63 carb replacement

[Harry S.]

Re: 63 carb replacement

I spoke with Bob last summer, he said he is taking a vacation from work and that it might be permanent. I would look for an alternative.

[Joe T.]

Re: 63 carb replacement

I have had my carb completely restored and tested by All American Carburetor. THe car starts well, choke sets perfectly, kicks down when warm and car runs great once moving. Unfortunately the stumble at start is the same unless I rev to 1600rpm and I am concerned about wearing out my clutch with all the feathering I need to do. One relative of mine suggests it is because I have a 3:08 rear diff, another relative(both mechanics ) suggested electronic ignition would solve the problem. DO any of you have a response to the gearing issue because I do have a loud clunk in the rear differential so I'm thinking a rebuild is coming and I wouldn't mind at that time changing to 3:70 gears. Thank you, JOE

[Timothy B.]

Re: 63 carb replacement

Joe,

From your description it sounds like a vacuum leak. Double check the rubber hose to the vacuum advance and PCV valve to make sure the ends are not egged shaped causing the leak. Did you adjust the emulsion screws after the engine warmed good.

[Duke W.]

Re: 63 carb replacement

Replacing points with an electronic switch will not do anything, assuming the points are in proper working order. As suggested, going through the idle speed/mixture adjustment procedure in the shop manual should mitigate the problem. The nominal setting on the idle mixture screws is 1.5 turns out from the seat. Find the ideal setting then turn them out and additional 1/4 turn. This will richen the mixture. Road test and try another 1/4 turn out if the problem isn't mitigated.

What you are probably experiencing is lean stumble. Idle/off idle fuel is metered based on manifold vacuum, not venturi vacuum. As you let out the clutch, manifold vacuum drops, so you get a momentary lean condition, which can cause a stumble. You should be able to engage the clutch without stumble without exceeding about 1200, but 1500 isn't that bad.

The 3.08 axle is a good combination with the WR four-speed, assuming that's what you really have. Have you verified?
The standard axle on the 250/300 HP was 3.36, and nothing shorter was available. A 3.70 is way too short for your application.

The 3721S was jetted leaner than the 3461S, but has larger idle jets, which are part of the primary venturi cluster, so if tweeking the idle mixture doesn't improve the situation, you need to audit the 3461S configuration to determine if it has been modified. Pay particular attention to the primary venturi clusters, and primary jets and rods.

This is another example of applying Occam's Razor. Tweeking the carburetor is a lot simpler and probably more effective than changing axle gears or buying a generic replacement carburetor that will introduce a whole new set of integration issues.

The axle "clunk" may be due to worn Positraction clutches, which is a common problem on high mileage cars. Check side yoke end play. Excess play from Postraction clutch wear causes excess differential gear backlash, which can cause the noise you report. Deteriorated front carrier mount cushions can also cause such a noise, so when you get the car up in the air to check yoke end play, pry on the front carrier mount to see if there is excess movement.

Duke

[Stuart F.]

Re: 63 carb replacement

Joe;

Since you ran a 3721 on it and it worked fine, it is almost certain to be a carb issue. However, make certain you are using the correct base gastket(s). I too recently had work done on my 3461s by All American, but I had to tell them exactly what I wanted. I had been fighting a lean surge condition for several years and I called them for advice. I told them my primary shaft bores were real loose, but they suggested that, if that were the case, I should spray starting fluid around the primary shaft to see if the engine picked up speed. If not, they said the shaft was ok. Well, I tried that and did not have any vacuum leaking there. As I had run out of every possibility for the cause, I bet my marbles on the loose shaft. I called them again and had to talk them into re-bushing it for me (they don't/won't do that normally). Wa La! That did it. It works now better than new. The loose shaft was causing a mismatch exposure of the low speed transfer slots.

I too run a hi gear; 3.36 with a close ratio tranny, so I'm used some issues with my L-76 (340 hp) engine. One other thing I got forced into doing, and that was with the low speed/idle jet tube on one of my primary clusters had been drilled from .035" to .037". As a result, I had to drill the other cluster to match. This actually seems to have helped my low speed off idle as now it practically idles away from a stop with little or no throttle. I don't recommended doing this to anyone as I'm sure my fuel economy will suffer, but who cares?

Stu Fox

[Duke W.]

Re: 63 carb replacement

The idle jet on the 3461S is .035" and .037" on the 3721S, so merely installing the 37... primary venturi clusters in the 34... will get you the larger idle jets, and I don't think the venturi cluster numbers are checked during judging.

Another thing to check is the O-ring between the choke housing and main body. It it's not there or deteriorated it can cause a vacuum leak that can upset fuel metering.

Duke

[Stuart F.]

Re: 63 carb replacement

That's a great idea! Put the primary clusters from the 3721 into the 3461. Even though I tried that during my attempts to sort out my lean surge, as noted, my problem responded only to the new shaft bore bushings and my change to the .037" low speed/idle tubes was a forced change and any gains are a by-product.

