Last spring I got my carb rebuilt because motor (1964 300hp) is running very
rich. My carb guy went thru the carb installing kit and new floats as well as insuring it was set and worked properly. The motor still runs very rich and I don't know if the carb has had any work done buy prevous owners as there is an aftermaket cam in car,or is the a common problem with these carbs. Looking for any suggestions or is it time to put the AFB on the shelf and buy a new carb. If so suggestions on new carb would be appreciated. Thanks again. :cheers:

 

Your 3721S AFB is very reliable and they normally work very well. Unless you can determine that something is not to factory spec, don't blame the carb. The choke system is probably the subsystem that causes the most problems on the AFB. Cam, ignition, and carb changes made over the years can cause problems. Bubba is everywhere!

How do you know it is running rich? Have you driven it with a portable exhaust gas analyser to get some data?

You need to start with some basic diagnostics like compression/leakdown, idle vacuum, verify that timing - initial, centrifugal, and vacuum is to spec IAW the '64 Corvette Shop manual. That's where you need to start. Give us some data we can work with to steer you in the right direction.

Throwing away the AFB and installing and aftermarket carb is not going to solve your problem, but it could compound it.

Duke

 

post the number on the front of the carb and i will post what the internals,jets etc should be. if you have a lower vauum at idle because of the aftermarket cam you may need to change the power piston springs to lighter one to prevent the metering rods from going on rich at idle. :chevy

 

...yup, back to basics, limit your assumptions 'till you know what's going on

 

Thanks for the reply's guys. Duke shelfing it was only a last resort. I just wanted to get started on this early before the -45 degree temps we are having right now turn around and warm up so spring doesn't catch me off guard. Didn't really know where to start but like always this forum points in the right direction. Clem numbers on tag are 3721SB and 3851761DD if this was what you needed. Your discription of the piston springs sounds like what I have been experiencing, a very strong unburnt gas smell at idle. If it is this where do I purchase new springs. Again thanks guys :chevy

 

BK64,

Springs are the same as what Edelbrock sells for their version of the AFB. I just bought a set from a local speed shop ($5 for a complete set of 10 springs - 2 sets each of 5 different calibrations). P/N is 1464. Also available from Jeg's under p/n 350-1464 for $5.49, but they will hit you with the extra cost for shipping!

Agree with Clem that first place to start is these springs for "tuning" for a "rich at idle & light cruise condition" once you have verified the choke is not partially closed and that the idle screws are adjusted for best idle (see Edelbrock manual mentioned below for best way to set idle).

If you need more info on tuning the AFB's, look at Edelbrock's web site under Automotive / carburetors / Edelbrock Performer Series (EPS) Owner's Manual (see below listed URL). They have a very good section on tuning the carb. Just beware that their rods/jets may or may not work in the Carter AFB. I bought an older Carter "Strip Kit" off E-Bay for additional rods & jets, since the Edelbrock rods available did not fall in the same range as my Carter rods.

http://www.edelbrock.com/automotive/index.html

Good luck,
Plasticman


[Modified by Plasticman, 9:51 AM 1/23/2003]

 

If the richness is only at idle, then the power pistons springs are not the issue.
It sounds like you have the original carb.

You should check ignition timing and proper operation of the vacuum advance. Also, the nominal idle mixture setting is 1.5 turns out from the seat.

Remove the air cleaner cover at idle and look for fuel dripping out any of the four venturi discharge nozzles. There should be none. If fuel is dripping, the float level is too high. This is why the power piston springs are not an issue at idle because no fuel should be flowing through the main jets - only through the idle jets.

With the OEM cam the engine should idle smoothly at about 500 RPM and pull at least 16" of vacuum.

You say it has an aftermarket cam. What is the idle speed and vacuum you observe. In '63 the same 3461S carb was used on both the 340 HP and 300 HP manual trans engines. Despite their very different valve timing and idle speed/idle vacuum, the same carb worked fine on both with about the same idle mixture adjustment. Being as how the 3721S is just a slight recalibration of the 3461S, the 21S should work with a bigger cam. One difference between the two carbs is larger idle speed jets on the 21S, which would indicate a need for the idle mixture screws being closer to the seat than the 1.5 turns nominal setting.

If the idle vacuum is less than 16" you need to install a NAPA/Echlin V1810 vacuum can in place of the OEM can. This is very important to achieving good idle characterisitics if you have and aftermarket high performance cam.

Duke





[Modified by SWCDuke, 10:34 AM 1/23/2003]

 

Thank you for the information, once it warms up a I will start doing the checks Duke you said at least I feel I can resolve this before spring. Once I have the information at least I know where to start. :cheers:

 

Duke,

What is the curve of the NAPA/Echlin V1810 can? At what vacuum level does it start to add advance, and what is the total advance?

Thanks,
Plasticman

 

The VC1810 is equivalent to the Delco "236" can that was used on 327 SHP/FI engines '64 and up. Nominal specs are start at 4" with a maximum advance of 16 cranshaft degrees at 8". This can should be retrofitted to '63 SHP/FI engines and used on any SB that has a high performance cam that idles at 14" or less manifold vacuum.

