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Discussion Starter #1
I was browsing the net and found this you tube video.

<table width="891" cellspacing="0" cellpadding="0" border="0"><colgroup><col width="891"></colgroup><tbody><tr height="20"> <td class="xl64" style="height:15.0pt;width:668pt" width="891" height="20">https://youtu.be/oBOrWUPhYto

It gives instructions on how to tune the idle speed by opening the secondary blade so the primary transition
slot remains at the recommended square configuration.

Has anyone tried this? Were your results positive or negative? Any other commentary is appreciated.

I might give this a try. On my engine, I need to have the transition slot open between 1/8 - 3/16 of an inch
and the idle air screws turned in to 1/2 + 1/16 turn in order to get an A/F ratio in the range of 13.9 - 14.2.

To me, this isn't how the carb was designed to be set up.

Thanks
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On my engine, I need to have the transition slot open between 1/8 - 3/16 of an inch
and the idle air screws turned in to 1/2 + 1/16 turn in order to get an A/F ratio in the range of 13.9 - 14.2.

To me, this isn't how the carb was designed to be set up.
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transition slot opened to 3/16" = more air than normal
idle screws turned in to 1/2 turn = less fuel than normal

so it seems like your idle circuit is super-rich, for some reason.
is this a new carb, or has it been around the block a few times? the standard 600 doesn't have idle adjustments on the secondary, so i'm not sure what would cause the idle circuit to be pouring so much fuel.
 

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because the tranfer slots are uncovered. the idle mix screws dont affect it
i think i'm following what you're saying-
since the transfer slots are completely uncovered, he's pulling fuel through the main circuit, instead of just the idle circuit?
 

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no, its
through the idle restrictor, but idle is usually reduced by the mix screws. the mix reduction at the screw is bypassed
 

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Discussion Starter #7
transition slot opened to 3/16" = more air than normal
idle screws turned in to 1/2 turn = less fuel than normal

so it seems like your idle circuit is super-rich, for some reason.
is this a new carb, or has it been around the block a few times? the standard 600 doesn't have idle adjustments on the secondary, so i'm not sure what would cause the idle circuit to be pouring so much fuel.
In order to get the car to idle at the recommended 850 rpm, I need to open the curb idle screw to where the transition slot is open too much. I should have made an exact measurement but it was open way too much. To combat this, I have to close the idle screws too tight.

It is a new carb. The float levels have been checked. The fuel pump is a manual and has the correct pressure so I don't think fuel is overflowing into the engine.

What I want to do is get more air into the engine at idle so the transition slot can be set correctly, which will allow the idle screws to be set correctly.

I thought about putting holes in the primary blades too. I'm not sure what size to start with. The problem with this solution is there isn't a means of reversing the change. Thanks
 

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Discussion Starter #8
i don't know how to search the forum for it, but the subject has been covered a bunch

I found a few threads that mentioned it can be done but none give any specifics on what someone did, like how many turns of the screw or how far they opened the gap from stock. I'll do some more searching to see what I come up with. thanks
 

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It's the way it should be done, it allows you to stay in the idle circuit, otherwise depending how deep you are, you could either have drippy transition slots, or even drippy boosters too.
 

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what engine is this carb on?
what happens when its set to stock?


found this on another forum:
really the only reason to adjust the blade positon is to correct the throttle blade/transfer slot relationship. since the transfer slots are not metered by the idle mixture screws, if there is too much of the transfer slot exposed the engine will run rich at idle.

if your idle mixture screws have little to no effect then likely too much of the transfer slot is exposed.

my holley rebuild manual states that .040 - .065 of the transfer slots should be exposed. but in my experience ive had to close them down to .020 - .025 to get the idle cleaned up.
 

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Discussion Starter #13
this is what i'm thinking too.
can you look down the carb with the engine running? use a big handheld mirror if necessary.

I looked at the carb at idle and didn't see any drips coming from the shooters.



