• When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.

AVS carb questions

FirstHemi

Member
Local time
1:49 PM
Joined
Jan 23, 2021
Messages
10
Reaction score
4
Location
Buffalo New York
Can someone tell me what the difference is between the AVS carburetor for an automatic transmission and a manual transmission?
Also can someone tell me what the difference is between the carburetor for a 383 and the 440?
From what I see in my service manual AVS 4736 is for 383 manual transmission
Avs 4732 is for automatic transmission, and
Avs 4737 is for 440 manual transmission
Avs 4738 is for 440 automatic transmission
Trying to understand what makes them different
Thanks
 
Sizes of everything would be the first thing I would think of.
The bare castings might be identical? I would think.
 
My 4 speed AVS has a dashpot. To slow down the rpm drop for shifting easier. I believe?
 
383 1 9/16 Primary (625)
440 1 11/16 Primary (750) generally
Dash Pot affects opening of secondaries as well.
 
Last edited:
Like above have said the sizes of venturis and throttle blades varied with the engine size. Also, IIRC the metering rods/jets and possibly the air valve spring were different.
 
For a given engine, the differences would probably be very subtle, slight change in Venturi booster, maybe air bleed size, primary rod diameters. Even a single, seemingly minor change would require a different part carb number.
 
Probably similar to what the difference in distributor curves are Manual usually rev up faster.
 
IMO.....the basic castings are similar but application vs application or auto trans VS standard transmission, the differences between the two are sometimes very subtle. Assuming for the same engine, the standard trans application will usually include an externally mounted DASHPOT, that slows the closing rate of the primary throttle plates to prevent a large RPM drop when shifting (other than WOT) and a subsequent lurch upon clutch re-engagement. USUALLY, beside the obvious differences in primary and secondary metering jets, the primary metering rods and step up piston springs will likely be different, but the most overlooked items are the primary booster venturii assemblies, which control ALL of the idle and off idle fuel transition circuits and are not adjustable. 1968 AVS designs had a single idle mixture adjustment screw a NON ADJUSTABLE off idle air bleed. Later models went back to dual idle mixture screws, but maintained the non adjustable off idle air bleed. Other subtle variations include the size and location of the off idle fuel transition slots and their size. As emission regulations became more stringent in later years, accelerator pump well size and pump stroke changed to accomodate emissions and transmission. Sometimes, throttle plate bevel angle changed to accomodate the engine's air flow fuel flow transition from idle to and including main fuel flow.....depending on transmission used. To be specific, careful disassembly and measuring all the components of the carb is the only true way to discern the differences between applications. Others will say it doesn't matter....but it depends on the individual's wants or needs.
BOB RENTON
 
Thanks again Bob. Knoledge on these should be passed down.
 
If you look at the booster venturis you will see some tabs on some of the
to correct Air Flow. I look for ones without those if I use aftermarket manifolds.
 
I have charted this for AFB and just started doing it for AVS. As stated the 440 HP had the larger 1 11/16" Primary bore while the 340/383 were 1 7/16". The calibrations between 340 and 383 are very close. And from what I can see the Man seem to be leaner than Auto which seems like a reversal from AFBs.

If you want to understand the detailed changes the carter service manual has the calibration data on the air bleeds, jets, etc... up through 69. After that MOPA probably did not pay to have that data and the service document are only about setting, not the detailed calibrations.

AVS only have primary venturis the secondary with the rapid opening of the air valve above the bore doesn't. You have a calibrated discharge tube with various sized hole. The other issue is the air bleed for the secondary is in the top, so having the correct top effects secondary calibration.

The dash pot does not hold open the secondary. It is an emission item. It prevents the primary throttles from fully closing on a throttle closed slowing down situation with transmission in gear so high RPM motor. IF the throttle blades went full closed in this circumstance the motor would go lean, misfire and have unburned fuel exit the exhaust. So it keeps primary valve open enough to prevent this.
 
Auto Transport Service
Back
Top