[OMC-Boats] Automatic choke info

Got this email/article from a classic car group I belong to and thought it might make interesting reading to supplement earlier carb/choke discussions.

If might be worthwhile to save the link below as a resource for carb issues.
http://www.thecarburetorshop.com/

-Bill
Dallas, TX
1970 Evinrude Explorer - 155 Buick V6 - OMC Sterndrive
http://www.photobucket.com/evinrude_explorer

--- On Wed, 10/7/09, Tate, Randy wrote:

From: Tate, Randy
Subject: [northtexasamc] good article on automatic chokes
Date: Wednesday, October 7, 2009, 10:23 AM

excerpts: “Operation – integral hot air chokes The integral choke, like the divorced choke, closes due to a coiled bimetallic strip that rotates with temperature change. The coil is located inside the integral choke housing. Also inside the choke housing is a vacuum piston assembly. An internal vacuum source enters the choke housing behind the vacuum piston, and escapes by the choke piston to cause a negative pressure (vacuum) on the entire integral choke housing. As the housing is connected to the hot air choke tube, this negative pressure pulls air heated by the exhaust through the heat tube. This hot air causes the tension of the coil to relax, allowing the vacuum piston (this may be thought of as an internal choke pulloff) to open the choke plate.” “In the late 1950’s, there was some concern about unfiltered air entering the engine through the hot air choke, as the supply to the heat tube was generally unfiltered. At this time, a tube was pressed into the airhorn of many carburetors. This tube was then attached to the supply end of the heat tube pressed into the manifold. As the air supply now came through the carburetor air filter, this improvement eliminated “dirty” air from entering the engine through the automatic choke. This modified hot air integral choke was and is the most functional and reliable of all automatic chokes.”  “In the mid-1970’s, the electric choke was reintroduced, due to government smog regulations. The electric chokes were timed to go off much sooner, thus somewhat reducing emissions because of a leaner mixture. As in the 1930’s, drivability suffered until the engine completely warmed to operating temperature. Being a service manager in a new car dealership in the late 1970’s was a frustrating occupation when one had to convince the customer it just didn’t get any better.” “Electric chokes are functionally the poorest form of automatic choke, as the choke is controlled by a pre-set time, based on the heating element rather than engine temperature control. They are not even consistent from engine to engine, as the voltage applied by the electrical system will have a bearing on the time. The only “pro” for the electric choke is that they may be used on modified engines with aftermarket manifolds that have no provision for either divorced or hot air integral choke. This issue has been addressed in recent years in the aftermarket, as a “tunable” voltage control is available. This improves the electric choke, but only in that the operator can set the timing. The choke still doesn’t know the engine temperature. If a control is available that senses engine temperature and choke plate opening position I am unaware of it.”  “Testing – all except electric chokes and the 1937-38 Delco unit We get calls daily from those who would replace a perfectly good choke unit because their choke doesn’t function. With the integral choke, the odds are at least 100 to 1 on a non-functional choke that the vacuum source is plugged rather than a bad choke unit. To test the unit, remove the choke coil from the engine or carburetor. Place the coil in an “indexed” position on your workbench. Measure the linear distance the coil must travel for the carburetor choke to move from the fully closed position to the fully open position. Now aim a portable hair dryer at the coil and watch it move. When it reaches its maximum deflection, measure the amount of travel and compare this with the needed travel you measured on the carburetor. If the choke coil travel equals or exceeds the carburetor travel, the choke unit does NOT need replacing.” “This article, as with all other articles found on this website, have been written to place in the public domain and copied if desired. We only ask that you credit The Carburetor Shop LLC when you copy the article.”     __._
Received on Wednesday, 7 October 2009