The carb has these components in it



66 mains, 33 shooters, dark blue pump cam (2nd position) and, silver (i think) secondary spring)
 

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Discussion Starter #14
what engine is this carb on?
what happens when its set to stock?


found this on another forum:
really the only reason to adjust the blade positon is to correct the throttle blade/transfer slot relationship. since the transfer slots are not metered by the idle mixture screws, if there is too much of the transfer slot exposed the engine will run rich at idle.

if your idle mixture screws have little to no effect then likely too much of the transfer slot is exposed.

my holley rebuild manual states that .040 - .065 of the transfer slots should be exposed. but in my experience ive had to close them down to .020 - .025 to get the idle cleaned up.

It is a 4160- 600, 80457-13. It's my goal to get the transfer slot set close to the factory recommendation. I don't have a feeler gauge that will allow me to make an accurate measurement. Going by eyeball.
 

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so for your application, factory specs dont work as they are for an engine that makes plenty of vacuum.. you do need to open the secondary plates some to allow enough air in with the idle mix without exposing the transfer ports. or... modify the primary and or secondary plates with holes
 

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One way to check, although not really precise, is to see if your ported vacuum is alive at idle. Passenger side of the front metering block on most Holleys. Has to be hot idle, choke fully open to check. If that port is dead at idle, you are likely fine

If it has vacuum, then there is enough airflow past the blades that it is also likely pulling on the T-slots. You want a little vacuum on the T-slots to precharge them so they act quickly when the throttle opens, but too much and you get enough fuel that when the blades open, at best you have a bit less available for transition, at worst, none because you were already using their fuel
 

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Discussion Starter #17
One way to check, although not really precise, is to see if your ported vacuum is alive at idle. Passenger side of the front metering block on most Holleys. Has to be hot idle, choke fully open to check. If that port is dead at idle, you are likely fine

If it has vacuum, then there is enough airflow past the blades that it is also likely pulling on the T-slots. You want a little vacuum on the T-slots to precharge them so they act quickly when the throttle opens, but too much and you get enough fuel that when the blades open, at best you have a bit less available for transition, at worst, none because you were already using their fuel
That's a great tip. I checked it today. There was no vacuum there at idle with a warmed up engine. I started tuning the carb using the secondary blade so this test might not be valid as I think you wanted to see the results with the secondary blade at the factory setting and how it was tuned before. Here is what I have done so far with the secondary blades and details about the engine.

Blueprint Engines 306, aluminum heads
cam lift, I/E 543/.554
duration, I/E 218/226
lob separation @ .050 - 112 d

Timing set to 16, Distributor MSD ready to run, BPP3023BL distributor and BPP82022BL coil, 2 smallest springs in distributor so the advance is all in at 2500 or less. Distributor is connected to the ported vacuum at metering block. The wire to the coil is 14 gauge and there is 12 volts at the coil.

Spark plugs Champion 792, gap set to .45. , plug wires moroso ultra 40

Idle speed at 850, vacuum at idle is between 13.5-14 or 14-15 depending on how lean I set the idle screws.
Cast iron exhaust manifolds with dual exhaust, 4 speed car, 9 inch 3.50 gears with trutrac.

Carb is Holley 4160 600 cfm, 66 mains, 33 shooters, dark blue pump cam #2 position, Silver secondary springs. The fuel economy has been a steady 10 mpg even when the A/F ratios are in the mid 14’s. I’ve been easy on the pedal when driving in stop/go traffic.

I attempted to tune the carb using the secondary screw. I measured the gap at .35 before so I could reverse the change if it didn’t work out.

I’ve pulled the carb 5 times and tried different gaps. I started at .77, which seemed to be too much. Then .50, which didn’t seem to be open enough. Then .59 and now .61. With all settings, I am finding the idle screws need to be turned in or out to get the A/F ratio in the mid 14’s.

