AF lean out on 2-3 change

[F-BIRD'88]

Loosen a/valve spring 1/4-1/2 turn & increase sec jets 2 sizes.

I'd also change the plugs before the next asphalt session. With no data (especially rpm trace) or time slips or plug readings or even fuel level in the tank, might as well go full old school on fixin' it with a screwdriver and an handful of brass. Since 2x4 426 Hemis had 3/8" fuel lines back in the day, I'm thinking that the pump feed line isn't the issue either unless the Carter vane pump truly can't suck.
This appears to be the combination, engine-wise, but we still don't know what car, what rear gear, weight, fuel tank setup and the configuration of the cold air setup. The 2x4 dual plane setup falls down up top and makes less hp than a single plane with a 4150 sized throttle body.
https://www.hotrod.com/articles/test-ed ... en-3-hemi/

I bet a different result would be seen if the dual plane was tested A. with bigger carbs... B. combined with added carb spacers. I don't recommend the 500 cfm carbs for anything where max top end power is desired.

Also be interesting to see the Mopar single plane tested with 2x4 carbs on an adapter...

[F-BIRD'88]

The larger vs smaller Edelbrock carbs almost always make more power and torque. Even in pairs.
Air cleaner filter elements have effect, . So does the contour shape of the air cleaner base "bell".
I'd jet it for max on track performance vs what the AFR guage says or should say... It is at best a "you're too lean "warning device as you tune for in car max perf.
By rights the feed side of the fuel pump should be bigger.
Fuel pressure in high gear is ?
I would not have chosen 500 cfm carbs if for max performance..
If at higher car speed the carbs are seeing a different relative air pressure, than at lower car speed the AFR in high gear at high speed will change..

Test for that by accelerating 1/4 mile using just high gear.

If fuel pressure change in high gear is a issue and it is a engine mounted mechanical fuel pump try adding a pusher pump at the tank. EG: Carter 4070.

[rebelrouser]

I have been racing NSS, using 2, 750, federal mogul AFB's for almost 30 years. I have O2's on my car and have had a couple different engine combinations on the dyno. My car now runs 9.70's 139 mph. All the AFB's I have ever used lean out at high RPM's. To help with this issue I have done the following which made it better. First AFB's have a very small float bowl, if you take the top off the carb, and just lay a float in the chamber you will see what I am talking about. My son is a mechanical engineer, and in discussing this issue with him, his work had a computer program for calculating flow and other variables in hydraulic systems. I gave him the specific gravity of the fuel I was using, and all the dimensions of the fuel system, and the demand for fuel we used was .5 lbs per horsepower. His results, said the needle and seat orifice and the orifice in the pressure regulator were the restrictions in the system. I changed out the pressure regulator, got new large orifice needles and seats, and drilled the carb bodies for dual feed. It still leaned on the top end but it was an improvement. I did notice that it became very consistent after the modifications, most of the time slips were nearly identical, unless the driver screwed up.
There are some older threads about the newer AFB clone carbs and the boosters etc. on speed talk, look them up, lot of good info. My next step was to put the carbs on a flow bench and use the velocity probe in the jet to measure how hard the venturi pulls on the jet. By polishing and trimming on the boosters, I got all eight to pull the same amount. Next step was to lighten the secondary weights, and measure how much vacuum it took to open them, and again grinding and swapping spare parts until I got exactly equal secondary opening. Also on the secondary opening, the amount the blades open can really affect the amount of pull on the jets, if the blade opens too far and goes slightly over center it really messes with the booster signal. I say this because lots of guys are cutting off choke air horns for more flow, and are letting the secondary blade open too far because they cut off the material that used to stop it. I also changed the way I cut off AFB airhorns, because of the turbulence you get on a flow bench around the air bleeds and top of the boosters if you cut them too short.
Then in reading the older speed talk posts on AFB's I made some thin sleeves to extend the primary boosters into the narrowest part of the venture. That also gave it another step in the booster which my idea was to improve fuel atomization. Not sure what helped the most, or if I even did everything right, but those carbs are worth 35 HP on my dyno over my old ones. And guess what, they still lean out some on the big end, so I am chalking it up to the small float bowls.
And just on my car, I have found it likes more ignition timing at the launch, and less on the big end, so I have an old MSD retard that works off rpm, to retard the timing at 5,000 rpm. When I first put it on, it was worth a tenth. I need to see if it still works with my current combo, that was 4 engines ago.
I know guys that are running NSS cars into the 8's with AFB's and I have seen their carbs apart and they don't have a ton of modifications, I am sure they lean out as well, I don't see how they could not. My car runs through 1st and 2nd right at 12.5, then in 3rd , it leans to 13 to 1 as I cross the finish line, if I richen it to run 12.5 across the line it slows down, and plugs read rich. On the dyno the air fuel remained pretty constant, but the length of the pull was not as long as a run, and of course the load on the engine is different than a pass down the dragstrip.

Very impressive effort and real results to show for it. Have you used smaller main air bleeds? Have you made new emulsion tubes with fewer and smaller E-bleed holes? You must have a set of original 409 carbs to study? Notice how simple the primary booster E-tube is and that there is no tube in the secondary boosters? Did you notice the discharge nozzles are larger than the FM and Edelbrock versions ?

