Stock Carburetor Jetting

[Dale M.]

I am trying to determine the "stock" jetting for my '64, 300HP, 4-speed vette, carb: 3721SA. I discovered that my metering rods do NOT have the same numbers. One is a 16-204 and the other one is a 16-199. I have not yeet confirmed the jets. It must be unusual for different metering rods? I know that my car was raced, but do not know what type of tuning might have been done.

My next question is regarding gas and jetting. Do I need to go richer with the use of ethanol gasoline? If I am getting 10% ethanol, can someone recommend how much to richer?

Thanks Dale

[Stuart F.]

Re: Stock Carburetor Jetting

Dale;

The 1964 Service Manual Supplement Specifications lists the standard main jets as .104" primary and .0689" secondaries. The metering rods are called out by their step sizes; .069" high step and .060" low step. You would have to measure them with a micrometer to confirm.

My records show the #16-204 rod(s) as being from a 1965 Model 3720SA (.063" x .073"). I'm sorry, but I don't have a record for your #16-199 as being used in a Corvette AFB application (that doesn't mean that it wasn't, rather that I haven't run across it on any 1963-65 application).

Maybe someone has a more extensive AFB record collection to include applications other than Corvettes and will favor you with a response.

Stu Fox

[Duke W.]

Re: Stock Carburetor Jetting

I have the following info for the 3721SA:

Primary main jet: .0995" (120-224)
Sec. main jet: .067 (120-185)
Metering rods: .069"/.063" (16-167)

The data in the 1964 Corvette Shop Manual Supplement is for the 3721S, which was replaced by the 3721SA as a running change.

The 3721SA was short lived before being replaced with the 3721SB that used .069/.060 rods (16-389) - same as the 3460S, 3461S, 3720S, and 3721S.

16-204 is a .073"/.063" rod, which was used on the the 3720SA/SB Powerglide carbs.

I can't find 16-199 on the chart, but it's arranged by size, not number and there are several score of part numbers. Measure the step sizes and maybe I can find it. Also, use a good magnifying glass to verify that the number is correct.

I believe Edelbrock offers a limited line of jets/rods that will work in a vintage AFBs, and Federal Mogul might still offer some parts (They bought Carter decades ago).

You could also contact the various vintage carb vendors to see if they have a couple of 16-389 rods.

You should not need to recalibrate a 3721SA to operate satisfactorily on E10 gasoline, assuming the engine exhibits good driveabililty quality with a propery adjusted idle mixture

Duke

[Duke W.]

Re: Stock Carburetor Jetting

The 3720SA/3721SA jet/rod data is in the 1965 Corvette Shop Manual Supplement. (The SB models replaced the SA models as a running change in '65.) The rods are listed at .069"/.063".

I found additional information in Carter Form 5456F (Jan '62, revised Nov. '67) It says as follows in the parts list:

"16-199 Step-up rod (.069"/.063") std. (3721SA - use 16-389)"

This part number is not listed in Carter From 5460, which lists part number/sizes of all AFB/AVS jets and rods, but there is a .069"/.0635", 16-167.

At this point you need to remove the bowl cover, verify the stamped numbers on the jets and report back. You should be able to see them without removing, but might need magnfication.

Based upon installed jets I can recommend what metering rods should work, and hopefully you can find some.

If the jets are OE either two 16-199 or two 16-389 should be okay, but the 16-204 (.073/.063) may be on the lean side even though it was used on the 3720SA/SB Powerglide carbs.

Are you experiencing any performance or driveability problems with the current setup, and what altitude do you live at?

I also need manifold vacuum at idle speed to determine if is has an OE cam or something close. The OE cam should pull about 18" at 500 in neutral.

Duke

[Dale M.]

Re: Stock Carburetor Jetting

Here is what I have found out from measuring the jets and metering rods.Primary jets: numbers 120 224, looks to be .0100" (not sure how accurate my caliper is on a hole this small). Metering rod 1: 16-199, .059 - .065. Metering rod 2: 16-224, .059 - .069. Secondary jets: 120-785 (maybe 7 should be a 1?) measured at .068. All this seems to be about the correct range. Hopefully it just needed a good cleaning.

I assuming the E10 and its evaporation can get deposits on the internals. It sure seems like the bowls go dry in a short period of time. I have it cleaned and putting it back together. It had been running fine for several years, then it suddenly started cutting out. First the fuel pump and then this miss.

I thank you for your replies and the part numbers for the stock carb.

Dale

PS: Maybe I should have mention that it has #3863151 CAM, the 350HP L79. Engine was rebuilt in 1983 at 100,000 miles.

[Duke W.]

Re: Stock Carburetor Jetting

The 3721SA main jets are 120-224, which are speced at .0995", so it appears you have the original primary jets.

The 3721SA secondary jets are are 120-185/.067", so those are probably original, too.

You didn't tell me what altitude you live at and normally drive your car.

