Here are my observations and comments on jetting my Stovebolt Holley 5200 carb with my new JAW wideband O2 sensor.
First off, let me say that just about everybody who tells you that all you have to do with a Weber 2-bbl to tune one is adjust the idle mixture and speed screws is
treating you like a dummy. I'm VERY frustrated with the LACK of good info on jetting these great carbs, it makes no sense to me why the main sources of them don't seem to want anybody to actually open one up and do anything with them. It's like they just assume that everybody who has a Weber is too stupid to do more than turn a screw, grrrr.....
/soapbox off
So: from you guys and the Pat Braden Weber book, the Weber circuits go something like this:
Idle Jet - controls the % of air/fuel going to the Idle Mix Screw and also the Progression Slot, and also affects the AFR's at cruise / low rpm.
Idle mixture screw - controls volume of mixed air/fuel being added to the air going past the throttle blades at idle (it DOES have an effect on the AFR at idle, not just the volume of AF coming from the Idle Jet.)
Progression Slot (Circuit) - aka "Off-Idle" - not adjustable, controls the transition from the idle circuit to the Main Jet.
Main Jet - controls the amount of fuel going to the venturi, has main effect on AFR at low/speed cruise and above (idle jet effect lessens with rpm/airflow).
Air Corrector Jet - controls amount of air mixing with fuel going to venturi, has main effect at high rpm/airflow.
Secondary Side - comes into play towards wot, most often the Idle and Progression circuits have little/no effect since the throttle on that side tends to open up so fast and go right past them into the Mains.
OK, let me say that I've decided that if you're serious about carb tuning, you NEED a wideband Oxygen setup. I'm pretty good with carbs (if I do say so myself), but with a wideband you can see a LOT finer detail of what's going on than you can with a vacuum gauge, tach, and a good sense of hearing / touch. I had no idea how much a difference in Air-Fuel Ratios (AFR) that 1/16th of a turn on the idle screw can make - like from 13:1 to 14:1. Plus, you can easily and clearly see things if they're not set up just right - like slooowly going from low rpm / low load to high rpm / low load (in 2nd gear, say) you can see when the progression slots come into play and then when the Main Jets come online and then when the Air Corrector Jets come into play - if they're rich/lean, you'll see it happen every time.
Anyways, I swapped the HW5200 back on. Here are the jets it had in it:
Primary side:
Idle = 80, with mix screw 2 turns out
Main Jet = 160
Air Corrector Jet = 180
Secondary side:
Idle = 70
Main Jet = 160
Air Corrector Jet = 180
This jetting was too rich all around. Here are the AFR's:
* Idle was rich, but the mix screw (and wideband) makes a tremendous difference. Adjusted it to 14.0 for best vacuum and AFR.
* Progression (Off- Idle) was 13.0-13.5
* Low-speed cruise (40-50mph) was 12.0-12.5
* High-speed cruise (65-75mph) was 12.5-13.5
* WOT was 12.5
Note that with these jets, It drove just fine and I was getting 20 mpg putting around town and some highway, and with my Autolite 1100 on, it was much leaner (16.5 at cruise), perfectly happy, and getting 20mpg around town.
Tonight I swapped some jets around, did some driving, and here's where it is now:
Primary side:
Idle = 60, with mix screw 3 turns out (like I care anymore!)
Main Jet = 142 (it originally came with this jet in it)
Air Corrector Jet = 180
Secondary side: (didn't change anything (yet) and don't think I'll have to.)
This jetting is much better. Here are the AFR's:
* Idle = 14.5 for best vacuum and AFR.
* Progression (Off-Idle) = 14.0-15.0
* Low-speed cruise (40-50mph) = 15.5-16.0
* High-speed cruise (65-75mph) = 16.5 +/- .5
* WOT = 13-13.5
It drives well, no stumbles or anything. The "Butt-dyno" says that it's a tiny bit peppier with better throttle response, but it's not as dramatic as I'd think. Should see a jump in mileage at this rate.
Call me a happy camper!
8)
