Tuner wrote: ↑Sat Apr 24, 2021 4:26 pm
That cleaner should be OK. Did you mention the jet size?
All I've changed so far is to go down 2 sizes on both primary and secondary. Current jetting is 70/78 and stock was 72/80. I should also mention steady state, level ground, 55 mph cruise is right at 12.5:1 with current jetting. And full throttle AFRs are in the low 10s.
I would drop that 6 (six) jet sizes. That should move the part throttle to near 15.5 and the WOT to about 13. I say again, the annular booster in a "750" with 1-3/8" venturi acts more like the booster signal of a 600 with standard boosters.
If it has a 3 or 4 emulsion bleed metering block it will not respond as much to the jet size change as it would with the standard old school Holley 2 E-bleed metering blocks.
Tuner wrote: ↑Sat Apr 24, 2021 4:26 pm
That cleaner should be OK. Did you mention the jet size?
All I've changed so far is to go down 2 sizes on both primary and secondary. Current jetting is 70/78 and stock was 72/80. I should also mention steady state, level ground, 55 mph cruise is right at 12.5:1 with current jetting. And full throttle AFRs are in the low 10s.
I would drop that 6 (six) jet sizes. That should move the part throttle to near 15.5 and the WOT to about 13. I say again, the annular booster in a "750" with 1-3/8" venturi acts more like the booster signal of a 600 with standard boosters.
Ok thanks. So just to clarify, drop 6 on the primary, and 6 on the secondary as well, for final jetting of 64/72?
All I've changed so far is to go down 2 sizes on both primary and secondary. Current jetting is 70/78 and stock was 72/80. I should also mention steady state, level ground, 55 mph cruise is right at 12.5:1 with current jetting. And full throttle AFRs are in the low 10s.
I would drop that 6 (six) jet sizes. That should move the part throttle to near 15.5 and the WOT to about 13. I say again, the annular booster in a "750" with 1-3/8" venturi acts more like the booster signal of a 600 with standard boosters.
Ok thanks. So just to clarify, drop 6 on the primary, and 6 on the secondary as well, for final jetting of 64/72?
Yes. Drive, observe A/F, rinse, repeat. An old school Holley moves about 1/2 A/F ratio for each jet size.
steve cowan wrote: ↑Sat Apr 24, 2021 7:14 pm
Do the plugs show rich??
Kinda surprised it that rich with that main jetting size.
Afr can lead you astray,been talked about alot lately
No, I haven't done any plug reading. How long of a WOT run do you think I'd need to do to get an accurate plug coloring? It would definitely give peace of mind to back up the readings of the wideband to be on the safe side. What did you think of my sensor placement in the stock manifold?
I would not worry about WOT at this stage,spark plugs should be relatively clean all the time.its a shame it was not dynoed with your exhaust manifolds etc .
Tuner has given you a starting point so stick with that.
As well as the booster size that tuner alluded to, you have choke butterfly that can mess stuff up as well,I noticed vacuum advance as well,hooked up??there is good advantage in vacuum advance to help the burn on cruise mode.if you can put some spark plug photos up when possible might help.
I just ordered the jets Tuner recommended from Summit.
Yes vacuum advance is hooked up. I still need to get a vacuum reading at cruise speeds to fine tune the power valve as well. I know the engine is making 16" vacuum at idle in neutral (manual trans) so the stock 6.5" power valve is most likely gonna have to be raised. I know I noticed a slightly lean "hole" when slowly easing into the throttle toward wot, most likely from the PV sizing.
And the leaner jetting required to get my cruise and wot in check is only going to exacerbate this hole I'm sure!
Electric choke is not currently hooked up. I have simply rotated the choke body to load the spring and keep the choke blade open all the time (I've never been one to use a choke)
eric8 wrote: ↑Sat Apr 24, 2021 10:10 pm
I just ordered the jets Tuner recommended from Summit.