FWIW, the 3721 series clusters have Venturi tabs where the 3461 does not. Carburetor engineers add similar tabs on certain manifold applications to direct the mixture in a manner to over come difficiencies in the manifold. I read that some where and have drawn a personal conclusion that all L-75 engines (or any engines with the cast iron intake manifolds) should have the Venturi Clusters with tabs. Only 63 L-76 engines, with the aluminum semi-high rise manifolds should have the clusters without the tabs. The 3460/61 carbs should not have been used on L-75 engines. That's my uneducated opinion.

Stu Fox

[Duke W.]

Re: 63 carb replacement

It's pretty amazing that the same carb was used on both 300 and 340 HP engines... same for later 300 and 350 HP 327s.

Considering that idle/off idle fuel is metered by manifold vacuum and the SHP engines have decidedly different idle and low load vacuum characteristics it's surprising that the same carb actually worked okay on both, but then the 3461S was probably richer than necessary on the 300 HP engine, which is likely why they leaned out the cruise and WOT calibration when it was only used on the 300 HP engines in '64 and '65, but the richer idle jets were probably necessary to have a smooth transition from off idle with the leaner cruise calibration.

Duke

[Timothy B.]

Re: 63 carb replacement

The best home brew way to tell if the idle calibration is close is after the throttle blade position (idle) is set, determine how far out the emulsion screws are after the adjustment. On my 300hp engine with the 3461S they are approx 5/8-3/4 turn out so I know it's probably fat in the idle circuit. It's been reported that approx 1 1/2 turns means the calibration is close

On a Holley you can finger the idle air bleed at 2500 rpm to see if the engine likes more or less fuel as the engine will speed up or slow down. That tells you if the idle feed restriction could be reduced. The Carter is different because you can't get to the bleeds so the only way I know to test at home is to introduce air below the throttle blade to see the same. You could do this with a tee in the V/A line and some sort of screw to meter the air.

Today most people would laugh at this because of the technology available to use a O2 sensor and log the information to view and see exactly the rpm and vacuum along with the A/F ratio.

It's a good point about the generic usage of the 3461 Carter and the 3810 Holley and I can tell you for a fact that 3810 is rich on a 300hp engine. The good thing is that they are not terribly difficult to tune if you have patience. I have reduced the IFR on my 3810 down from .031 to .028 and the engine likes it with more response from the throttle plus the emulsion screws are out around a turn and a little bit. That's the area I work on because the idle circuit stays active up near 3000rpm and overlaps the main circuit somewhere around there.

Plus, I think the unleaded gasoline is different and makes the engine idle faster than the old leaded fuel so don't be afraid to tune the carburetor.

[Joe T.]

Re: 63 carb replacement

Thank you all again! THe idle mixture screws were 2 and 1/4 turns out from seed. I adjusted both to 1 and 1/2 out from seed and the problem is 90 percent better! Smooth start at 1300 rpm and no stumble! All that frustration. Uggh. How big a job are the posi clutches and can a local transmission shop do them? As you can tell I'm not good at fixing Vette's, just paying for them! Joe T

[Timothy B.]

Re: 63 carb replacement

Joe,

FWIW, the best way to adjust the mix screws is the set the warm idle speed where you want it (600+-)rpm and with a fully warmed engine adjust the screws to get the highest vacuum and idle speed. You want them both to be approx the same turns out. Any time you change the idle speed by giving the engine more air (idle speed screw) the emulsion screws should be checked. There main purpose is to meter (lean) the curb idle A/F ratio because the idle circuit calibrations are richer than a engine needs at curb idle.

[Stuart F.]

Re: 63 carb replacement

Tim;

For "a screw to meter the air" how about a fish tank valve? I have a bunch of them thinking some day I would find ause for them.

Stu Fox

[Timothy B.]

Re: 63 carb replacement

Hi Stu,

I am glad you got your 3461 carburetor straightened out. Any kind of valve would work if you have the patience to test. Most of our cars live in the idle circuit rpm range so I think it's worth getting close as far as A/F ratio. A good thing to keep in mind is that the fuel today is different than the fuel these carburetor calibrations were built to.

[Joe T.]

Re: 63 carb replacement

3461 still stumbling badly, I thought I had it fixed but it was my imagination. so far have had carb completely restored by All American Carburetor, they ran the carb on an engine to verify it was working. Last night mechanic and I (together) advanced the timing, reset the dwell to 30 degrees, checked vacuum, re-adjusted idle then idle mixture screws. Idle now set to 800, choke sets, car starts warm and cold, runs great once rolling but hesitation,bucking and stumble at start is driving me nuts. My 66 does not do this( I thought it was me but that was also my imagination). NOt sure what next step is. Spoke with All American Carb who are willing to re-eval it but that's a pain in the ass with shipping and time without the car in running condition. Any thoughts? JOe

[Timothy B.]

Re: 63 carb replacement

Joe,

Sounds like a vacuum leak, and you say it ran fine with another carburetor? Accelerator pump OK?