aftermarket 6pak jets/sqirters

[Meep-Meep]

I just pulled my 6 BBL carbs off my 440 to make adjustments and here is what I found: Both outboard metering plates are stagger jetted with the smaller orifice on the diaphragm side. Best I can measure I have .086" and .095" on both front and rear carb and at WOT in high gear on a dyno pulling up to 6000 RPM it was big time lean. The center carb had 61's both sides. My plan is to open up the outboards a little while maintaining the stagger jetting. The front carb will be jetted to 63.

What I noticed about the idle when I first put the 6 BBL on is the small amount the center carb needs to be open to get a normal idle. I'm running a very mild cam and get 12" Hg at idle so for the most part this is an OEM carb friendly engine. The outboard carbs do have an idle circuit and I found they contribute quite a bit to the idle fuel. I also found it extremely important to make sure the safety linkage puts pressure on the outboard carbs to ensure complete throttle blade closure. If you can wiggle the outboard throttle shaft with the center carb throttle closed you will not have a consistent idle. My carbs are NOS later model replacements and the mixture screws on the outboards are still sealed with lead plugs and at this point I see no reason to tamper with them.

Over all my car runs great on the center carb but is weak when you mash the pedal. It will still roast the tires down to the rim if you let it but I think there needs to be something more, and the dyno run showing a nearly 18:1 AFR proves it wants more fuel. I was also fighting a huge hesitation at the track so that's more proof that I'm not picking up enough fuel.

 

[poppa]

I remember setting the outboards closed. I got a little to focused on the center I think. Hopfully I will have time tonight to redo the idle settings.

 

[lewtot184]

I just pulled my 6 BBL carbs off my 440 to make adjustments and here is what I found: Both outboard metering plates are stagger jetted with the smaller orifice on the diaphragm side. Best I can measure I have .086" and .095" on both front and rear carb and at WOT in high gear on a dyno pulling up to 6000 RPM it was big time lean. The center carb had 61's both sides. My plan is to open up the outboards a little while maintaining the stagger jetting. The front carb will be jetted to 63.

What I noticed about the idle when I first put the 6 BBL on is the small amount the center carb needs to be open to get a normal idle. I'm running a very mild cam and get 12" Hg at idle so for the most part this is an OEM carb friendly engine. The outboard carbs do have an idle circuit and I found they contribute quite a bit to the idle fuel. I also found it extremely important to make sure the safety linkage puts pressure on the outboard carbs to ensure complete throttle blade closure. If you can wiggle the outboard throttle shaft with the center carb throttle closed you will not have a consistent idle. My carbs are NOS later model replacements and the mixture screws on the outboards are still sealed with lead plugs and at this point I see no reason to tamper with them.

Over all my car runs great on the center carb but is weak when you mash the pedal. It will still roast the tires down to the rim if you let it but I think there needs to be something more, and the dyno run showing a nearly 18:1 AFR proves it wants more fuel. I was also fighting a huge hesitation at the track so that's more proof that I'm not picking up enough fuel.

the out board jetting for the repops is staggered on the back carb .093/.086. the center carb is usually #62 but needs to be #64 for stock exhaust and #65 or #66 with headers, no big jets! the linkage from the end carbs to the center carb is to keep the carbs opening and closing in unison. tight linkage isn't good. the carbs are designed with the shaft offset and the throttle blades positioned so that engine vacuum keeps them closed. check the throttle blades to make sure they aren't sticking on the end carbs. float level on the end carbs should be set with a light trickle out the sight plug. i use yellow springs for the track and purples to drive on. setting the idle mixture on the end carbs is very important because the center carb can't compensate for all 6 barrels. theres an idle feed tube in the main well in the center carb and the restriction is small and can't be easily serviced. take the plugs out and back the screws out 3/4 turn from lightly seated. in my experience fuel delivery is very critical. all power problems i've ever had related to low fuel pressure or some restriction in supply side. in run .093 on all 4 corners and #65's in the center (headers and mild solid cam) with a 6.5 power valve. i use a modified carter 6903 fuel pump that in my tests is just adequate. all this applies to vacuum carbs not the aftermarket mechanicals.