No I have not tried smaller air bleeds on the high speed side, may be my next project. My car idles very nice, but I have drilled and tapped the booster idle air bleeds and made several AFBs idle really pretty with a small camshaft. I wish I had a set of 409 AFB's to look at. I am not sure how Federal Mogul, and then Edelbrock decided on which and what AFB to copy, it would be interesting to hear from an engineer that was involved. I am sure stock and mild engines was all they were concerned with, and not racing applications when they decided.

[rebelrouser]

I changed mine ages ago to the AVS/Thunder design because of the air doors pulling open too soon.

I may be making too much if the lean out....especially as I have not resolved the timing curve yet.

I know another friend with a 600HP G3 Hemi 390 cube stroker who made 35rwhp with an xtra 2 deg timing - these motors don’t need much but respond big time to minor timing adjustments - his is EFI so its easier to tune at the track with the Holley software.

Anyhow, Ive already pulled the trigger on the Tanks Inc 67 Belvedere EFI tank and pump - so if the problem continues I’ll have somewhere else to look.

People may tell me that I am all wet, but the rod stroke ratio has a lot to do with how much timing a engine needs. The rod stroke ratio determines the amount of time that the piston dwells, and maximum torque is developed if you fire the mixture right before the piston starts its downward motion. My engine program calculates a timing curve for an engine, and it seems to prove this theory out. RB Mopars like a lot of timing in my experience. I run mine at 38 degrees, and on cool days I have run as much as 42 degrees. It is always faster with more timing.

[ClassAct]

I changed mine ages ago to the AVS/Thunder design because of the air doors pulling open too soon.

I may be making too much if the lean out....especially as I have not resolved the timing curve yet.

I know another friend with a 600HP G3 Hemi 390 cube stroker who made 35rwhp with an xtra 2 deg timing - these motors don’t need much but respond big time to minor timing adjustments - his is EFI so its easier to tune at the track with the Holley software.

Anyhow, Ive already pulled the trigger on the Tanks Inc 67 Belvedere EFI tank and pump - so if the problem continues I’ll have somewhere else to look.

People may tell me that I am all wet, but the rod stroke ratio has a lot to do with how much timing a engine needs. The rod stroke ratio determines the amount of time that the piston dwells, and maximum torque is developed if you fire the mixture right before the piston starts its downward motion. My engine program calculates a timing curve for an engine, and it seems to prove this theory out. RB Mopars like a lot of timing in my experience. I run mine at 38 degrees, and on cool days I have run as much as 42 degrees. It is always faster with more timing.

That’s what I have found as well.

[F-BIRD'88]

I've found that with the Edelbrock AFB csrbs that throttle response idle to WOT is good from a dead start. But when cruising ,go WOT and there is a brief secondary hesitation.
Then pulls strong.. The AFB secondary air door cannot be all things in all situations.. It is more of a querk nature of this carb series vs a performance problem. Customizing the secondary air door and counterweight mass and offset should fine tune that. If you care too. Would be nice if there was a selection of different AFB air doors / weights available for end user tuning.
At the track compared to my best holley VS and double pump carbs the ET was a toss up, so close, how you drive ghe car made more difference. The MPH was 1 mph higher with the Edelbrock carb. These aluminum carbs really benefit in getting the carb body running temp right.
A heat isolation gasket or spacer helps a lot.

[F-BIRD'88]

Since its a different car check the fuel pickup in the tank.
I had this problem on my Firebird. Once corrected the Eddy carb pulled much better in high gear.. It mattered.
This feed line tank pickup fault effected the Eddy carb more than the holleys. The Eddy carb ended up 1 mph faster. than the holleys, but all were improved.
Do you still have your AFB carbs to test compare against the perf of the AVS Thunder carbs?
116 to 113 mph (¼ mi) is a big loss if same weights.

Didn't you have 600 or 750 AFB carbs on that engine before?

[F-BIRD'88]

Any all 3/8" fuel system fittings must be checked for intrrnal passage size. A few may need hand "blueprinting" with a ¼" drill bit. If using the Eddy Banjo style fuel inlet fittings these can use some hand porting beveling passage entraces also . How you clock them matters. The fuel filter matters. I've found that the pretty chrome hot rod stuff is not the best for function on many.. If when using any 3/8" stuff all has to be checked for internal passage flow.

[Chargermal]

Wow...where to start!

1 - They were 500 AFBs - now 500 Thunders.
2- Point on feed line diam is noted, however that will all change with my new pump and tank
3 - Probably too late to conduct any track diagnostics - although I like the idea of the top gear run.
4- I’m running 1/2” line from the filter with pushlock fittings all thru to the carbs.
5 - The old car picked up 1.4 mph and 12rwhp with the 1” open spacer stacked below the 4 hole - so mph loss could well be down to plenum size along with the tune (although the tune is the same)

Gen3 NA Hemis rarely require more than 24deg at WOT....27 deg showed no imorivement on the dyno - which is NOT surprising. Boosted motors usually top out at 19-20 deg total.

[rustbucket79]

Why such an oddball combination, is this a class racing deal?

[Chargermal]

If you can define ‘oddball’...I’ll try to answer.

[rustbucket79]

Kickass engine, ancient fuel metering? How’s that as a definition?

[Chargermal]

I get ya. Well, for a start I know Carter/Brocks.....with EFI I have no clue.

2nd, I went with a G3 Hemi because they make as much power as a SB stroker but are more fuel efficient, start easier, need less cam for = HP and weigh the same.

3rd - My car is a tribute to a 67 Plymouth RO23 super stocker - which had in line dual quads - EFI would have looked naff.

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