Electric Choke Conversion

[insert name here]

I have a 1980 Chevy C10 with a 305 engine. It's a "Big 10" if that matters. The carb (2-barrel) has some issues, and I have a new one I was going to put on, but it has an electric choke (original is hot air). I've been putting off doing the swap because I didn't want to convert to the electric choke, but it's really time to get this job done.

My understanding is the electric choke should be wired to the oil pressure switch. Thing is, my truck doesn't have one. It has an aftermarket pressure gauge that is completely mechanical. In place of the original switch is just a fitting that allows oil to run through a tube into the dash. (Note that I'm referring to the pressure switch on the top driver's side near the back of the engine. If there's ANOTHER oil pressure switch elsewhere, please let me know.)

As far as I've read, wiring directly into an ignition source (such as the wipers or an open spot on the fuse block) is NOT the correct way to wire a choke, because it will still draw power and disengage the choke when the key is on but the engine is off (draining the battery and making the truck hard to start).

What options do I have for wiring that's only energized while the engine is running? Alternator exciter wire, maybe? I feel like I've heard that there's a wire on the starter that's only energized once the engine is running?

I did find an unused plug dangling down from a wiring harness recently. Someone had tied it to the transmission bellhousing to keep it from flopping around. I'm curious what that is (blurry picture attached), and if it could be useful.

 

[75gmck25]

I’ve seen recommendations to tie it to the ignition-on wire for the wipers, but I’ve never tracked it down. My ‘75 K25 has a brown wire under the hood that is used for the relay that disconnects the aux battery when the ignition is off. I don’t know if the same brown wire is on all trucks, but it works fine for my electric choke.

 

[Ricko1966]

Buy a ps 126 pressure switch. Install it at the port over the oil filter or tee it into the port under the distributor. Hook 1 side of the ps 126 to power,battery power is fine. Hook the other side to your choke. Ps126 is normally open it will not let power to the choke until the engine has oil pressure. This is the reason you can hook it straight to the battery,starter whatever is convenient. Just like the wall switch at your house has constant power but the lights only come on when you flip the switch. Clarifying before the rush of everyone saying you have to Hook it to keyed power. You can,but you don't have to.

 

[insert name here]

Sorry it took me awhile to come back to this. I did not see a spare brown wire on my truck, but I found that the black wire connected to the alternator only has power once the engine is running. So I opted to add another wire at that connector to control a relay (to keep the added draw on the existing circuit low) so I could add a new circuit for the electric choke. I also tested the heating element in the electric choke to size the wire for the new circuit, and to my surprise it drew less than 2 amps. Almost makes the relay and new circuit feel pointless.

Even though I already have a solution, I'm curious about that port over the oil filter you mentioned, Ricko. I took a look at it on my truck, and it was there but empty. I figured it would have a plug installed if there were no pressure switch, but nope. Just a hole with some threading at the tip. For trucks that have a switch there, did they drill the hole out at the factory so oil could reach the switch?


I also have one other general question (though maybe it should be a new topic). The new carburetor has very strange idle mixture screws (picture below). I've never seen these before. Is the implication that the manufacturer doesn't intend for them to be adjusted? I was under the impression that all new carburetors need the idle mixture adjusted to match the engine they're installed on.

You must be registered for see images attach

 

[Radiohead]

The manufacturer most certainly didn't want you messing around with them because then the government could bitch about Johnny Sixpack getting ahold of them and upsetting the emissions applecart. But its not 1985 any more, so those have been rendered adjustable and I think the type of drive is called double D. I think I have a tool for those from Thexton but any tool will work if the drive part is right.

 

[75gmck25]

The original idle screws have a double-d head, and when assembled by GM they also had a tamper-proof cap over them. Your caps have been removed/drilled out. You can get an adjustment tool made specially for that screw head, or buy replacement mixture screws with a standard screwdriver head.

 

[Sad Sack]

Consider what @Ricko1966 posted.

 

[Ricko1966]

I think you were looking at the hole for the clutch ball,you'll see the plug I'm talking about and it was the factory location for the ps126 I was talking about on trucks that used them. One pic shows the switch for 87 up in place.
As for your DD adjuster screws in a jam I have done both of these. Taken a bolt and a cut off disc to cut a channel through a 5/16th bolt,just big enough to hit the 2 flats. I've also taken brake line/fuel line tubing and squished the end just a little so it would hit the 2 flats.
And adding a relay to terminal 1 at the alternator was absolutely the right move,it's the other way I tell people to do this circuit. Make sure the relay trigger is on terminal 1 on the alternator,but by using a relay it keeps the charge indicator light isolated from the choke coil so the charge light works as intended. Both these methods the ps126 and the relay were factory methods, relays on Gbody's, pressure switch on trucks.

 

[Matt69olds]

The original idle screws have a double-d head, and when assembled by GM they also had a tamper-proof cap over them. Your caps have been removed/drilled out. You can get an adjustment tool made specially for that screw head, or buy replacement mixture screws with a standard screwdriver head.

Take the original mixture screws out, clamp them in a vise (use a couple pieces of wood to protect the threads) and carefully cut a groove in the head to allow adjustment with a “ “primative” screwdriver.

 

[Matt69olds]

Take the original mixture screws out, clamp them in a vise (use a couple pieces of wood to protect the threads) and carefully cut a groove in the head to allow adjustment with a “ “primative” screwdriver.

another idea, slip hose vacuum hose over the ends of the mixture screws. Home made handles!!

 

[insert name here]

Thanks for the replies, everyone. I got a set of specialized screwdrivers today for a few bucks, and it included the double-D type. New carb is on, idle mixture is set, and the electric choke works as intended with the new wiring.

(And Ricko, you are correct about plug above the oil filter. I took a quick look again today out of curiosity, and I was looking at the wrong hole. The plug was there when I looked in the correct spot slightly lower.)