 

[poppa]

I was just looking at air/fuel meters ,which should I get ?? Some say narrow band others say wide ,a couple don't specify???? How do they hook up? They say electrical and I was guessing (never had/used one) they would be vacum. All I have seen look like something you would "mount" in the car ,I was picturing a handheld device. Thanks

 

[lewtot184]

I was just looking at air/fuel meters ,which should I get ?? Some say narrow band others say wide ,a couple don't specify???? How do they hook up? They say electrical and I was guessing (never had/used one) they would be vacum. All I have seen look like something you would "mount" in the car ,I was picturing a handheld device. Thanks

i don't use them but they could be a nice tool for tuning. i believe wide band is preferred. you have to weld a bung in the exhaust pipe and install an oxygen sensor plus buy the analyzer. a little pricey for what i do.

 

[poppa]

I don't want to do that. More work ,not enough time. Someone else suggested that but didn't explain all that. I thought it was just a handheld thing.
Yep ,you can turn the center carb's idle screws out if you turn the outboards in. LOL I hope to "test drive it tomorrow".

 

[Meep-Meep]

the out board jetting for the repops is staggered on the back carb .093/.086. the center carb is usually #62 but needs to be #64 for stock exhaust and #65 or #66 with headers, no big jets! the linkage from the end carbs to the center carb is to keep the carbs opening and closing in unison. tight linkage isn't good. the carbs are designed with the shaft offset and the throttle blades positioned so that engine vacuum keeps them closed. check the throttle blades to make sure they aren't sticking on the end carbs. float level on the end carbs should be set with a light trickle out the sight plug. i use yellow springs for the track and purples to drive on. setting the idle mixture on the end carbs is very important because the center carb can't compensate for all 6 barrels. theres an idle feed tube in the main well in the center carb and the restriction is small and can't be easily serviced. take the plugs out and back the screws out 3/4 turn from lightly seated. in my experience fuel delivery is very critical. all power problems i've ever had related to low fuel pressure or some restriction in supply side. in run .093 on all 4 corners and #65's in the center (headers and mild solid cam) with a 6.5 power valve. i use a modified carter 6903 fuel pump that in my tests is just adequate. all this applies to vacuum carbs not the aftermarket mechanicals.

When I first put my carbs on they had the yellow springs and I only found that out when I looked after noticing from the drivers seat the outboards were opening too quickly. My engine is quite mild compared to most you run into. I have about 12" Hg at idle and it's very crisp.

I made a few dyno runs with different springs and the black springs produced the best AFR and that is because the outboards never opened all the way. The plain springs allowed them to open all the way, but with a huge lean spike, followed by a slow recovery to the rich side but never got to ideal. Still lean no matter how you slice it. On one test with the black springs I hooked a wire and manually pulled the outboards open when they stopped opening on their own. This was about 5K RPM and it liked it! Power was up but the mixture was still lean.

In those few runs what I observed about the opening of the end carbs and seeing the A/F data was interesting. The opening rate is not linear. When the signal from the primary is strong enough to open the end carbs you get a very quick initial opening until about half way then it slows way down. This is the exact opposite of what I want!! That quick initial opening is exactly where the huge lean spike is, and with a soft spring it never fully recovers from it (lean).

I dug into this deeper by checking the size of the secondary bleed hole on three sets of 6 BBL carbs. This is the bypass port that runs in parallel with the vacuum diaphragm passage. The carbs on my engine are .043" and on one set I found .067" if I recall correctly. It was somewhere in the .06" range. And on one set I had both sizes!! To slow my signal down I opened mine up to the .06" dimension, and that with a brown spring (whatever one is under the black one) made a noticeable difference as felt from the drivers seat. I have yet to try this on the dyno and will as soon as I get my new metering plates.

I stand by my comments about the safety linkage. I was getting a very inconsistent idle after a spirited run that allowed the outboards to come in until I tightened up the linkage to make sure the outboard throttle bladed were seating with repeatability. These carbs are newer replacements and I saw no evidence of tampering. The throttle blades seat well and do not stick and seem to be as good as any slightly used Holley. It's the play in the shaft that seems to be causing the difficulties so I just let the linkage eliminate it at idle. It's not too tight but just enough on both carbs to avoid sticking or binding. The idle is consistent after multiple full throttle passes.

Another comment about the idle. Again, my engine is pretty mild and I feel the effect of all the carbs at idle to the point that the outboards are too much. My center carb idle stop screw on the solenoid is barely holding the shaft above the screw in the base plate. I can pretty much close of the center carb and let the outboards do all the work but prefer it the other way around, hence my inconsistent idle. Once I got a handle on the outboards my idle is fine.

