It's a 6361 metering block. I believe its in the idle well, maybe cast in or a pressed in restrictor. How hard to modify or replace ? thanks

The dimples in the slots on either side of the PV plug do not show the idle feeds drilled. Is that the secondary block?

Yeah, that's what is puzzling. It's the primary. Carb is 4668.

Are the 4 brass plugs at the top all “cups”, or are the inner two “flat”?

If they’re flat like in the pic I posted, those are likely “idle tubes”, like what are used in 3 circuit 4500 carbs.

If they’re cups, I’d just pull one out and see how the main well is configured.
Replacement plugs are readily available.

The “If it were me” answer is........ if I wanted to hit the easy button to do some experimenting, I’d just buy one of these and see what happened:
https://www.holley.com/products/fue...metering_blocks_and_plates/parts/134-155

I might have another one I could sacrifice if I want to open it up.

That's an early 70s emmisions carb.
Lot's of things were done different and very specific for the application.
If you have the whole setup, and are running close to stock, you can prob do some minor tweaks to increase the range of adjustment.
That stuff was all set up for fine adjustment of relatively lean idle (14.2:1) and high idle rpm.
Shop manual and theMTSCs cover most of those changes.

If you want it to be more like a normal pre-emissions carb. Take a look at what Mike did here.
https://www.tapatalk.com/groups/racingfuelsystems/list-4668-t1344.html

Take note of the booster notch and other staggered restrictions Mike found - that's specific to work with the intake manifold for best distribution.

Might be an option. Does this have replaceable idle jets ?
The metering block in question is off the 383 stocker engine

Thanks for that link !

If you want it to be more like a normal pre-emissions carb. Take a look at what Mike did here.
https://www.tapatalk.com/groups/racingfuelsystems/list-4668-t1344.html

Well well........ that is an odd duck.
It doesn’t look like the idle well is drilled at all....... at least, not from the top.

IMO, anything “oddly” done(compared to “normal” performance metering blocks), was to address something emissions related.

For what you’re doing........ I’d def be experimenting with a more traditional and user friendly metering block.

Edit:
There’s a lot of good info in that link Mattax posted.

However........ I don’t see how all those mods would put you further ahead than just using a more tradition block.

I see in the specs for the original block that the ifr in the idle tube was .022”.
That’s not going to work at all in a Stocker build.

I’d also be very careful examining both the primary and secondary idle feed circuits, since there was a mention in the link of the primary idle curcuit interacting with the secondary side.
I would insure those were isolated from each other.

It has to be done as a package with the main body. A lot of work.
When I was looking at the late 69-70 predecessors to the 4668, the transfer slot circuit was seperated from the idle circuit.
Transfer slot got its own air bleed in the main body and IIRC fixed restriction.
The idle circuit has an adjustable but hidden restriction after the air bleed. The air bleed in those cases was externally adjustable.
IMO its best to use that carb for its original use, with relatively minor mods if there's no reason/desire to keep the emissions characteristics. I'm not suggesting emissions control is always bad, but some of those early setups were tradeoffs for fuel economy and even sometimes NOx in favor less HC and CO. I wouldn't blame anyone who wanted to run a richer stronger idle at lower rpm and a more normal initial timing to go with it.

I don't have a '71 service manual, but that's probably the one with the details of this carb's secrets

Did you see there is also an older thread where Hank is working on a 4668
https://www.tapatalk.com/groups/racingfuelsystems/viewtopic.php?p=12160#p12160

Thanks for the info guys. Mattax, this is a stock eliminator race engine.

I've done a fair amount of working on this carb and it's functioning "OK" but could use quite a bit of fine tuning. As far as the IFR goes, the engine idles decent, but has wanted to die when put in gear. So I'm pretty sure it's lean. The idle rpm is pretty hign, 1300ish. I wonder if I could simply reduce IAB and have the idle mixture screws a little further out ?

The OP is running the carb on an NHRA stock eliminator car.
The factory low speed calibration is totally out to lunch for that application.

The rules dictate the correct carb has to be used, and that fuel metering mods are legal(but not the addition of 4 corner idle).

Like I said previously......if I were doing it, it would get a set of new std performance style metering blocks, and then I’d do the same types of mods to the main body/baseplate that were done in that link.

There is no need to try and work on “part throttle cruise” in this application.
It needs to start well, idle decent in gear, do a burnout okay, and have decent manners when staging........along with providing a good fuel curve at WOT.

I’m pretty sure the main issues the OP is having are related to how the motor runs when in other than WOT operation.
Which after seeing how they’re calibrated...... makes perfect sense.

After looking thru those two threads....... for even modest performance use...... it appears the low speed circuits on those carbs are a mess.

I need to better understand the circuit before suggesting anything permanent.
It looks like the IFR is in the bottom of the tube and from what you're saying the IAB's are in the normal location.
Are the idle mix screws in the standard location etc?
A relatively easy experiment would be to stick some wire in the IABs.

Are you using the Direct Connection 'Tach Drive' distributor?
Could be losing timing with rpm if idling at 1300 with any distributor, but the race distributors its a little worse.

I just took a quick look at a '71 Plymouth FSM.
A couple questions I have now is
1. how the hot idle compensator works. The text says this opens an air bleed, but does say how.
2. Is there a second air bleed for the transfer slot or not. Description says there is one but I'm not seeing it or a third circuit. Of course the text is covering 4 different 4160 carbs...


As far as the idle mix screw goes, take one out and see if it has a needle valve tip or looks like the ones Mike found.
My guess is that the flat tip worked to increase the effect of restriction within the 270 degrees it was originally limited to.
I'd take a note of how the flat runs, and also be very gentle when closing it to the bottom. It probably doesn't shut off flow completely like a needle tip would. Not that you need to shut off the flow, but when experimenting its common to run the screws in until engine slows.

The IAB and mixture screws are in the normal locations. not sure about the transfer bleed. timing is locked out at 37

The hot idle compensation is a curved piece of sheetmetal the bends as it get hot and opens an air bleed to dump below baseplate. I've plugged it.

unplug that hot idle compensation and let it suck air,jet up as needed and your car will go faster.
but fix your problem first or you will have more trouble

Thanks. So the compensator is not a bleed in the traditional sense (into the circuit) but more like a variable on the PCV or bypass air.

I think if there is a second transfer bleed it will be visible near the regular IAB's, and seperate path to the transfer slot. That would be visible on the body facing the metering block.
37* locked out is, IMO going to make it more difficult to get the idle producing more power. I'm sure that's very close to what is needed at WOT during the run.
But whether that much is really needed at idle I don't know. A richer fuel mixture at idle will be stronger but will also burn quicker.