Ok, one more question on the Holley jetting.

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TrailBeast said:
The transitions show near a perfect square
Click to expand...
Square was never a specification. It was always a ballpark description for a quick check and for guys who had no means to measure. You are absolutely free to give the idle position screw another 1/8 or quarter turn. Then reset the idle mix screws to trim the idle mix.
It's a free experment (other than burning fuel).

The other thing you could do with the carb off is measure the t-slot width and length to see if its relatively normal (old school) or extra long or wide. Being a Holley my hope is that its in the range established back when millions of carbs were going into new cars.

The range of exposed under the throttle for an old school carb was expected to be .020 to .040" see Mike Urich's Holley Carburetor's and Manifolds
 
Mattax said:
Square was never a specification. It was always a ballpark description for a quick check and for guys who had no means to measure. You are absolutely free to give the idle position screw another 1/8 or quarter turn. Then reset the idle mix screws to trim the idle mix.
It's a free experment (other than burning fuel).

The other thing you could do with the carb off is measure the t-slot width and length to see if its relatively normal (old school) or extra long or wide. Being a Holley my hope is that its in the range established back when millions of carbs were going into new cars.

The range of exposed under the throttle for an old school carb was expected to be .020 to .040" see Mike Urich's Holley Carburetor's and Manifolds
Click to expand...

That's exactly how I viewed it (as a general indication) so no specific measuring done.
I know it's more than .020 and not nearly .040 just by looking at it though.:D

I did raise the idle and re trim the mix screws leaner as a test.
It made a slight difference, but mostly just raised the idle RPM.
 
Have you tried smaller IFRs? Reduce IFR size, and back out the idle mixture screws to get your idle where you want it. That will lean the idle circuit.
 
I am having a similar problem to the OP (too rich off-idle) since I replaced my baseplate with a Quick Fuel.
It's simpler (once all the orifices have been converted to replaceable) to enlarge the IABs.
Wouldn't that be worth trying first?
 
DrCharles said:
I am having a similar problem to the OP (too rich off-idle) since I replaced my baseplate with a Quick Fuel.
It's simpler (once all the orifices have been converted to replaceable) to enlarge the IABs.
Wouldn't that be worth trying first?
Click to expand...


Me personally when it’s that far off I prefer to get the IFR where it needs to be or really close and fine tune with the IAB.

Others may have a better way to do it.
 
No problem setting the idle mixture where it likes (12.8-13) with the current IFR.
I have a .081 TSR too... could drop it to .075. Would that be an option too?
 
10.5 will come in pretty early depending on his cruise vacuum...
AJ, how big are your PMJs and PVCR?
 
mopowers said:
Have you tried smaller IFRs? Reduce IFR size, and back out the idle mixture screws to get your idle where you want it. That will lean the idle circuit.
Click to expand...

yellow rose said:
Me personally when it’s that far off I prefer to get the IFR where it needs to be or really close and fine tune with the IAB.

Others may have a better way to do it.
Click to expand...

This is the plan so far guys.
Just have to wait till it warms up a but again.
 
The IFR controls solid fuel. It's the main determinant
IAB is a correction on the IFR, and if not too much is added, the air injected lowers the density. If too much is put in it stalls the flow or the air bubbles coalesce and it produces inconsistant spurts of air and fuel bubbles.
The TSR is a variable air bleed acting on the lower density mix of air and fuel.
The TSR is a variable fuel restriction on the lower density mix of air and fuel.

When the pressure difference on the IFR is high enough, flow is choked. More pressure will not flow more fuel. This is part of why the idle system leans out (on a stock engine) as thottle opens from idle. On a hot rod with low vacuum at idle, its will increase fuel flow with increasing manifold vacuum before choking.

If the idle chokes fuel too early relative to the mains starting, then the IFR needs to be made bigger. and vica versa.
The same AFR can be achieved with different IAB/IFR combos, but the larger IFR will carry the idle system's contribution further into the mains starting.
 
Mattax said:
The TSR is a variable air bleed acting on the lower density mix of air and fuel.
The TSR is a variable fuel restriction on the lower density mix of air and fuel.
Click to expand...

You lost me here... it's both? If the idle is fine but the t-slot is too rich, won't making the TSR smaller also fix it?
 
Mattax said:
The IFR controls solid fuel. It's the main determinant
IAB is a correction on the IFR, and if not too much is added, the air injected lowers the density. If too much is put in it stalls the flow or the air bubbles coalesce and it produces inconsistant spurts of air and fuel bubbles.
The TSR is a variable air bleed acting on the lower density mix of air and fuel.
The TSR is a variable fuel restriction on the lower density mix of air and fuel.

When the pressure difference on the IFR is high enough, flow is choked. More pressure will not flow more fuel. This is part of why the idle system leans out (on a stock engine) as thottle opens from idle. On a hot rod with low vacuum at idle, its will increase fuel flow with increasing manifold vacuum before choking.

If the idle chokes fuel too early relative to the mains starting, then the IFR needs to be made bigger. and vica versa.
The same AFR can be achieved with different IAB/IFR combos, but the larger IFR will carry the idle system's contribution further into the mains starting.
Click to expand...