What is interesting, the .77 combo produced the most responsive engine for part throttle driving with A/F’s above 15 but secondary throttle response was poor and I couldn’t get it to idle under 950 (too much air coming from the secondary’s) At all settings, the exhaust smells rich even when the A/F is in the mid to high 14’s or over 15. The plugs are probably fouled again. The exhaust at idle still smells rich. I’m replacing plugs every 200 miles or less.

The settings now are: secondary gap = .61, Transition slot more than square. (see the attached photo), idle screws at 1 ½ + 1/16 turns out. A/f at idle 14.2-14.9, A/F at steady cruise (2000=50mph, 2900= 60 mph) is at 14.5- 14.9.

Another thing that happens is, as soon as I drive the car after tuning, the idle rpm increases. Normally it increases to 1000 or so but today it was up to 1300 rpms. When I got back I shut off the engine but could not find a reason for the increased rpm. I restarted the engine and it was back at 850. There must be a problem with the linkage.

I wonder if I should drill the primary blades (or buy pre-drilled blades) instead of adjusting the secondary blades?
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ahhh - lumpy cam? no opinion from me on drilling vs adjusting the blades, but I learned something today:


https://www.hotrod.com/articles/carburetor-tuning-cure-big-cam-sag/

Problems begin when engines with big camshafts create idle vacuum lower than 10 Hg. With this reduced idle vacuum, the throttle blades must be opened farther in order to allow the engine to idle at a given rpm. When the throttle blades are cranked open, this uncovers too much of the idle transfer slot. This circuit isn’t metered by the idle mixture screws, delivering fuel over and above that set by the idle mixture screws. The result is the engine idles much richer. The unsuspecting tuner then compensates by leaning the idle mixture screws. Unfortunately, once the throttle is opened further under mild acceleration, there’s no more fuel available from the transfer slot and the engine stumbles.
The key to improving this situation is to introduce a metered amount of bypass air into the engine. This allows the tuner to close the primary throttle blades back to what Holley considers a stock location, uncovering only 0.030 inch of the transfer slot below the throttle blades.
The time-honored way to accomplish this task has been to drill holes in the primary and secondary throttle blades.
 

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Discussion Starter #19
ahhh - lumpy cam? no opinion from me on drilling vs adjusting the blades, but I learned something today:


https://www.hotrod.com/articles/carburetor-tuning-cure-big-cam-sag/

Problems begin when engines with big camshafts create idle vacuum lower than 10 Hg. With this reduced idle vacuum, the throttle blades must be opened farther in order to allow the engine to idle at a given rpm. When the throttle blades are cranked open, this uncovers too much of the idle transfer slot. This circuit isn’t metered by the idle mixture screws, delivering fuel over and above that set by the idle mixture screws. The result is the engine idles much richer. The unsuspecting tuner then compensates by leaning the idle mixture screws. Unfortunately, once the throttle is opened further under mild acceleration, there’s no more fuel available from the transfer slot and the engine stumbles.
The key to improving this situation is to introduce a metered amount of bypass air into the engine. This allows the tuner to close the primary throttle blades back to what Holley considers a stock location, uncovering only 0.030 inch of the transfer slot below the throttle blades.
The time-honored way to accomplish this task has been to drill holes in the primary and secondary throttle blades.

I read that article too. My vacuum #'s aren't that low but I have all of the symptoms. I think I'm going to place an order with Summit and give it a try. It can't hurt as I can always put the original parts back in.



Thanks for the link and comments.
 

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I thought I would update the board just in case someone needs this info in the future.

I opened the secondary blade to a gap of .73. This required the idle mix screws to be opened to 1 3/4 turns. More air needs more fuel. Idle A/f is in the mid 14's. Vac at idle is 14.5 - 15. A/F at 2000 (50 mph) and 2800 (60 mph) is in the high 14's to 15. Throttle response is good and the idle is steady. I think this is a good as I am going to get the carb tuned. I'll drive awhile and see if the performance drops as the plugs get some time on them.



Thanks for everyone's help. Much appreciated.
 
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