E-brock 1902 modifications

[CJK440]

New guy here. I have a 74 Camaro that I built a nice 383 for. I built it stock appearing and thinking that a later built E-brock 1902 would be similar in appearance to the original carb but likely would have less miles than an OEM unit, I picked up 3 of them in decent appearing condition.

I rebuilt one without changing much of anything as a starting point. I got a copy of the book and followed a couple of things. I made my own high flow seat, I drilled the idle screw holes to .092 and installed a different accelerator pump that uses the garter spring (but retained the other stock springs). Choke was removed for now. This carb has the baseplate adjustable APT and when I cleaned it I eyeballed the adjustment back to where I thought it was (although as I understand it, the APT should not affect my idle issue that I will explain below).

My engine is a 9.6:1 383. It has 200cc Iron Eagle heads, a ZZ4 dual plane and factory iron exhaust manifolds. I had a custom hyd roller cam made up with 272/276 advertised duration 224/228 @50. LSA is 110.

I got the fresh build running with the aforementioned 1902. It was timed to 12 initial with 35 total, the vac advance and PCV were plugged for now. At 800 rpm I'm seeing 12 in/hg vacuum.

The problem I am facing is that fully warmed up, at 12* advance and 800 rpm I am lean as heck. 19:1 on my wideband and lightly hand choking the carb confirms lean condition. I have the idle screws backed out a full 7 turns (which makes them wiggly) without any significant change. Whats the typical range for idle adjustment screws on a properly configured Q-jet?

Figuring that the 1902 is likely a stock replacement and not tuned like a 1910 performer I am thinking, the current calibration is just not up to the task for the combo.

Although I understand the dangers of making too many changes at once, what I was thinking about doing next, is putting the one on the car off to the side and rebuild another one but follow one of the high performance recipes in the book. I was thinking "recipe 2".

Am I headed in the right direction?

[Mudsport96]

What are your idle air bleeds at, and your down channel restrictors? I had to open them up quite a bit on my 1901 Edelbrock.
They are more designed for the trucks of the mid-70s so the cam with less than 112 to 115 degree lobe separation doesn't give a great vac signal at idle.
I have the comp 268he cam in my 350 and pull around 13 to 14 inches at 800 rpm, so I would imagine a 383 should do at least that even with a slightly larger cam like yours.
Also, I would see what bumping timing to around 14 or so would do for vacuum

[CJK440]

One the first carb everything was stock, not sure what they measured. I'm just going to put that aside or perhaps part with it.

I ended up going for it and built a 2nd 1902 following the recipe #2 in the book this AM. What a difference. The mixture screws do something now at about 3 turns out. Warmed up I'm at 14-14.5 AFR at idle. With vacuum advance disconnected I get a tiny bit of vacuum at the distributor port, when I hook vacuum to the diverter/manifold vacuum port, base + vacuum advance is 32 degrees and I can back the curb idle screw out all the way and its at 900 rpm. My bypass orifices were larger than what the recipe called for already, maybe i need to restrict them if I want a lower idle.

I can't drive the car yet but happy in how it runs now with the recipe in the book. Hopefully that carries over to the other circuits once I get behind the wheel.

This carb has the large main air bleeds (.120) in the main body. I compensated as much as I could per the book with 77 jets and 45 rods. I ended up still drilling the main air bleeds in the air horn to .070. Not sure if that was the intent. I suppose I could plug the .120 holes and redrill to the smaller size.

Looking forward to see how it goes on the street.

[Cliff Ruggles]

No need for 32 degrees timing at idle speed, if I read that correctly.  You are only compensating for not enough idle fuel to the mixture screws (idle system too lean).

If you are working with an Edelbrock divorced choke model with 4 MAB's it will need restrictors driven in both places and a different jet/rod combination plus correct PP spring.

I can put everything together in a custom kit for you to take the guesswork out of the equation.

That cam will need much larger idle tubes than stock, and a pretty decent amount of bypass air to be happy.

Use the vacuum advance but hook it up to the ported vacuum source.

Also use the PCV system to purge the crankcase.....Cliff

[CJK440]

I've been reading up on vac advance ported or manifold with interesting points about both, but regardless, I originally got it idling nice with ported vac to the dizzy and when I switched it to manifold all I really had to do was unscrew the idle stop. The mixture didn't change much.

I drilled my idle tubes to .037 and down channels to .054. idle air bypass holes measure .102. I am running a low vacuum PCV Fram FV181 which doesn't rattle at idle.

I will contact you about a custom kit but regarding the large main air bleeds can you clarify the passage in the book regarding the subject? The recommendations were to compensate to jet up to 76/77 and an 026 tip metering rod .030 smaller than the jet, which is what I did. Is this not as good as restricting the large bleeds and sticking to the letter of the recipe? You mention restricting both locations, did I make a mistake by drilling the upper main bleeds or do you mean both .120 bleeds?

[Cliff Ruggles]

For sure there is some very good reading on manifold vs ported vacuum to the advance. 

There are also many flaws in most of those articles or comments from folks regurgitating that information...FWIW.

Anyhow, with a very well thought out engine combo where the CID, compression ratio, quench distance, combustion chamber efficiency and camshaft chosen compliments the combination the engine will need LESS timing everyplace to be happy, especially at idle speed.

Some engines actually buck and kick in protest when you try to add a lot of timing at idle, they refuse to idle down, or just don't like it.  Others can go either way, some don't like or want much at all, and folks who do a piss-poor job of choosing parts, cam too big, LSA too tight, not enough compression, etc, usually find themselves having to run the base timing WAY off the balancer and adding a TON of idle fuel and bypass air to make things happy........Cliff