It'd be nice if I can figure a good way/place to make the IFR manually adjustable for only the transitions from the outside of the metering block with a needle similar to the idle mixture only smaller.:D
 
Might be possible with really long needle screws that come out the end of the fuel bowl (with a low-mounted IFR anyway). Think there would be clearance problems with the float, and you'd have to seal them to the bowl too...
 
DrCharles said:
Might be possible with really long needle screws that come out the end of the fuel bowl (with a low-mounted IFR anyway). Think there would be clearance problems with the float, and you'd have to seal them to the bowl too...
Click to expand...

Since I see people putting the IFR jets in the top of the metering block runner it might not be that complicated.
It would depend on if there is a straight shot from the top of the block into that same port.
I just don't feel like pulling it all apart again at the moment to investigate it.:D
Might see if one of the local guys has a metering block I can study.
 
Mattax said:
The IFR controls solid fuel. It's the main determinant
IAB is a correction on the IFR, and if not too much is added, the air injected lowers the density. If too much is put in it stalls the flow or the air bubbles coalesce and it produces inconsistant spurts of air and fuel bubbles.
The TSR is a variable air bleed acting on the lower density mix of air and fuel.
The TSR is a variable fuel restriction on the lower density mix of air and fuel.

When the pressure difference on the IFR is high enough, flow is choked. More pressure will not flow more fuel. This is part of why the idle system leans out (on a stock engine) as thottle opens from idle. On a hot rod with low vacuum at idle, its will increase fuel flow with increasing manifold vacuum before choking.

If the idle chokes fuel too early relative to the mains starting, then the IFR needs to be made bigger. and vica versa.
The same AFR can be achieved with different IAB/IFR combos, but the larger IFR will carry the idle system's contribution further into the mains starting.
Click to expand...


As always, Mattax explains it way better than I do.

Thank You Mattax.
 
TrailBeast said:
It'd be nice if I can figure a good way/place to make the IFR manually adjustable for only the transitions from the outside of the metering block with a needle similar to the idle mixture only smaller.:D
Click to expand...
Actually there are a few versions of a system where this can be done.
It's pretty cool.
BLP is the current manufacturer, and only on a limited basis. Give 'em a shout if you want one.
Something a little different
RapidJet blocks revisited
 
DrCharles said:
You lost me here... it's both? If the idle is fine but the t-slot is too rich, won't making the TSR smaller also fix it?
Click to expand...
Yup its both. That's another erason why TSR need is determined later in the tuning process. Of course I had to learn this the hard way myself.

The transfer slot is both a variable air bleed and fuel restriction.
I'll just snip quotes from Tuner here.
the full original is archived here Innovate Motorsp-rts Archive: How do you know what stage of the carb is working?

"The magazine articles tell us the slots obviously deliver more fuel as more of the slot is exposed to the vacuum at the edge of the butterfly and slot is exposed to the manifold vacuum below it. The real deal... is that the portion of slot above the butterfly is air bleed and that below the butterfly is fuel feed. As the throttle blade changes position up and down the slot, the ratio of air bleed to fuel feed changes. As the butterfly opens air bleed is being cut off and fuel feed is being increased. Usually (almost all Holley’s & clones), the slot and the curb idle adjustment are fed by the same idle jet and air bleed.

Ideally, the mixture is richest at the dead idle and then gradually leans as the throttle is opened."

Your engine may be radical enough that a t-slot restriction will be helpful. It should help if its too rich as it transitions into the start of the mains. Trailbeast's engine doesn't seem to be that radical. Rather its pulling hard on the fuel right off of idle.

The full explanation in the link covers the physics.
 
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It's interesting that Chrysler Tech says the same thing in 1966.
upload_2020-3-27_16-3-6.png

They're more subtle about it so it could be missed on first reading. And you may notice its general but Carter oriented (hence the mention of the balance tube.
Carburetor Fundamentals (Session 222) from the Master Technician's Service Conference
 
Your engine may be radical enough that a t-slot restriction will be helpful. It should help if its too rich as it transitions into the start of the mains.
Click to expand...

Today I made the IFR smaller (maybe). Didn't make much noticeable difference!
I don't have the precision equipment to drill .033 instead of .034 with sufficient accuracy. Will increase the IAB slightly. If still too rich, then I'm going to make the TSR smaller.

Trailbeast, any progress with yours?
 
You can put a strand of wire in the IFR and bend it over. The gasket will hold in there. When you get it right you can calculate the area and drill a permanent IFR.
 
True, but I'd be looking at a very thin strand of wire since I am not trying to make major changes now. And the calculation doesn't always agree with the actual flow once drilled, either!
After going to the trouble of removing the metering block, it's not hard just to drill a new set screw (I have lots of them) and thread it in and try it. When it's right i can just leave it :)
 
DrCharles said:
Today I made the IFR smaller (maybe). Didn't make much noticeable difference!
I don't have the precision equipment to drill .033 instead of .034 with sufficient accuracy. Will increase the IAB slightly. If still too rich, then I'm going to make the TSR smaller.

Trailbeast, any progress with yours?
Click to expand...

How do you hold the brass to drill it?
 
very carefully :p
I drilled and tapped a piece of 1" aluminum bar stock for 6-, 8- and 10-32.
Setscrew threaded into the bar, bar located on tabletop drill press, then drill very gently (fingertip pressure).
I'd like to have a mini lathe but I don't.
 
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