Convert to replaceable idle air bleeds?

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DrCharles

DrCharles

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Do I have enough metal here
Dart Carb 3_zpstm1vhfla.jpg
to drill and tap the primary idle air bleeds?
(Not sure what the thread size is on the replaceable jets, but I could use a brass set screw and make my own).
 
DrCharles said:
Do I have enough metal here View attachment 1715470593 to drill and tap the primary idle air bleeds?
(Not sure what the thread size is on the replaceable jets, but I could use a brass set screw and make my own).
Click to expand...


I have not do it, but I've read of others doing it. I do know the sizes of the Power Valve Channel Restrictors I've changed are less than consistent. I used a decent quality drill index to gauge the size of the holes. Many were a size or 3 off or different than their markings indicated.

Are you using a wide band gauge to target the A/F ratios you are going after?

There is a lot of good carb tuning info on the site Taptalk. Much of it from military studies made for aircraft carb tuning starting in WW1.
 
Yes, I have a wideband gauge on the dash ;)
Once I get a clean transition, then I'll start going after the PVCRs. Cruise on the mains is good and lean (14.5-15.5).
First step is to put an 8.5 or 9.5 PV in, since I now know that Holley's rule of thumb is completely wrong (there's a 4.5 or 3.5 in there now!)
 
DrCharles said:
Yes, I have a wideband gauge on the dash ;)
Once I get a clean transition, then I'll start going after the PVCRs. Cruise on the mains is good and lean (14.5-15.5).
First step is to put an 8.5 or 9.5 PV in, since I now know that Holley's rule of thumb is completely wrong (there's a 4.5 or 3.5 in there now!)
Click to expand...

I am currently basing my power valve selection on 1/2 idle vacuum, but am going to change it based on 1/2 cruise vacuum.

I have a good article regarding drilling, tapping and relocating the power valve channel restrictors to the bottom of the metering block, but I will have to dig for it. It made a noticable improvement in stabilizing my A/F ration at idle with the large cam I am running.
 
Interesting. Why would the PVCR have any effect at idle, since that circuit is not drawing fuel (through the boosters)?

My idle vac is 7-8" but cruise is around 15. So I'll start with an 8.5 PV for now.
 
DrCharles said:
Interesting. Why would the PVCR have any effect at idle, since that circuit is not drawing fuel (through the boosters)?

My idle vac is 7-8" but cruise is around 15. So I'll start with an 8.5 PV for now.
Click to expand...


Yes there is enough metal there to do replaceable brass.

I get mine from McMaster-Carr and if you are going to go all the way (I do) you'll need 6/32, 8/32 and 10/32 brass set screws 3/16 long.

The 6/32 is for the emulsion holes, IFR and maybe one orther, the 8/32 in the PVCR and T slot restricters and the 10/32 will be your idle air and main air bleeds.

I say go for it. You'll need a small drill set, and you should have some pin gauges wouldn't hurt and a tap set.

I'm down for you working this through!!!!
 
DrCharles said:
Interesting. Why would the PVCR have any effect at idle, since that circuit is not drawing fuel (through the boosters)?

My idle vac is 7-8" but cruise is around 15. So I'll start with an 8.5 PV for now.
Click to expand...
It only adds fuel when the boosters are following. If it adds at idle it is blown.
 
Good catch. Sorry, I'm calling them the wrong name. The ones I changed are actually the Idle Feed Restriction of IFR. Here's a picture.


IFR jets.jpg
 
Before drilling, are you measuring what is in there now? :poke:

A reference for the locations on a 4150/60
Holley Carb Passages Identification Pictures

Map out the entire carb.
Holley Carb Sheet
not sure if that speadsheet is still downloadable. If not, find some posts from "GNTKllr" and cut and paste his maps for a start.

Tapping the air bleeds is a pain with the choke tower in place. One reason I suggested experimenting with wires first.
Machining main body for screw in air bleeds

making set screws into restrictors
drilling restrictors

Alternatively, the air bleeds can be removed and an aluminum, brass or slteel restrictor can be pressed in. I've done that on carbs with the choke tower

A demonstration of why PV enrichment should not effect idle mixture. This is the way it should be when all the restrictions etc are in good relation with each other.
Effect of an open or closed PV on idle

An explanation of why PV opening can cause a change in idle mixture is in Shrinker's posts here
Power Valve Question

Your older carb probbaly has the IFRs in the lower position.,
If so, keep them there. Although YR I think I thiunk has done better with them high. But he also is further along in tinking with carbs.... I would keep it low in your case so you are making the least changes
Relocating the Holley Idle Jet

Lots of good stuff in here. Racingfuelsystems - Stickies
You can join, but you do not have to. We all HATE the Tapatalk takeover of the hosting service. If you don't want to join, just close the box[X] and continue browsing like normal.
 
Nearly worthless eyeball estimate of the primary IABs in the photo, they look to be on the larger side, maybe .074 to .078.
If so, then the primary IFRs ought to be at .033 or larger.
I assume you have pin gages or drill bits with shanks that aren't buggered up. Use them to measure.
 
Yes, I know to measure what's there before I start drilling :poke:
I do have appropriately sized drills to use as pin gauges.

