poorly from the start. 

2) Condensation in the fuel tank, which then gets into the fuel. 
More likely. Winters in Victoria are very humid and certainly the boat 
gets a lot of condensation in winter. But I have an over-sized 
fuel filter/water separator which I change regularly, so I'm 
moderately sure that's not it. 

3) Carburetor icing. If you put your hand on the carb throat 
when the engine is running, it's a lot cooler than the ambient 
temperature, and certainly a lot cooler than the nearby manifold, 
which gets quite hot. On cold humid days, I've noticed a thin coating 
of frost on the outside of the carb. My guess is that frost is 
forming on the inside of the venturi, restricting the flow of the fuel/air 
mixture into the manifold and into the cylinders. Eventually, however 
the manifold fully warms up and starts conducting heat to the 
carb. The ice melts, the venturi clears and the engine starts 
working normally again. 

SG 



-- 
------------------------------------------------------------------------ 
Stephen Gwyn                       | Tel: 1-250-363-3136 
Canadian Astronomy Data Centre     | Fax: 1-250-363-0045 
Herzberg Institute of Astrophysics | Cell: 1-250-885-6969 
5071 West Saanich Road             | E-mail: Stephen.Gwyn at nrc-cnrc.gc.ca 
Victoria, BC, V9E 2E7, Canada      | http://astrowww.phys.uvic.ca/~gwyn/ 
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