3461S Carb Choke Problem (1963, 327/340HP)

[Peter L.]

My 63 convertible, has a 327/340 HP engine, with the correct 3461S carburetor. I've had the same problem since I finished the restoration 5 years ago, and the carb shop I used to rebuild it has been un-able to fix it. When I start the car cold (2 to 3 pumps of the gas peddle, one won't do) to set the choke and pump some gas into the manifold, the car starts right up, and goes to high idle (1400 RPM) perfectly. The problem is the once the engine is running, and the vacuum (16 to 17 inches) opens the choke from full shut, it doesn't open it enough (approx. 1/8"). If I do nothing, the engine will idle down over a 15 to 20 second time period, and stall. If I goose the throttle a few times, and let it run at about 2000 RPM for 20 to 30 seconds, the choke will open due to heat at the choke bi-metal spring, and it will work fine from there on. Or, if I manually open the choke right after cold start approx. another 1/8", the engine will smooth out, and run fine at the high idle speed. I have to hold it until the choke spring is warm enough to take over, and then it will run fine. The carb rebuilder is well known and highly respected by all NCRS members on both clubs I belong to in my area (Central New Jersey). I just got it back after he had it and completely went over it (dis-assembled, compared all parts to factory measurements, etc), and tested it on a flow stand with test fluid. I'm looking for help to see what he could be missing (I've had it to his shop both on and off the car 6 or 7 times in the last 5 years) that could be preventing the choke from opening enough during initial start, before heat takes over and opens the choke that way.

[Timothy B.]

Re: 3461S Carb Choke Problem (1963, 327/340HP)

Peter,

Two things come to mind, first is the choke piston vacuum break adjustment which is done by bending the rod that runs from the choke piston lever to the choke blade, shortening this will pull the choke blade open further.

The second is to make certain the small vacuum feed hole that runs to the choke piston assembly and through the carburetor to under the throttle blades up from where the vacuum advance tube connects is open thus providing vacuum draw to the piston assembly. There is also a small gasket where the choke piston assembly bolts to the main body that seals that vacuum hole.

The 1963 corvette shop manual has a very detailed procedure of all the 3461 adjustments. Don't give up on it, these are very nice carburetors.

[Peter L.]

Re: 3461S Carb Choke Problem (1963, 327/340HP)

I'll be taking the car to the carb shop for a rod adjustment next week. Perhaps this rod adjustment is something that must be done on the car, not the flow test stand. I will also question him on the second possible problem you talked about.
Thanks.

[Harry S.]

Re: 3461S Carb Choke Problem (1963, 327/340HP)

I'll be taking the car to the carb shop for a rod adjustment next week. Perhaps this rod adjustment is something that must be done on the car, not the flow test stand. I will also question him on the second possible problem you talked about.
Thanks.

Peter, the small gasket that Tim refers to is actually an O-Ring. It needs to be there to ensure there is enough vacuum on the choke. Also, clean the brass piston in the choke. I use 12000 grit wet black sandpaper and that works well. You can also put the sandpaper on a Q-Tip and ream the hole the piston slides in.

Good luck.

[Peter L.]

Re: 3461S Carb Choke Problem (1963, 327/340HP)

I think to piston and the holes it goes in are fine, from my observations. The choke moves manually with no binding. And when I cold start it, and manually open it the extra 1/8" to allow more air in,, if I let it go, it will return to the "not open enough" position. There is no resistance to opening enough, or releasing it back to it's original position.
Also, quick question. Can this piston be accessed with the carb on the car, or must the carb be removed?
Thanks to both of you for our suggestions. I'll let you know how it turns out next week.

[Harry S.]

Re: 3461S Carb Choke Problem (1963, 327/340HP)

I think to piston and the holes it goes in are fine, from my observations. The choke moves manually with no binding. And when I cold start it, and manually open it the extra 1/8" to allow more air in,, if I let it go, it will return to the "not open enough" position. There is no resistance to opening enough, or releasing it back to it's original position.
Also, quick question. Can this piston be accessed with the carb on the car, or must the carb be removed?
Thanks to both of you for our suggestions. I'll let you know how it turns out next week.

