Automatic Choke

[Robert William Gray]

I have a 1972 21.5 Grew with a 188 Horsepower Mercrusier inboard-outboard.<br />In the past I never experienced any trouble with idling speed or with starting up when warm.<br />Now I find that I have to pump the throttle serval times, even when the motor is warmed up, to get it started. I don't want to set up the idling RPM's much more than what I have done to date.<br />I am told by reputable dealers and service men that the boat has an automatic choke on it. <br />I have looked at what I think is the automatic choke on the side of the carburetor, and it appears to have a bi-metal coiled spring inside of it, but it also has three electrical wires feeding into it. I haven,t as yet, tried to trace these wires, and won't do so until next spring because the boat has now been put into winter storage.<br />Can anybody give me the theory of operation of these types of chokes. I am aware of the bi-metal spring system, but I don't know without taking it apart what the wiring is supposed to do.<br />HOw does this type of choke system kick in when cold and kick out when hot?

 

[BigBabyLou]

Re: Automatic Choke

I had a similar problem with starting my engine and that's when I found out that the choke on my carb is also bimetal coil operated. The coil is enclosed in some kind of box which has one wire attached to it. I traced the wire to the alternator where it was interrupted (oxidation, age, mechanical tension). I replaced the wire and that fixed it.<br /><br />This is simply an electic heated coil which, when the engine is started up, warms up and opens the choke. I've heard of other systems when the choke heating coil is wired to the ignition key, that means it would open the choke after a while if you just leave the ignition on, not even running the engine.<br /><br />Hope this helps you track down your problem. Check the wires, trace them to their source and what do you know, they might be interrupted somewhere too.<br /><br />Good luck<br />Lou

 

[petryshyn]

Re: Automatic Choke

Because of the spiral shape, the bi-metal spring will apply tension on the choke valves when the temperature drops off. This tension will attempt to close the choke. <br /><br />The bi-metal spring and housing assembly should be adjusted (rotated) to "just" close the choke valve (no preload) when the ambient temperature is around 70F.(the throttle should be half open when checking this) <br /><br />Once the engine is started, voltage from either the alternators stator or the ignition switch will be applied to a heating coil behind the bi-metal spring. This will result in the choke valve slowly opening as engine requirements for a rich mixture decrease with warm up. <br />

 

[Robert William Gray]

Re: Automatic Choke

Thanks Gentlemen for your help in understanding the workings of automatic chokes. Now I al least know what to look for next spring when I take my boat out of winter storage.