Hey guys, trying to get the tune just right on my Holley carb. Let me tell you what I have so far...388w with afr185 heads and a 238 degree cam and a 2500-2800 rpm stall. It's a 750 cfm double pumper. Read David Vizard's book and it got me pretty close. Running a 67 jet on the primary and 80 in the secondary. I have an autometer wideband on the car and cruise is a nice 14.0 to 15.0 and that makes me happy. Had to put a 50cc accelerator pump diaphragm and the yellow pump cam with a #40 nozzle - finally cleaned up a gas pedal stab and my AFR doesn't go lean or stumble. Part throttle was pretty bad at first, would go super lean at about 25-30% throttle - 17.0, so I went from a 4.5 power valve to a 5.5 valve to a 6.5 valve and even now it still goes up to 16.0 briefly and then cleans up to 14.0 and keeps falling to wot the car runs 12 - 12.5. Any suggestions? If I cruise or go balls to the wall then it runs very clean, at part throttle is where my issues is.
While Vizard's methods apply readily to older carbs, not sure they can be used verbatim on newer models. I'll guess you've read through Holley's carb tuning? https://www.holley.com/blog/post/how_to_tune_the_power_valve_in_a_holley_carburetor/
Agree with Tom. Get a vacuum gauge and tape it to the windshield, see what is going on while it happens.
Yes. I just can't imagine it needing a higher numbered power valve. I am using the 6.5 now...maybe I should drill the fuel passages? The car doesn't have the best vacuum and I am a little bit thrown off by the responsiveness that it has based on cam size. I ended up with a small head (comparatively speaking, still flows 285 cfm, but only 185cc intake runner) and large-ish cam, but it's not sluggish anywhere...so I am wondering if the carb is getting a very strong signal at part throttle, my cruise vacuum gets up pretty high (13-15), although I am using my mityvac as a gauge...which isn't ideal.
One would think a 6.5 would be in the ballpark. Still, lean at part throttle suggests a higher number PV is needed. I'd try a 8.5.
I'll order an 8.5 and a 7.5 and try them out. Will get a vacuum gauge from HF tomorrow and see what I get at cruise too. This carburetor and engine combo has been putting me to work. Had a super rich idle and cruise and couldn't figure it out...tried a bunch of combinations of power valves and jets only to figure out the fuel needle wasn't seating and while driving my accelerator pump nozzle would dribble and the boosters would pull fuel through. Even went as far as drilling the butterflies just to get more air in there (thank God I had a parts carburetor with the same base plate).
Took it for a spin with the vacuum gauge attached, cruise is between 12.5 and 17.5 depending in how cracked the butterflies are. My lean spot is definitely between 5 and 10. Starts at 8.5 inches at about 16:1, so I will definitely put a 8.5 power valve in it. It also seems a little lean at 6.5 with an AFR of 15.0 and starts dropping as soon as I hit 5 inches, so I guess the passage should be drilled out a bit as well.
Your combo has greater fuel requirements than mine, and it sounds like you are very close. In my case, I had issues with a slight stumble when slightly accelerating from a cruise. I used an accelerator pump cam that delivered the pump shot quicker/sooner. I also took care to not use any of the T-slot to gain my desired idle. I figured out a way to adjust the secondary stop screw (while carb was installed) to raise idle RPM instead of the "idle" screw. Lol.... if you read Vizards book, then you know every trick I know.
T slot woes are not fun, I ordered a Brawler carb for this car a few weeks ago and it looks like Quickfuel moved the t slot up a little so you can open the butterflies and not expose the t slot.
50cc accel pump is way too much for a street car. You have other problems and that pump is just a band-aid...
Would be helpful if you suggested what the other problems might be. The 50cc pump does seem big, I did run my previous engine (milder 331) with the smaller pump shot but with the biggest cam and #35 nozzle. I went with a 14 degree bigger cam and 57 more cubes so the bigger pump didn't seem out of the ordinary. As a matter of fact I originally had the 50cc on my 331, the issue there was a leaky metering block and part of my shot would end up on the intake manifold, took a minute to find that on a brand new carburetor as it would evaporate by the time I stopped. Either way, what problem do you think the 50cc pump is masking?
Is the stumble at cruise, or accelerating from cruise, and at what rpm? Does it act the same at a higher speed cruise?
No problem at cruise, it had a stumble if I punched the gas let's say from 5% throttle to 30% throttle, the bigger pump and cam cured that. The issue I am having is under moderate acceleration (pedal held steady at 30% or so throttle) the afr goes lean (15-16). I hooked up a vacuum gauge and cruised with it and it seems to be happening at 7.5-10 inches of vacuum so I will be putting in a 8.5 power valve and seeing what that does.
What altitude are you at? I'm no "guru", but have been running Holley carbs for over 3 decades. It's unusual for a 750 on an engine like yours to need larger than s #35 discharge nozzle and a 30cc accel pump. A sudden lean condition could be a vacuum leak. Is your distributor vacuum advance? And if so, have you tested the diaphragm? Your power valve rating should be half that of the vacuum reading at idle in-gear (auto) or in neutral (manual). You should be able to run the orange pump cam in the #2 position. A 30cc accel pump properly adjusted. Squirter size should then depend on altitude. I'm at 6,000 feet, so I had to reduce mine from the stock #31 nozzle to a #28...