The carb can stay where it is. There are about five screws that need to be removed to take the choke off the carb.

[Frank D.]

Re: 3461S Carb Choke Problem (1963, 327/340HP)

If you hold the throttle completely open and work the choke valve plate by hand back and forth and there is no binding then the piston is most likely not your problem... Did you (or someone) set the choke unloader adjustment properly ?

[Peter L.]

Re: 3461S Carb Choke Problem (1963, 327/340HP)

Guys,
Just an update to let you know I haven't forgotten your efforts on my behalf. As it stands, the rebuilder completely rebuilt the carb again, and the problem is still there. However, it's hard right now to document it, as our temperatures have been 55 or higher at night for the past 2 or 3 weeks (last night it went all the way down to 66 degrees). I'm waiting for a really cold night (45 or less) to really give it a test, then go back to the rebuilder and ask for a replacement.
BTW, when I force it by hand setting the choke fully closed the choke does open about 1/2 way (approx 1/8") to where it's supposed to (1/4") right after start before heat gets to the choke spring. and runs rough. If I open it by hand to about 1/4". it smooths right out. So who knows. It might be the choke assembly, but I find it hard to believe two (I also have a 64 carb he also rebuilt) carbs would both have bad choke assemblies, as they both do the same thing.

[Edward J.]

Re: 3461S Carb Choke Problem (1963, 327/340HP)

Peter, I am reading the posts trying to make head or tails os the problem, the 3461 carb. When working properly is a great carb. The choke is a little temperamental. Depending on the climate, it simple with a cold engine, set the choke maybe one or pumps to floor with pedal and this will close butterfly completely shut, put the fast idle cam on it highest lobe, When started the little piston inside choke housing will use engine vacuum to slightly open the butterfly to keep the engine from overloading with fuel or not enough air. The adjust on the choke housing shows rich or lean, by Turing clockwise or counter clockwise.
I would start by pulling the chrome tube off the housing and start up and put you finger over the fitting you should be able to feel a slight vacuum, if you Do that’s good.The best way to attack the choke is remove the black choke housing (3) screws, then there should be a metal shield facing you remove, the piston and link are behind shield, see if it moves freely,(they have been known to bind up from carbon because the hot air tube fails inside the manifold rusts through, causing to much hot air and this will also cause the choke to open to quickly. Also note that there should be no black carbon inside housing this is a sign off a failed tube. If in your first step no vacuum you will remove choke housing the remaining 2 screws, behind the housing the is a recess for a o ring seal that may or may not be there. The a seal for the vacuum port,without choke will not operate properly. Just follow each Step, with a little patience you can run these tests is about half Hour.
hour.

add. Choke housing adjustment is around 1 step rich. Keep in mind you can go either way a step or two. the bi-metal coils do weaken through. The heating and cooling cycles. And tinkering with the adjustments are required sometimes.

[Frank D.]

Re: 3461S Carb Choke Problem (1963, 327/340HP)

The above post just about covered the gambit of choke operation and the failure points. I have seen pistons stuck so badly that I took the Welch plug out and used Hoppes #9 solvent and a brass brush of the proper size and cleaned the inside of the cylinder like a rifle bore, I'm not a fan of abrasives like emery cloth and the like. Also, I set my choke on a typical cold morning by having the black cover loose with the throttle linkage held wide open and turn the cover until the choke just completely closes than a "scooch" more, maybe another 5*-10* turn.

[Peter L.]

[Chris D.]

Re: 3461S Carb Choke Problem (1963, 327/340HP)

One question: Does your fast idle rod attachment to the choke shaft look like the picture earlier in this thread? If so, that is wrong. Pull the clip and flip the cam to the forward position and reconnect. Ref figures 14 & 15 in the 63 shop manual. I've made this mistake and it does prevent proper operation.

[Peter L.]

[Edward J.]

Re: 3461S Carb Choke Problem (1963, 327/340HP)

Peter, the setting is 1 notch rich this can be adjusted 1 notch either way.Outside temp.or engine will effect just how much the butterfly is open or closed.

[Peter L.]