Something else you need to look at is the VAC. What's the number stamped on the mounting bracket. The original for a '64 300 HP does not meet the Two-Inch Rule with a L-79 cam.

Without the above information there's nothing more I can do to help you get the spark advance and fuel maps properly set up.

If you really want to get your engine dialed in look at the following thread and download the pdf link in post #37:

https://www.forums.ncrs.org/showthre...rb-replacement

The L-79 cam requires a different spark advance map than the 300 HP cam, but bubba doesn't understand this, so since you seem to be willing to tinker, get the spark advance map dialed in first, then you're ready for grad school - the fuel flow map.

Also, what is the current initial timing? Is the VAC jammed up against the manifold or spark plug wire bracket or is it somewhere in between with plenty of margin to the physical rotation limits. And while you're at it remove the dist. cap and pull the rotor up and down and give me an estimate on how much end play the shaft has.

It won't take that much to get the engine dialed in so it runs better than ever.

[Dale M.]

Re: Stock Carburetor Jetting

I do like tinkering and really understanding the details. My brother and I had a 283 pushing a 3410 lb. car to achieve 138 mph in a quarter mile in the late 60's. We built a lot of our own parts and experimented, but that was before I had a family and worked about 90 hour weeks. I retired and now have this '64 to keep up.

My Vacuum Advance is stamped with a B1
I live in Cincinnati, OH at about 485 feet of evaluation

I am downloading the article you referenced and will be looking into it.

I don't know how to thank you for all you have done for me, but I really appreciate everything.

Dale Maris

[Duke W.]

Re: Stock Carburetor Jetting

So you're near sea level. I think the OE installed jets along with two 16-389 or 12-199 rods should be okay with the L-79 cam with proper idle mixture adjustment. At worse it might be a little lean that might show up as some lean surge, but you don't worry about that until the spark advance map is optimized.

The B1 is a boat anchor. It doesn't even meet the Two-Inch Rule for a 300 HP engine.

For L-79 I recommend the 12" B26 - NAPA VC1765 or equivalent in another brand.

Since you like to tinker, it should be pretty easy to dial your engine, but rather than typing we should talk on the phone at this point, so send my an email (not a PM) from my profile after you go through the presentation.

Duke

[Joseph R.]

Re: Stock Carburetor Jetting

Duke, I looked for that presentation, and it was at post #28 of the linked thread in my hybrid view instead of #37. Joe

[Duke W.]

Re: Stock Carburetor Jetting

...don't understand what you mean by hybrid view, but here's the link, again.

http://stlouisncrs.org/news_files/Du...omySeminar.pdf

Duke

[Stuart F.]

Re: Stock Carburetor Jetting

Dale;

Glad you have hooked up with Duke about your carburetor issues. He has been very helpful to me over the years during my struggles with AFB's, timing MAPS and his 2" rule regarding VAC's, the Hinkley-Williams valve adjustment procedures for solid lifters, as well as replacement tires and rear wheel alignment, and many other tips that I've picked from his advice to others. He and Joe Lucia on our TDC are two of the most knowledgeable fellows with the best information records. If you get advice from either of these two Gentlemen you will be well served.

Some little items I'd like to impart to you about AFB's, specially those that appear to have had Bubba or a rebuild shop inside of them; check the part numbers on each set of venturi clusters (primary and secondary). Typically, they should be one number apart, i.e. #789 and #790, and appear to be the same except for right and left configurations of their air bleed holes. Also, check to be certain there is a hot idle air valve mounted on the secondary clusters (a "T" shaped device with a bi-metallic spring and a valve button on the underside). If it is there, check it's condition and function (I have seen junk ones put in by rebuilders with broken, bent or missing parts). It is possible, if the valve is not seating properly, the carburetor will have a major vacuum leak.

Stu Fox

[Duke W.]

Re: Stock Carburetor Jetting

Thanks for the kind words, Stu.

The part numbers I have for the 3721SA venturi assemblies are as follows:

Primary, choke side: 58-793S
Primary, pump side: 58-794S

Secondary, choke side: 58-620S
Secondary, pump side: 58-621S

The big difference in the primary clusters is the larger idle jet - .037" versus .035" on earlier models, and this was combined with slightly leaner cruise and power calibration.

That little valve with the bimetallic spring between the secondary venturi assemblies is called a hot idle compensator. The valve should open at some temperature to purge excess fuel vapors and prevent an overly rich mixture, which can degrade idle quality.

It was never clear to me if the damned thing ever worked. If it opens, idle speed should increase, but I never noticed that.

I should put one in a pan of water with a meat thermometer, heat it up on the stove and determine when it opens - same easy test that one can do with an engine thermostat, but I don't have a spare laying around. Do you?

Who knows how many times these 50-year old caburetors have gone through some rebuild cycle, and it's not uncommon to find a kluge that has mismatched parts relative to its original configuration.

Hey, if it fits - parts is parts!

Duke

[Stuart F.]

