General AFR Guidance (Holley Tuning)
I agree with Goldduster that using vacuum at idle as a guide for Power Valve opening is not useful. Best Power valve opening point for a particular combo is hard to pinpoint without lots of testing.
The graph from Larew in
this thread shows that engine needed to start getting richer mixture at just 60% of maximum load, whereas the engine in Taylor's experiments didn't need richer mix until 95% load! Different engines, different needs.
Making part throttle accelerations holding different vacuum levels is one way to try to find best opening point. Lets assume no dyno but there is a way to measure the acceleration, either an accelerometer, a stop watch or a video which can be timed later. Acceleration should improve with each pass that uses more throttle (lower vacuum). Lets say say all runs start at 55 mph. Holding 14" of vacuum the car accelerates to 70 mph in 8 secs.
Then at 12" of vacuum the car accelerates to 70 mph in 7 secs
Squeezing 10" of vacuum the car accelerates to 70 in 6.5 secs. H'm seems like a flat spot.
Then at 8" of vacuum it hits 70 in 5.5 secs and maybe under
that it just can't be held at a steady vacuum.
So in this example if it had a 6.5 or even an 8.5 power valve, it would definately be worth trying a 10.5" Power valve. If the flat spot went away, that would be the proof.
It would be important to establish that the wide open throttle jetting is about correct before doing this experiment. And since you have a way to measure AFR, the WOT run will show if the AFR stays consistant. Once those are established, then work on PV opening point.
According to my AFR gauge, it goes really lean when I lean into the throttle in high gear, which causes the stumble. pump nozzle is a 40 with orange pump cam. I may see if a blue cam would help as well.
If the fuel is coming entirely through the main boosters, the accelerator pump should not need to cover. Accelerator pump is to cover the delay in the slow responding idle/transisition circuit. The faster the throttle is opened while the idle/transistion circuit is supplying fuel, the more pump shot is needed.
->To seperate whether the problem is pump or idle circuit, very slowly increase throttle. Any stumble, look at the idle circuit. Probably needs to be a little richer off idle; experiment with wires in air bleeds to see if that helps. No stumble, then work on pump shot amount or timing.
- > In general, the idle circuit's continue to contribute a significant amount of fuel while cruising at most street speeds. In my experience, main circuits don't fully take over until cruising at interstate speeds. This will of course vary with engine, gearing, carb size etc etc but most people, including me, have a hard time beleive the typical Holley 'idle circuit' is in play at speeds well above idle. Well, then I accidentally proved it to myself by running primary main jets one size too small. :LOL: Good learning experience.
I've had this issue with a holley before. Maybe it's just a street avenger thing. Good thing I've got a couple double pumpers to try out as well.
It does seem like some of the newer 4150 Holley's and clones have odd calibrations and problems compared to the older versions. That's one reason those guys created the racing fuel systems forum. Take a look at the Stickies in the Holley subforum as well as the number of threads on fixing 'avengers', declawing Demons and so forth. If I had summarize the areas that seem to need addressing it comes down to location of idle feed restriction, too much air in the emulsion tubes and transfer slots too big. All fixable as they show, but its a bit of work and requires patience and learning curve!
I've got timing set at 28 initial and 36 total. Seems to run well at that timing combo, other than the annoying stumble of course.
I trust if it starts and works, its probably close. Occassionally a little too much timing can cause something that seems like a stumble at part throttle. I just had that with my Jeep and I thought it was fuel. If you think thats a possibility, back it off a little to check.
