Need help with carb adjustments

[Richard M.]

Need help with carb adjustments on 63 327/340 with Carter AFB 3721 SB. When warm - engine is idling low and at moderate crusing speeds it is lunging. Also get an occasional backfire when revived. Appreciate help with how to adjust idle speed adjusting screw and idle mixture adjusting screws. Do these symptoms mean the carb is running lean?

[Timothy B.]

Re: Need help with carb adjustments

Richard,

To adjust the air/fuel screws, warm the engine then set timing at approx 12*BTDC at the slowest reasonable idle you can get. Next. turn A/F screws in-out to obtain fastest idle and max vacuum. You may have to go from side to side and when your finished both sides should be opened approx the same amount. This info is in the 1963 service manual as well as other adjustments for the AFB

If your 340 is original, can you verify the vacuum advance #'s on the can attached to your distributor. If the three #'s are 201 then you may want to consider up grading the advance can to #236 which is the 64-65 advance and it allows for max advance at lower vacuum which is what you car needs.

[Richard M.]

Re: Need help with carb adjustments

I am still learning and know just enough to be dangerous so any advice you can give is really appreciated. Assume you mean vacuum can attached to distributor. Don't see any numbers on it. Looks like a replacement part.

[John H.]

Re: Need help with carb adjustments

You need a vacuum gauge to properly adjust the idle mixture screws for highest steady vacuum; I removed the plug and added a fitting to the existing hole in the intake rearward of the carburetor just for that reason, and cap it when not in use (I have no options that require vacuum). If you already have a fitting there (for Powerglide, power brakes, or A/C), you can tee into one of those lines or use a fitting with one more nipple.

[Stuart F.]

Re: Need help with carb adjustments

Tim and John;

My experiments with replacing the #201 VAC with a #236 are not going as well as I had hoped on my 63 340 hp w/AFB. I am getting better part throttle response at low to mid speeds, perhaps better mileage, etc., but it seems that with the increased Vacuum advance at off idle low speeds I am getting much more stumble. Due to the more advance at idle, I have had to adjust my idle speed screw to close up the primary's in order to attain the 800 rpm at idle and, as a result, have changed the relationships of the carburetor adjustments (ports, perhaps accelerator pump, etc.). I'm not sure yet, and I can't look into until after T.S. Fay passes as I buttoned up the car and the garage today. It seems possible that this #236 can is hunting, and with my ball bearing plate reacting freely, they are dancing all over the place. It looked good on the distributor machine, but then you can't duplicate a Duntov's gyrations, Ha!

Stu Fox

[Timothy B.]

Re: Need help with carb adjustments

Stuart,

The 236 V/A can specs should be 15*@8" so the timing should not be searching at idle. On my AFB, with the primary throttle plates closed against the housing, the transfer slots are still shightly present under the throttle plates so I don't think you can make a case for the plates to be closed too far. Open the A/F screws another 1/4 turn and see if the stumble goes away. Is the accellerator pump giving a good shot?

I hope everyone in Florida is OK and this storm does not get bad.

[Stuart F.]

Re: Need help with carb adjustments

Tim;

You may well be right. I know it didn't make much sense as the change is not all that great. I have not/will not have a chance to do much about it until later this week. We're in the midst of storm prep today. Doesn't sound too bad, but we want to be prepared in case it changes. We are concerned about being in the NE quadrant where a likelyhood of tornadoes is the greatest.

I'll try the idle mixture screw adjustments first as you recommend as that is the easiest. I had a horrible amount of gas smell in the garage yesterday after filling the tank (worst ever). I usually let it sit with the hood open and garage well ventilated, etc., but this time I buttoned things up right away with two covers on the car and storm bars on the garage doors. It smelled to high heavens afterwards!

Thanks for your advice.

Stu Fox

[Jim T.]

Re: Need help with carb adjustments

Stuart your smell of gas after filling up and parking in the garage. I had that on my car. I could actually see the wetness on the front throttle plates. My discovery was a sinking float in my Holley 4777 650cfm double pumper. The original float in this carb is plastic/nylon and had fuel inside, a leaker, keeping the needle and seat open allowing additional fuel to enter. Replaced the float with a metal one, also replaced the same float in the secondary fuel bowl. This fixed the problem. I looked at the leaker float I kept, it still has just a little bit of fuel, removed it June 2006.
I don't know how the gas we get at the pump these days affects the performance of needle and seats after a period of time, but this is something to check.

[Stuart F.]

Re: Need help with carb adjustments

Thanks Jim. I'll keep that in mind. 63's w/ AFB's have always been "stinkers" and are even more so with today's gas. I'm also going to look into a replacement gas cap with a two way valve. I currently have a dealer applied locking gas cap (chrome) from back in the 70's with a straight vent hole.

Stu Fox

[Clem Z.]

Re: Need help with carb adjustments

Tim;

You may well be right. I know it didn't make much sense as the change is not all that great. I have not/will not have a chance to do much about it until later this week. We're in the midst of storm prep today. Doesn't sound too bad, but we want to be prepared in case it changes. We are concerned about being in the NE quadrant where a likelyhood of tornadoes is the greatest.

I'll try the idle mixture screw adjustments first as you recommend as that is the easiest. I had a horrible amount of gas smell in the garage yesterday after filling the tank (worst ever). I usually let it sit with the hood open and garage well ventilated, etc., but this time I buttoned things up right away with two covers on the car and storm bars on the garage doors. It smelled to high heavens afterwards!

Thanks for your advice.

Stu Fox

be careful filling the tank as the fuel is at 55 degrees from the under ground storage tank and if it get hotter after filling the tank the expansion can cause a over flow

[Stuart F.]

Re: Need help with carb adjustments

Tim;

That's probably what happened, or at least a Florida version of it. I can't imagine anything being 55 degrees down here, but I'm sure there was a difference. The station where I filled is about 2 miles away and I didn't top it all the way up like I might if I were going to drive a distance after. I pulled it in the garage right away and covered it up, which I don't usually do either.

Thanks again;

Stu

[Duke W.]

Re: Need help with carb adjustments

Need help with carb adjustments on 63 327/340 with Carter AFB 3721 SB. When warm - engine is idling low and at moderate crusing speeds it is lunging. Also get an occasional backfire when revived. Appreciate help with how to adjust idle speed adjusting screw and idle mixture adjusting screws. Do these symptoms mean the carb is running lean?

The 3721SB is tuned specifically to the 300 HP configuration as it was no longer used for SHP. The cruise calibration is leaner, which may cause surging at steady cruise depending on other factors, like typical air density and the camshaft. Are you absolutely positive it is the OE Duntov cam? Does it have the OE pistons for proper compresssion ratio? Where is the car domiciled? Have you done a compression or leak down test to verify the engine's general internal health?

Pop off the dist. cap to see what ID is stamped on the VAC. The OE "201 15" VAC will cause idle stability problems. If it's not "B28" or "236 16" it's needs to be changed to a B28.

Also verify that the total idle timing is stable somewhere in the range of 26-30 degrees at about 900 and the vacuum reading is about 12".

Do you have a 1963 Corvette Shop manual ? Set the idle mixture screws to the recommended initial setting and follow the procedure using 900 for the target idle speed. Use the tee to connect the vacuum gage to the VAC signal line.

Check all the various carburetor adjustments and make sure they are to spec - the various linkages including the accelerator pump and float levels.

If all the above does not correct the problem and the engine is otherwise OE equivalent and in good health consider the larger main jet used on the OE 3461S.

Duke