Re: Stock Carburetor Jetting

Duke;

Actually, I do have a hot idle compensator on the bench. I'm doing a summer long test with the compensator removed and a block off plate in it's place. Last summer, I used it. This summer not in order to do a longer hot summer evaluation.

I continued to use the same hot re-start procedure, I.e. Crank the engine over for a few seconds with foot off the throttle, then apply about half throttle. The reasoning being to evacuate the overly rich charge from the manifold and cylinders, to be replaced by a more proper fuel/air mixture. Last summer, this procedure worked great. This summer, it worked, but not as well.

This was just a part of my on-going search for ways and means to reduce the gas smell in my attached garage.

I've also been testing my 3461s carb in an effort to sort out the lean surge condition, on which I finally found success. The 3461s also has "vapor vents" which I have sealed off with rivet heads, also as part of my gas evaporation containment effort.

My conclusion is that the compensator aids the hot re-start procedure on the road, but contributes to the gas smell in the garage. So it's a trade off.

As a side light, I thought I might see some better secondary tip in performance with the compensator out of the way (specially the tabs), but if there is I have no way to measure it. I could not feel it in the foot or the seat of my pants, Ha!

Stu Fox

[Duke W.]

Re: Stock Carburetor Jetting

Hot soak percolation can cause an excessively rich mixture - to rich to ignite - otherwise known as "flooded".

The flooded start procedure calls for flooring the throttle and then cranking the engine until is starts. With the throttle wide open there is no manifold vacuum to pull fuel through the idle passages and insufficient venturi vacuum due to low air flow to pull fuel through the venturi nozzles. The engine is just pumping air, so the mixture should rapidly lean out to an A/F ratio that will ignite.

Have you tried the above procedure on hot restarts?

The 3461S was set up richer at cruise than later AFBs, and I never had any lean surge problems with my 340 HP engine despite running up to 40 degrees total WOT advance, all in at 2350 and the very aggressive 236 16 VAC in Seattle which is at sea level and can get down to the 20s during winter, so air density was high. Total cruise advance at 60 and up was up to 56 degrees, and it would knock down 22 MPG on 70-MPH freeway trips with the 3.08 axle.

Does your 3461S have the OE jetting?

I'm not sure what you mean by "vapor vents". Are you talking about the the bowl vents? Those are referred to as "inside vents" in the '63 Shop Manual spec section. Those need to be open because they act as pitot tubes to maintain dynamic air pressure in the bowls. The spec section also say there is one "outside" vent, but I've never found it.

I doubt if the hot idle compensator does much for the flooded condition because the opening is too small. Also, for the same reason, I doubt if it has any effect on vapor release to the garage. What you smell is fuel vapor escaping through the air cleaner opeining. As vapor is formed in the bowls it escapes to the air horn and then out the air cleaner opening as more vapor escapes from the bowls.

Also, can you run the the test I described on the stove and determine the temperature that the hot idle compensator begins to open.

Duke

[Stuart F.]

Re: Stock Carburetor Jetting

Duke;

I will run a test on the hot idle compensator as soon as my wife lets me invade her kitchen. Will let you know what I find.

The vapor vents I refer to are exclusive to the 3461s (not on any 3720 series carb). They are small holes (4) immediately above each throttle butterfly, clear through to the outside of the carburetor. Use a mirror held either in front or the back of the carb and you will see what I mean. The last carb I saw with such vents were some Rochester 2 barrels. They used slots in the gasket (between the body and cast iron base) to vent the fuel vapors from the throttle bores to atmosphere after shutdown.

I am aware of the 4 bowl vent tubes in the top plate of AFBs. These are different. I plugged the vapor vents using rivet stems cut to length. I ground off any excess that protruded into the throttle bores with my Dremel tool. The stems have a slight swagging (?) immediately under their heads which, in itself, holds the rivets in place, but I still used a dab of super glue to ensure they stay.

Duke, I have all but eliminated hot soak percolation, both during shut downs on the road or in my garage. FWIW, I do not use OE jetting, float settings or accelerator pump squirter. I have also made a number of changes to isolate the carb from heat, as I have described in detail in previous threads on the subject.

The hot start procedure I use is not for "flooding", rather for typical hot soak and hot under hood conditions. My intent is to avoid flooding situations if at all possible by not adding any more fuel until the engine has cranked long enough to clear the over rich mixture. It works very well for me, and I have been using this method for several years now.

The lean surge condition that I had with my 3461s was finally cured by re-bushing the primary throttle shaft. It was so sloppy that I had uneven exposure of the transfer slots from one bore to the other. In addition, because someone had bored the idle/low speed jet tube in one cluster to .037", I had no choice but to drill the jet tube in the other cluster to match. It turned out ok and actually improved my off idle response (3.36 rear with CR trans). My 3461s is a carb I picked up a number of years ago. It was a complete basket case put together by a rebuilder in the Northeast (who shall be nameless). Turned out to be quite a challenge, but it works perfectly now.

Stu Fox