Because full vacuum advance is locked in at 8" it will allow big cammed engines to idle with more advance and stable advance. This will improve idle stabillity and reduce any tendency to overheat at idle or low speed driving.

Chevy missed the mark when they added vacuum advance to SHP/FI engines beginning in '63. The '63 can didn't provide full advance until about 16", but the Duntov cam only pulled about 12" and it varied, so the advance wandered and this leads to idle instability. Very early '64 SHP/FI distributors had carried over '63 distributors until the new calibrated distributors were available. This vacuum can carried through to at least '67 L-79s. Not sure about '68s, but that was the beginning of the emission era, so spark advance maps were typically reconfigured to meet emissions as a first priority, not performance, driveability, or fuel economy.

Duke

 

Duke,

Thanks for the info. My current vacuum can starts at 7", and max's out at 13" with a total of 18 crank degrees.

I have an old Sun Machine that allows me to play with these things a bit. Just wondering how it compared with the NAPA/Echlin can.

One of the methods I use is to run the engine at idle with the dist. loose for easy adjustment (after the cent. advance has proven to be "adjusted & curved" for best performance without detonation. Rotate and note where the dist. position is where the engine runs the smoothest. Then disconnect the vacuum advance and read the actual initial advance (without vacuum). If the initial advance varies much from what I think it should be (knowing what the cent. advance curve is), then I look for a vacuum can with the offset in degrees/inches from my parts box. This is of course only good for idle comparison, so then I rev. the engine in steps to check if any roughness enters into the picture. If it does, then back to the parts box and the Sun Machine. Sometimes I will break out the exhaust gas analizer, but have not seen a correlation between slight roughness and dead smooth. Probably the analizer needs a new sensor!

My current can with 18 crank degrees is a little more than I should have running 12 degrees initial and 36 total, all in by 2500 (54 deg. with vacuum added at cruise). Don't know if 16 deg. max will make any significant diff. (but it won't hurt either) I know that Crane use to make an adjustable vac. can. Might look at getting one to try out (if still available).

Always looking to fine tune (that's all part of the fun!).

Plasticman

 

Assuming you have a correct idle mixture, best total idle timing for a SHP/FI or equivalent cammed engine will usually be in the range of 25-30 degrees, and 20-25 for a medium performance engines like the 300 HP. If you have a quick centrifugal you might get a few degrees centrifugal advance at your idle speed, which is usually okay.

Timing is optimized when EGT is at a minimum, so if you have an IR gun to check exhaust manifold temps, that can be a help in optimizing total idle timing, but it's tough to see much difference within a range of about two to four of degrees of timing.

With a quick advance curve as you have, a SHP engine should have in the range of 50-54 degrees total advance at cruise. This is the sum of initial, full vacuum, and full centrifugal. This amount of advance should minimize specific fuel consumption (maximum fuel economy). It's best to get the centrifugal all in as soon as possible. The limitation of how quick it can be is usually low rev WOT detonation.

These numbers are for SBs, but BBs have similar advance requirements - probably on the high side of what equivalent performance level SB engines like or may a couple of degrees more.

Another way to determine optimum idle timing is to tie up the centrifugal and disconnect the vacuum and then go back and forth between rotating the dist. and adjusting the idle speed and mixture. Once you complete this iterative process, you probably have everything optimized. The amount of timing you read is what you should have at idle as the sum of initial, vacuum, and centrifugal, if any.

Optmized total idle timing will minimize the tendency to overheat and minimize idle fuel consumption. As we've found in many threads, idle and low speed overheating have been eliminated in many cases by selecting a vacuum can more suitable to the engine's idle vacuum characteristics - one that provides full vacuum advance at idle.

Duke

 

Duke,

Thanks for the further info. I'll have to try the IR gun on the headers!

Never have had a problem with overheating!

Thanks again,
Plasticman

 

with a after market cam you may have the transfer slots open to far to get it to idle and this will cause your ricness but make sure your power pistons are all the way down at idle because low vacuum will cause them to pulsate up and down. remove the power piston cover,have someone hold the power piston in the well and then start the engine,the power piston should stay down all by it self at idle,if not you need weaker springs. i will scan you a parts list of the correct jets and metering rods for your carb so you can check to see if any thing has been changed. i also would set the floats 1/8" lower than the specs to start with because of the type ofgasoline we now have. :chevy

 

Had the problem on my '63 340 HP with the OEM vacuum can that didn't provide full vacuum advance at idle. I could actually hear the power pistons rattling against the cover. Once I installed an 8" vacuum can, idle vacuum increased due to the additional idle advance and the power pistons stayed down at idle.

Duke

 

Had the problem on my '63 340 HP with the OEM vacuum can that didn't provide full vacuum advance at idle. I could actually hear the power pistons rattling against the cover at idle. Once I installed an 8" vacuum can, idle vacuum increased due to the additional idle advance and the power pistons stayed down at idle.

It still amazes me that Cheverolet was able to use the exact same carb on the '63 300 HP manual and 340 HP - same number - same calibration, and they worked fine on both engines despite the very different valve timing, though the 340 needed the 8" can to achieve acceptable idle stability.

Duke