L78 Choke Setting

[Joel T.]

Hi All;

I'm getting ready to take my 1965 up to Mass in September for a PV. Pretty much everything is working but I am still having an issue with the choke setting... it basically starts out a bit too rich so the engine stumbles as it warms until the choke opens a bit more and the high idle (like 2000 RPM per the book) gets reached.

Question, how much should the choke be open once she fires?? I don't see that setting listed anywhere in the manuals I have on hand. Also, when you bend that rod to the high idle cam, which way to you bend it so as to not interfear with anything??

Thanks!!

Joel

[Wayne M.]

Re: L78 Choke Setting

Joel -- just a quick review of the specs for the 3124. Choke unloader 0.375"; choke bakelite cover set to 2 notches lean on scale. What color is your plastic fast-idle cam ?

[Joel T.]

Re: L78 Choke Setting

Wayne'

.375 would be a 3/8" drill if my math is correct.. On the 3124, the high idle cam is red in color...

The carb was rebuilt (it is actually a service replacement). The choke thermostat (the black part which rotates) has no index mark on it.. I did scribe one on it which corresponded to the center mark on that back plate with all the adjustment marks. The high idle would not set so I rotated the thermostat CCW two marks. The cam now engages but the butterfly does not open quite enough as best as I can determine.

Thanks for the response!

Joel

[Wayne M.]

Re: L78 Choke Setting

Joel -- yes; same as 3/8" drill. Here's a pic on my service 3124 (date 653) with a red cam, showing the index on the bakelite -- you can see it almost 1 line to the left (ie. lean) of the cast index scale. It's a very fine line and easy to miss.

I have another 3124 service (dated 874) with a neutral (or white) plastic cam -- that was the reason I asked the color.

When you fire up the cold motor, does the choke piston immediately open the choke plate somewhat from the pre-start position ?

[Joel T.]

Re: L78 Choke Setting

Wayne;

Hmm, the carb I have does not have that marking on the black plastic as yours does... I scribed one which matched up to the triangle center mark... (based upon the initial set up by the rebuilder) and I am now two marks to the right of that triangle based upon your picture.

Did you ever have to bend the rod to the cam or did you get proper adjustment via rotating the thermostat?

Thanks again,

Joel

[Larry M.]

Re: L78 Choke Setting

Joel:

For 1966 and 1967 cars, the Chevrolet Service Manual and the Chevrolet Overhaul Manual both have all the specs and the adjustment procedures. For 1965 cars, you may only need the 1965 Service Manual Supplement.

The Section you want is 6M ENGINE FUEL. This contains all the adjustment procedures. The 6M SPECIFICATIONS SECTION at the back of the manual(s) has all the specs/settings in tabular form.

Holley probably has similar info on their website under the TECHNICAL HELP heading at the top of the webpage.

Years ago when I worked more on these carbs, I remember if you got the wrong tension choke coil spring, or a spring that was damaged or stretched, it would significantly affect proper choke operation. May want to checkout that you have a good/correct one. I believe that they are also came in clockwise (CW) and CCW rotations.

Larry

[Wayne M.]

Re: L78 Choke Setting

....For 1965 cars, you may only need the 1965 Service Manual Supplement.

The Section you want is 6M ENGINE FUEL. This contains all the adjustment procedures. The 6M SPECIFICATIONS SECTION at the back of the manual(s) has all the specs/settings in tabular form.
Larry

Larry -- 1966 was the first year for divorced chokes (coil installed in manifold stove, with a cover, and protruding rod). The 1965 Corvette Shop manual was too early for the 396 engines, although settings for the Holley List 2818 are similar yet different.

I took the numbers from the 1965 Chev INTERIM shop manual, covering the mid-year introduction of the Turbo-Jet engine. As you say, section 6M for procedure, with specs at the end of the book.

[Jack H.]

Re: L78 Choke Setting

Generally, the right direction, BUT, 1965 is an exception! The 396 made its debut mid-way through the '65 model year and the BB engine is NOT covered by the 1965 Corvette Shop Manual Supplement. You have to go get the 1965 Chevrolet Interim Shop Manual for details on the L78 and its 3124 carb...