Miracle Carb

Rob Bromfield

GT40s Supporter
I have a Holley carb with 1850S stamped on the airhorn which makes it a model 4160.

However,sometime in it's life it's been converted to a model 4150 presumably by using the Holley 34-6 kit. I say presumably because the secondary metering plate has been swapped for what looks like a metering block and this new metering block doesn't have any idle screws so it's not some attempt to get four corner idle adjustment.

Because I'm an engineer I'm curious (or pehaps because I'm curious I'm an engineer) so I thought I'd try adjusting the two idle screws to get the maximum manifold vacuum at idle.
To my surprise I was able to screw both idle screws gently on to their seats without the engine missing a beat!

Aha! I though the powervalves leaking. So I took the primary side apart (someone has used the blue reusable gasket so this side has been taken apart several times in the past), extracted the power valve and used the not very scientific test of sucking on it.

There's no tear in the diaphragm as the valve actuates.

I'm assuming that there's no power valve in the secondary side as I haven't taken it apart (yet).

So out of curiosity, how does the engine continue to run with the idle jets screwed in?
 

Randy V

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The 1850 and 6619 carbs of the day (mid 70's) were being shipped with different metering blocks in the primary side which were emissions oriented. You could not shut off all the fuel from the idle circuits, but you could shut off most of it. Some of those metering blocks actually did not control the flow of fuel, but air. I think by 1980, Holley gave up on the emissions metering blocks. I have a few stashed away somewhere out in the abyss (otherwise known as my shop) along with a note that says something like DO NOT USE taped to them.
That said, there are a number of other fuel paths and a leaking power valve is a primary culprit many times. The rest is in the metering block gaskets being used. Holley went for many years with various metering block configurations and they used the gaskets for active channels to divert vacuum here and fuel there - so what I am saying is - make sure you have the RIGHT gasket for the metering block. You said it was converted to a 4150. Is the secondary metering block a true secondary block or has someone fitted a primary block to it?
 
Perhaps a quick visual inspection of the idle mixture needles and seats are in order. If they are grooved they will allow fuel to leak by them.
 
You don't mention large amounts of black smoke so we can probably rule out a power valve leak, but you might still have an incorrect rated power valve. All holleys have an idle circuit on both primary & secondary barrels, the idle mixture screw should be considered additional to these, also if a prior owner has used the main body from an application designed for a big block or large capacity engine in a previous life the air bleeds just inside the air horn at either primary or secondary ends may be to large ( or possibly too small for your particular engine).
Now as Randy mentions there are the metering blocks from the emission carbs- these bleed only air and there is a further restriction elsewhere in the carb to make this system operate as designed..sooo if you happen to have a combo of parts from any of these carbs you might just end up understanding a lot more than you ever intended about Holley carbs.
You could try a blast of carb cleaner in the air bleeds, or if your problem suggests that they may be to large try a piece of fuse wire bent into a hairpin shape & into the air bleeds to see if that helps
happy experimenting..
 

Rob Bromfield

GT40s Supporter
Thanks for the info guys. Here’s a few more details:

The engine is a 351 Cleveland and comes from a 1974 Australian Ford Landau.
I’ve replaced the open combustion head with one of the quenched heads. The engine’s been rebuilt and has a Competition Cam 280H cam

The secondary metering block doesn’t have any idle screws so I assume that a Holley kit was used for the conversion. The box of bits I was given with the car contains an old metering plate with “6” stamped on it and the remains of a packet that used to hold size 68 jets.

I’ve adjusted the float levels so that fuel just spills from the sight plug holes when the engine is shaken and adjusted the accelerator pump linkage to the recommended gap. The primary butterflies are fully closed when the accelerator pedal isn’t used and I’m getting 15 inches of Mercury at an idle speed of 750 rpm. I’ve settled on 750 as this is the rpm which, with all the auxiliaries on e.g. AC, is about 100 rpm above the rpm at which the engine wants to stall when fully warmed up. The power valve I inspected from the primary side was stamped with “65” so it will open at 6.5 inches of Mercury which seems to be in the ballpark.

There doesn’t seem to be a Holley expert in my neck of the woods, North Hampshire UK, so now that I’ve played with the rest of the car I thought I’d better learn how carbs actually work. It’s interesting to see how the theory you read about in the books varies from reality!
 
Rob, If you have closed the primary throttle blades completely that is the reason your not getting any response from the idle screw adjustment, attached pic on rh side of diagram shows what to aim for. should also set the rear/secondaries in the same manner but with only a small amount of transfer slot showing there, read up on the 'idle ezi' mod for holleys and obtain or make a system for your car- you can make a 'fixed' setup by installing a jet/orifice in the cruciform of the 4 barrels in the base plate and drilling holes that allow air above the throttle blades in the main body to reach that cruciform area. You then adjust /change the size of that jet to obtain a suitable idle speed without altering the throttle blade setting. Added a second pic of an old very tricked out holley to give you some food for thought. Be aware that the air sourced from above the venturi in that carb might also have been intended to enlarge the overall flow capability- otherwise known as rule bending!:)
Also many holleys were made without idle screws in the secondary block, 3310 variants as fitted to factory Boss 302 & 302 SBC being a couple.
 

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Rob Bromfield

GT40s Supporter
I’ve taken the Holley off the car and can now confirm that the Secondary metering block doesn’t have a power valve and looks like the pictures of the metering block supplied with the 34-6 kit.

I thought the primary throttle blades were fully closed but looking at them from the underside I can see that instead of the transfer slot being visible as a little square below the throttle blades it was visible as a definite slot. From what I understand this means that at idle the majority of the fuel will be coming from these slots. No wonder adjusting the idle screws all the way in didn’t have any effect!

Now I’ve set up the carb as Holley suggest with the primary throttle plates adjusted so that the transfer slot appears as a square and the secondary idle set so that the secondary throttle plates just moved and then gave it one more turn.

Tonight I’ll reinstall the carb and tomorrow I’ll fire her up.
 

Rob Bromfield

GT40s Supporter
She fired up first go and I spent about an hour and a half adjusting the timing, idle screws and curb idle screw until I got maximum inlet vacuum at 650 RPM.

I then took her for a drive...

Whoa! 0 to likely to lose licence in a couple of heartbeats!

What's the next step?
 
You can play with the cam that operates the accelerator pump and the size of the pump discharge nozzles, run the engine up thru the rev range with continuous light throttle increase to check for flat spots etc ( these will be wrong main jet size, secondary diaphragm spring rate or power valve-combos of those ). Be careful, licence loss territory. Check plug heat range and dist advance curve also, how successful will depend on you and observation skills, plus whether you have any test equipment to help diagnose... small steps and one thing at a time to avoid confusion.

Must have built around fifty motors similar in spec to yours for Ford Falcons here in NZ, they would run around 150mph in front of a C4 auto, give most big port 351c shakers a run for their money.
 
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Rob Bromfield

GT40s Supporter
There's a new rolling road set up just down the road but they only seem to know about cars with injection and ECUs.

I asked them if they knew a good carb man and they replied that they'd been looking for one for years and if I found one would I pass on his name!

What could I reasonably expect to get from a session on a rolling road?
 
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