kalli

Hi all,

can't drive because of ice outside :-( So I removed the carb to change a few things around.
The last time I was driving I had the effect that when partially accellerating (foot down just as far as the secondaries won't open. In the double pumper you can feel that).
it would lean out more and more the higher the revs went

So basically at cruise I'd hve ~14:1 and then it would climb slowly with every increase in rev to 16:1 where I stopped the testing.

I guess it could be the PV not opening. So I pulled the PV (which seems to be 4.5) and replaced with 7.5 (that worked well in my holley I had before). engine idle is around 15 vacuum.

Should that be it, or is there anything else that could cause it?

Just asking since I have the carb apart anyway ...

Kalli

Stepman

Sorry, I don't have your answer. I'm curious to see how this turns out though because I have a very similar issue.

tx65coupe

Subscribing.

67mustang302

Sounds more like an emulsion circuit issue. If the PV weren't opening it would just plain be lean. If the AFR starts out ok but leans out as revs climb without anything else changing it sounds more like a main circuit calibration problem. If the main circuit is properly calibrated it should keep the same AFR pretty much throughout the rpm range, even if it were wrong.

I'm assuming you were using an AFR gauge? I've heard that the non race Demons have issues leaning out. Have you changed air bleeds? What bleeds are on the high speed size right now? Are you sure the jetting is correct, and isn't clogged? What's the AFR at WOT(I understand if you don't know, I'd be hesitant to WOT on a carb that's leaning out myself). How many circuits does the emulsion have on the metering block?

kalli

Thanks for reply, always very much appreciated.
The carb came with a jetting of ~72 in the front and although there was no hesitation anywhere it was running pig rich everywhere. I liked the way it drove since there was no hesitation anywhere, but it was sooting up my sparkers and I wanted to get it where it should be. so i changed the front ones for something like 62 and it was running at a better A/F during cruise. I didn't change the rear ones yet. so at cruise (no matter if 40,50 or 60mph) the ratio is ok, but as soon as I partially accellerate (all without the secondaries it starts getting leaner and leaner). so i was hoping it was the fact the PV wouldn't kick in (still too much vacuum).
When I fully floor it (secondaries opening) it goes rich all thge way again, otherwise (and fully agreed) I wouldn't do that to her.

When I get at home I'll check on the carb to answer your questions in full.

btw: fuel pressure is constant 6psi and float levels were correctly set.

kalli

just to make sure we're on the same page. i didn't mean the higher the revs the leaner the a/f. rather when i accellerate (without using the secondaries), which already moves the car a lot it is getting leaner and leaner, the longer i do that.
WOT is fine again since I still have big jets in the secondary.
So during cruise (no matter what rpm) the a/f seems rather stable

as to your question I attached pictures as I wasn't too sure.
a) the main jets currently are 67 (i put in bigger ones as iu didn't like that lean situation). at the time of testing they were 62 i believe
b) i didn't change any bleeds (not sure what the PO did). carb is 2nd hand. but from what I can see there is actually nothing that can be changed (no adjustable idle feed restrictors, no adjustable high speed air bleeds, no adjustable PVRC)
c) as to the amount of emulsion circuits. I don't know. did you mean one idle circuit, one main circuit, or did you mean the amount of emulsion holes in the wells.
See picture ...

67mustang302

Well, firstly don't jet JUST ONE SIDE. Always jet the front AND rear up or down in equal amounts. The reason for this is so that the front and rear are balanced to provide the same AFR at WOT. Otherwise if the front is lean and the rear is rich, then the front cylinders will run lean and the rear cylinders will be rich(the cylinders on an engine closest to any given carburetor barrel, are fed primarily by that barrel). And your AFR reading with an O2 sensor will be inaccurate, since it's going to be reading an average of the AFR on one side. If it's pig rich at WOT, jet the front and rear down 1-2 sizes each(but the same amount).

And the emulsion circuit is where fuel and air pre-mix before it comes out of the booster. On your metering block if you look at the 2 vertical channels on either side of the power valve that go up into the "J" shaped channel at the top, you'll see 3 holes drilled in each channel at different heights. That's the emulsion circuit, the "J" channel at the top is where air comes in from the high speed air bleed and down into the vertical channel which is the main air bleed well. As the engine rpm/throttle increase and it draws more fuel out of the main well(also a vertical channel, but behind the main air bleed well inside the metering block) the fuel level in the main well drops, uncovering the holes that lead to the air bleed well(those 3 holes in each channel are cross drilled from the main air bleed well and into the main fuel well, which comprises the emulsion circuit).