Yes vacuum advance is hooked up. I still need to get a vacuum reading at cruise speeds to fine tune the power valve as well. I know the engine is making 16" vacuum at idle in neutral (manual trans) so the stock 6.5" power valve is most likely gonna have to be raised. I know I noticed a slightly lean "hole" when slowly easing into the throttle toward wot, most likely from the PV sizing.
And the leaner jetting required to get my cruise and wot in check is only going to exacerbate it.
You sure it is not so rich it is misfiring? Misfire for any reason will make a lean number because of the unburned oxygen. When you get the steady state numbers within reason you probably will have to mess with the accelerator pump squirter size and pump cam.
eric8 wrote: ↑Sat Apr 24, 2021 10:10 pm
Electric choke is not currently hooked up. I have simply rotated the choke body to load the spring and keep the choke blade open all the time (I've never been one to use a choke)
A carb with proper A/F for a fully warmed up engine will need some choke for a few minutes when it is cold.
This brief lean spot happens way before full throttle. It is probably around 50% throttle position as you slowly ease into the throttle in top gear for instance. AFR's go 12s, 13s, and then mid 14s until it comes around and goes back to the 12s. Which I'm assuming is around 6.5" Hg and the PV reels it all back in.
I definitely need to get a vacuum gauge in the cab so I'll know what's going on.
F-BIRD'88 wrote: ↑Sat Apr 24, 2021 10:36 pm
With what you stated about the choke are you sure the choke is staying wide open at WOT?
It sounds like it may be is closing up @WOT.
Wire it open , retest.
Good point... I'll double check to make sure that isn't happening. Spring in the choke is wound up tight but stranger things have happened.
eric8 wrote: ↑Sat Apr 24, 2021 10:26 pm
This brief lean spot happens way before full throttle. It is probably around 50% throttle position as you slowly ease into the throttle in top gear for instance. AFR's go 12s, 13s, and then mid 14s until it comes around and goes back to the 12s. Which I'm assuming is around 6.5" Hg and the PV reels it all back in.
I definitely need to get a vacuum gauge in the cab so I'll know what's going on.
You definitely need a vacuum gauge for carb tuning so you can judge when the idle/transfer circuit is affecting things at light throttle 15" and higher,or the main jet at near 10" or so. After a few changes of idle air bleed and main jet you will get a feel for how much each affects various throttle positions.
A normal carb should progress lean as load is applied and intake vacuum drops, just as you describe. The number means not so much as long as it doesn't sag or stumble.
The Carter BBD 2bbl on a '72 318 Mopar will cruise near 17/1 on level road load between 18~15" Hg and progressively move toward as lean as 18+/1 at mid-load between 10" and 5" Hg before the metering rods lift at 5" or so and it goes back rich to 13/1. I had the pleasure of fiddling with one such unit recently.
Did you do any changes on the dyno?
I am only guessing but pretty mild engine combo??
When I drive home from the track I drop fuel pressure down and it also effects cruise AFR 16s light throttle.
steve cowan wrote: ↑Sat Apr 24, 2021 11:22 pm
Did you do any changes on the dyno?
I am only guessing but pretty mild engine combo??
When I drive home from the track I drop fuel pressure down and it also effects cruise AFR 16s light throttle.
It's a fairly mild 383. 9.5:1, 195cc heads, 224 cam@050, 110 LSA. Numbers looked pretty decent on the dyno with headers. Especially considering that once I had the carb apart, I discovered nothing had really been "tuned" other than the idle mixture. All air bleeds, ifr, jets, etc were out of the box settings.
Screenshot_20210424-233033_Gallery.jpg
Fbird really has me wondering about this choke. I'm gonna look like a real idiot when I check it tomorrow and find out it was sucking the choke blade closed against the spring tension
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Checked it again today on some pulls and it definitely wasn't the choke getting sucked closed. Warmer temps today and saw 9.9:1-10.4:1 afr full throttle and around 12.2:1-12.6:1 steady state cruise at highway speeds. Hoping that the smaller jets will bring everything right in line nicely!
An old school Holley moves about 1/2 A/F ratio for each jet size.