This afternoon I'll be putting the distributor back in (probably not for the last time!) so I need the carb to run the engine :D
 
Mattax said:
Before drilling, are you measuring what is in there now? :poke:

A reference for the locations on a 4150/60
Holley Carb Passages Identification Pictures

Map out the entire carb.
Holley Carb Sheet
not sure if that speadsheet is still downloadable. If not, find some posts from "GNTKllr" and cut and paste his maps for a start.

Tapping the air bleeds is a pain with the choke tower in place. One reason I suggested experimenting with wires first.
Machining main body for screw in air bleeds

making set screws into restrictors
drilling restrictors

Alternatively, the air bleeds can be removed and an aluminum, brass or slteel restrictor can be pressed in. I've done that on carbs with the choke tower

A demonstration of why PV enrichment should not effect idle mixture. This is the way it should be when all the restrictions etc are in good relation with each other.
Effect of an open or closed PV on idle

An explanation of why PV opening can cause a change in idle mixture is in Shrinker's posts here
Power Valve Question

Your older carb probbaly has the IFRs in the lower position.,
If so, keep them there. Although YR I think I thiunk has done better with them high. But he also is further along in tinking with carbs.... I would keep it low in your case so you are making the least changes
Relocating the Holley Idle Jet

Lots of good stuff in here. Racingfuelsystems - Stickies
You can join, but you do not have to. We all HATE the Tapatalk takeover of the hosting service. If you don't want to join, just close the box[X] and continue browsing like normal.
Click to expand...



The link for your Holley carb mapping on the RFS forum says it's dead.

Do you have another link to it???

TIA
YR
 
yellow rose said:
Which block do you have??? The holley or the billet 5 emulsion block?
Click to expand...

I have the Promax billet block on my center carb, and was getting wide swings in idle A?F ratios. The IFRs were located up above the fuel level on that metering block.

I read that moving them to the bottom of the passage below the fuel level would stabilize that. A fact that was discovered in WW1 airplane carburetor research. It worked, and my idle fuel level stabilized. I also went with a .035 restriction in place of the .033.
 
yellow rose said:
The link for your Holley carb mapping on the RFS forum says it's dead.
Click to expand...
For the xls sheet? No. Tapatalk must have lost it as they did many attachments when they took over.
"Admin" is Mark W. If your on any forum with him, maybe PM him for a copy.
I actually have a copy of the entire backup for RFS. Problem is none of us have any real knowledge of the backend of php, and what we need to do to unpack it on a new host server. Of course we have no assuranec that all the attachments got properly saved with their links until we can unpack the backup. Sucks.
 
Thanks for the Tapatalk links. Lots of good reading there.
I don't think I'll have the guts to drill the passage for the 4-corner idle mod, either :rolleyes:
 
DrCharles said:
Thanks for the Tapatalk links. Lots of good reading there.
I don't think I'll have the guts to drill the passage for the 4-corner idle mod, either :rolleyes:
Click to expand...
The pics are gone but Bruce aka Booster just marked with a pen and eyeballed it. I'd was afraid of the bit walking especially when held on an angle, and also whether I was aiming at the down passage. Vizard wrote Brad Urban's shop had a jig to do it. So way backwhen I sent my 3310-3 to QF to have it converted (Urban had sold or closed his shop and before Innovate had a forum). Eventually I got it back, but they hadn't been able to convert because the outer wall isn't thick enough on -3 to do it the normal way. In the long run I was glad they hadn't been able to make it 4-corner idle. My combos aren't radical enough to make good use of it.
 
I do them all the time. Just take a little guts to get through the first time..

6-32 for IFR
8-32 for air bleeds

8404079E-9902-4846-B741-EB2505CCE7B6.jpeg

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99C4AF7D-F4FC-4D19-8295-6E162437CBA6.jpeg
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6EEDAB41-F301-4790-8B24-5B8817FB0E61.jpeg

013BB903-8F0F-49FF-BD49-7EAD8B2C9B0E.jpeg
4449B6A9-CC84-4F8D-8014-EA995D3AAC94.jpeg
 
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DrCharles, you are correct, Pv circuit has nothing to do with idle vacuum (or affecting the idle). I tune mine to come in at a vacuum where I need the enrichment. Say like going from cruise to passing. Engine in my roadrunner idles at 6.5”, I run a 10.5 in it. There is always somebody that has zero understanding of the circuits in a carb that will argue this.
 
Actually there are some limited circumstances under which the PV can affect the idle mixture somewhat (Mattax also provided the racingfuelsystems link). But normally it does not, unless damaged (blown) :)
 
DrCharles said:
Actually there are some limited circumstances under which the PV can affect the idle mixture somewhat (Mattax also provided the racingfuelsystems link). But normally it does not, unless damaged (blown) :)
Click to expand...

Care to explain? Unless the boosters are active at idle it cannot affect the idle. (Providing it’s not blown or leaking)

but I’m all ears
 
That's all straight drilling and tapping.
The one that is risky for those of us with less skill is plugging the idle weeps and drilling the idle ports.

upload_2020-2-16_14-13-28.png


upload_2020-2-16_14-14-56.png
 
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