Edelbrock carb folks, please read...

[Scott_Tucker]

Originally Posted By: morris
i have found VERY few people that can realy tune an engine.


After re-tuning hundreds of carbs that so-called experts tuned, I would totally agree with that.

I had a Dakota 3.9l, I wish I could have tuned it too. I've got the skills to reverse engineer the ECU now but I've moved onto other cars. Great engine though.

 

[morris]

one think about edelbrock carbs is that most people dont know. the mettering rods, steps are to much. i compered them to the rods in a chrysler O.E.M., that had smaller steps. after changing the edelbrock like the mopar rods. then it started running much better. i have on idea why the edelbrock rods are steeped so deep. it might be fine on a very hi performance engine, but not a mild engine

 

[Chris142]

Originally Posted By: morris
one think about edelbrock carbs is that most people dont know. the mettering rods, steps are to much. i compered them to the rods in a chrysler O.E.M., that had smaller steps. after changing the edelbrock like the mopar rods. then it started running much better. i have on idea why the edelbrock rods are steeped so deep. it might be fine on a very hi performance engine, but not a mild engine
After we couldn't get mine jetted correctly we resorted to hand sanding the rods.......still didn't work.

 

[Scott_Tucker]

Originally Posted By: morris
one think about edelbrock carbs is that most people dont know. the mettering rods, steps are to much. i compered them to the rods in a chrysler O.E.M., that had smaller steps. after changing the edelbrock like the mopar rods. then it started running much better. i have on idea why the edelbrock rods are steeped so deep. it might be fine on a very hi performance engine, but not a mild engine


Most carb 'tuners' don't have a good assortment of rods on hand to properly tune them and resort to sanding one's that are fat. It was once considered some sort of tuners secret but all I have seen is problems with doing it.

 

[morris]

to do jetting, ill try to keep it short. change a jet OR a rod only. you can NOT guess at the change if you change both. i did wright a computer program to change both. but iam not sure where i is. ill try to round it up. iam not sure iam smart enough to post it. we"ll see.

 

[Scott_Tucker]

I'd love to see that program.

 

[mechtech2]

Edelbrock has already tuned the mixture circuits .
We should be able to hammer one on , and just set the idle.

 

[Scott_Tucker]

Originally Posted By: mechtech2
Edelbrock has already tuned the mixture circuits .
We should be able to hammer one on , and just set the idle.


That would be nice and I have had that happen more often on AFB types than say Holleys, but no carb is one size fits all. However, the AFB types are very good at it and that is why I recommend them when somebody is looking for a good universal carburetor for an American V8 where drivability is more important than absolute power.

 

[morris]

TO:mechtech2 NO NO NO edelbrocks are NOT all ways tune to work out of the box. the OLD chrysler AFBs are 10 times the edelbrock could me. i would NEVER buy a edelbrock. i would search the world for a chrysler AFB first.

 

[SkyGod]

Carb screws are idle adjustment only...adjust screws for maximum engine vacuum at the correct idle rpm with vacuum advance plugged so not to advance the timing.
These screws control the transfer from the idle circuit to the power circuit...may or may not fix the usual quadrajet bog.
Everyone do their self a favor and buy a Holley...The Famous Smokey Yunick and friends were responsible for the time tested design.

 

[Scott_Tucker]

Something I just remembered: Some of the older AFB's had an air bypass screw to adjust idle speed rather than a traditional throttle stop screw. Basically there was a knob on the back of the carb that opened up passages in the carb to raise/lower idle speed. They are a little more tricky to get the idle speed adjusted because everytime you adjust it, you also have to adjust mixture. This is not always true on the throttle stop screw type.

 

[morris]

yes scott i know what you are saying. i though it was in the front, but no matter. i can remember them on Pontiacs. and you are right they were VERY hard to tune. on one i drilled a hole and put a screw to open the plates, it helped a lot. i send you a pm, if i did it wrong let me know.

 

[mechtech2]

Modern carbs are tuned for mixture at all air flows, when they leave the factory.
But manifolding can make a cyl lean or rich. We have to get the lean one up to the others.

I wish someone would invent a system whereby the mixture was feedback controlled, and the carburetor was eliminated n favor of a little squirter for each cylinder.

 

[440Magnum]

Originally Posted By: sasilverbullet

So bottom line is, should I leave it unplugged or plug it?


This is going to sound rude, but... whoever said that's a defect or should ever be plugged is an IDIOT. That is an intentional air bleed that serves two purposes. First, it provides air flow through the "leaky piston" type choke pull-off contained inside the phenolic choke housing. Secondly, it provides engine-bay temperature air to keep the choke turned off, or turn the choke off quicker, once the engine gets warm.