 

[poppa]

I don't have the vacum outboards ,I only turned the outs curb idle in about 1/2 turn and backed the center out about as much maybe 3/4. Just a quick screw in and I could notice a difference with the mixture screws where I didn't notice a difference before. It was in too much and probably into the transition circuit where the mixture screws are useless. I hope to test and make more adjustments tonight. I think I will have to get a more acurate vacum gauge soon also.

 

[lewtot184]

meep, i'm very familiar with your cam, used it for years. the smaller kill bleed in the end carbs was '70 and later upgrade. the '69's used the larger kill bleed size. i have used factory carbs and aftermarkets. my '69 r/t used the aftermarkets for years and i ended up putting them on my '65. i did a strip test with different springs in the secondary carbs. for every step heavier above the yellows the car slowed down about a tenth. this was with my r/t, 3.23 rear, 9.3 compression kb184 pistons, stock heads and exhaust, 272 cam, 225-70x15 t/a radials, 4060lbs, best e.t 13.13, best speed 106mph. i firmly believe you have a fuel delivery/jetting issue and that causes your lean spots, but ultimately it's your car.

 

[poppa]

Is there a ballpark for setting the curb idle on all three carbs? I don't want to have the outs to far in.

 

[lewtot184]

Is there a ballpark for setting the curb idle on all three carbs? I don't want to have the outs to far in.

i don't think there is anything definitive. i used to use a feeler gauge to set the throttle blades. all that did is make sure all 4 blades were an equal distance from the throttle bore. just don't uncover the transfer slots and your good to go.

 

[Meep-Meep]

meep, i'm very familiar with your cam, used it for years. the smaller kill bleed in the end carbs was '70 and later upgrade. the '69's used the larger kill bleed size. i have used factory carbs and aftermarkets. my '69 r/t used the aftermarkets for years and i ended up putting them on my '65. i did a strip test with different springs in the secondary carbs. for every step heavier above the yellows the car slowed down about a tenth. this was with my r/t, 3.23 rear, 9.3 compression kb184 pistons, stock heads and exhaust, 272 cam, 225-70x15 t/a radials, 4060lbs, best e.t 13.13, best speed 106mph. i firmly believe you have a fuel delivery/jetting issue and that causes your lean spots, but ultimately it's your car.

I'm running a Holley 7.5 PSI mech pump and pretty sure it's as good if not better than the OEM pump. I will check the fuel pressure now that you mentioned it but I'm not having the typical signs of fuel starvation. With the 4 BBL it was about 3 PSI at WOT. 6 BBL might be a bit lower because of the extra needle and seat. I'm thinking jetting issues.

- - - Updated - - -

Is there a ballpark for setting the curb idle on all three carbs? I don't want to have the outs to far in.

My outboard carbs seat in the bores and the throttle blades have holes in them. Not sure if someone put the holes there or if they are stock. In either case the outboard carbs on my engine contribute heavily to my idle. So much to the point that the center carb is almost not needed for idle. My 440 had a much cleaner and crisper idle with the 750 AFB.

 

[poppa]

In either case the outboard carbs on my engine contribute heavily to my idle. So much to the point that the center carb is almost not needed for idle. [/QUOTE]

How can you adjust your mixture then??? Mine are now where they should be, I pulled them off and they are just uncovering the slots and they is noticeable change when I turn the mixture screws. Tomorrow ,hopefully, I will recheck everything and play with the timing some more and make more adjustments.

 

[Meep-Meep]

In either case the outboard carbs on my engine contribute heavily to my idle. So much to the point that the center carb is almost not needed for idle.

How can you adjust your mixture then??? Mine are now where they should be, I pulled them off and they are just uncovering the slots and they is noticeable change when I turn the mixture screws. Tomorrow ,hopefully, I will recheck everything and play with the timing some more and make more adjustments.[/QUOTE]


Noticeable change is a good start. The idea is all the blades leak air and you need to dilute that air with the proper amount of emulsified fuel to support combustion, hence you noticing change. Ideally you don't want the outboards to over power the idle, so let's say you have the center carb out of the picture, the engine should barely idle. Then cut in the center carb to bring the idle to where it belongs.

 

[noosacuda]

Remember if you're tuning mech-sec carbs you have to treat as 3 carbs or a double pumper, as in all your idle screws should be adjusted the same. Worked for my triple 500's.

 

[Meep-Meep]

Remember if you're tuning mech-sec carbs you have to treat as 3 carbs or a double pumper, as in all your idle screws should be adjusted the same. Worked for my triple 500's.

Yes, I can see how this would work out. In my case the outboards are OEM and the idle passage is pretty small, thus allowing the center to do most of the work.