That's there for several reasons. One is that by allowing some air to bleed into the fuel well, the airflow across the circuit and over the top of the fuel in the main fuel well helps to lower the surface tension of the fuel, allowing it to break up into smaller easier to lift droplets. That makes it easier to lift the fuel up out of the well and to the booster, which helps to create a more responsive main circuit, and it also creates a frothy emulsified fuel mix that's easier to vaporize for combustion. Also what it does is by putting multiple holes at different heights and allowing some air to bleed into the main fuel well and pre-mix, you can alter the fuel ratio AFTER the fuel has passed through the jet. The size of the main fuel and air well as well as the size and height of the cross drilled circuits effect at what rpm/throttle position the AFR will change. It's what allows the carburetor to maintain a consistent AFR across a wide range of rpm and throttle position. Also by having multiple circuits at differing heights it allows air into the main fuel well sooner(the fuel doesn't have to drain all the way to the bottom of the well almost like in a single circuit) which helps to lift the fuel more easily and rapidly(the reduced surface tension). This gives better throttle response and shift recovery(the re-establishing of fuel flow through the main circuit when you get back on the throttle and under load after a shift). It gives you a carburetor that can establish good fuel flow even with little booster signal such as at low rpm and throttle position.

Yours is a 3 circuit emulsion, which is pretty much standard these days among most high performance and quite a few race carburetors. Some use 4 or even 5 circuits, but those are very high dollar carbs for ultra fine tuning of the fuel curve. Most carbs won't see any benefit beyond 3 circuits, and a 3 circuit setup provides very good fuel emulsion and control, as well as excellent throttle response.

As far as your specific problem, try jetting it back up to factory, and then stepping both ends of the carb down at the same time. In reality the jet doesn't actually set the AFR, what it does is restrict fuel flow into the main fuel well, effecting how rapidly the fuel level is drawn down and uncovering the emulsion circuits. The entire setup working together is what determines how much fuel is lifted out of the main well, which is what determines AFR. It could have just been that the jet was small enough that the fuel level was drawing down over time and uncovering more emulsion than it needed at the time, and was leaning it out.

Stepman

Wow!....That was a beautiful explaination! I think I really understood that and it made a lot of sense. I'm going to have to reread that a couple of times and then go play with my carb.

kalli

lucky you, i have to re-read this for another 5 times ...

But hey, 67m302, Thanks a MILLION for that writeup!
the reason I started with the front was as follows (and after reading your writeup I'll scratch the idea):
basically i knew to keep the two of them balanced (ront and rear jets), as in if i jet the front down 3 the rear should do the same.
But I thought, why don't I do it in such a way that I do the front first. once the front is ok I'll jet the rear the equal amount down or up and i should be in the ballpark. I thought that would be a _lot_ easier than doing always both sides (i hate emptying the fuel bowls and see all the petrol go to waste and make a mess to change the jets).
But the info with the cylinders taking the fuel from whichever barrell is closest is quite an eye-opener. Might be running rich back and lean front ...
Anyway, i checked on my sparkers and the good news is they're all rich still.

So from all this I'll change my practice on carb tuning.
I have bought a long sturdy rubber hose I can attach a vacuum gauge to while driving. That will tell me what PV I need, that should be the easiest. For the jetting, I'll just go back to factory and start from scratch. Maybe just make bigger jumps 3 jetsizes instead of 1 for starters.
I'm not too worried, I'll get it right eventually.
For now I have to wait for the bloody ice to leave.

Thanks a million again for your writeup, which I'll have to re-read a few times i guess *g*

67mustang302

Some carbs can get nutty in terms of the calibration when you make huge jet changes, like more than 4-5 sizes, others you can make a 7 or 8 size change even and have the calibration remain consistent. Check your vacuum at idle, and pick a power valve that's at least 2" lower than your idle vacuum, 3-4" less puts you in a good ballpark, but every engine is a bit different, and you may find you need the power valve to come on sooner or later(depends on engine setup and the particular carb). Also, make sure the PV isn't leaking either from a torn diaphragm or from the gasket around it. Check the recess in the main body when you pull the metering block, it should be dry(there may be a tiny bit of fuel that gets in from when you pull the block, but if the PV is leaking into the intake, that recess in the main block will be pretty wet or at least have a fuel trail going to the hole where the vacuum signal comes from). If the PV is leaking then EVERYTHING will be off.

Also make sure your accelerator pump shot isn't too huge, if it is what can happen is as you accel you dump a ton of fuel in the intake and some will sit on the bottom of the plenum in the manifold, and as you continue down the road the intake will clean out that fuel and when it's gone you get lean. That happens when the jetting is too lean but it's masked initially by the extra fuel sitting in the intake.