As for where the air goes, it dumps in between the two primary jets, aimed at the idle ports and transfer slots, so the incoming air mixes very well with the delivered fuel. Its just like the PCV port- everything else is built assuming that flow is present and the mixture screws have enough range to compensate for it. It just allows the throttle plates to be closed a little further. That actually improves idle quality since instead of some of the air going around the back side of the plate and never getting close to where the fuel is coming in over on the other side by the transfer slots, more of the air for idle goes right past the idle ports.

Older OEM electric-choke Carter carbs routed a hose from that fitting up to the air horn at the base of the air cleaner so that filtered air would go through the choke. As the aftermarket Carter and Edelbrocks got more generic, they just left the hole open. Since I don't like the idea of dirty air going into my brand new carb, I cut a plastic vacuum tube fitting so that its a press-fit in that hole, and then routed the line to where I could tie it down and put a paper fuel filter (one of the little cheap plastic ones) in-line as an air filter to make sure the insides of the carb stays clean.

 

[440Magnum]

Originally Posted By: Scott_Tucker
I'd love to see that program.


This thread really hits me where I used to live. Its only been in the last 5 years that I've started seriously (and not all that seriously...) tinkering with EFI. From ~74 to ~2007, I just played with carbs. Most of them Carters. I love Thermoquads... do you think I'm crazy yet?

As for tuning... when I was setting up my last couple of engine builds and tuning the carbs, I just made an Excel spreadsheet with the dimensions of all the jets in my strip-kit along one axis, rods (with an entry for each step on the rod) on the other. Each cell computes the area of the hole that remains with the rod (rich or lean step) poking through the corresponding jet. Print out the sheet, take it to the garage...

To make a change, you see what is in the carb, and if you need to (for example) go richer on the power step but stay the same on the cruise step, you can quickly find the new rod/jet pair that increases the "rich step" hole area while changing the "lean step" hole area as little as possible.

Armed with a tool like that, tuning Carter/Edelbrock carbs becomes as easy as tuning a Holley.


Oh, and the other tunable parameter in Carter carbs which everyone seems to forget is the SPRING under each metering rod piston. The stiffer the spring, the quicker the carb enriches when you step on the throttle. Weak springs tend to give you a nasty flat-spot coming off idle, or even demanding power suddenly when just loafing along. A good strip kit will have a spring selection, and the factory errs toward too weak on the springs (in my opinion). I've also seen some strange shade-tree kludges too. I once bought a used AVS (in SUPER condition) at a swap meet. When I took it apart, I figured out why it was available so cheap. Someone had tried to speed the transition to the power step not by changing springs, but by shimming the existing springs with spare jets.

 

[440Magnum]

Originally Posted By: mechtech2
Edelbrock has already tuned the mixture circuits .
We should be able to hammer one on , and just set the idle.


Depends ENTIRELY on the cam you're using. Yes, its true that if the only variable is volume of air flowing smoothly through a carburetor, then you can tune it once and it should work perfectly on a small engine, a large engine, a low-revving big engine, a high-revving small engine, etc. But exhaust/intake overlap combined with intake runner length and other parameters does very weird and RPM-dependent things with reversion through the intake valves, and the way the carb responds to pulsed airflow is different than the way it responds to perfectly smooth airflow. That's why "big" cams have such a hard time developing manifold vacuum at idle- all the reversion at low net airflow, and the back-pressure pulses hitting the carb are not trivial.

 

[SteveSRT8]

Originally Posted By: mechtech2
Modern carbs are tuned for mixture at all air flows, when they leave the factory.
But manifolding can make a cyl lean or rich. We have to get the lean one up to the others.

I wish someone would invent a system whereby the mixture was feedback controlled, and the carburetor was eliminated n favor of a little squirter for each cylinder.


Made me laugh there... "modern" carbs? Think about it!

And a little squirter for each cylinder sounds a lot like port fuel injection!

 

[mechtech2]

Steve -

Yes, modern carbs are supplied by Holley, Edelbrock, and others for cars. Lots of HP and vintage stuff is around. And a carb/manifold is easy and cheap compared to upgrading to FI.

They are tuned quite well from the factory. [idle must be set]

 

[SteveSRT8]

Thank you for that pretty well known info. If I'm not mistaken I still have 4 carburetors in service in my stable right now!

But seriously, you missed the humor completely.

The word "modern" and the word "carburetor" are nearly comical when used together.

 

[mechtech2]

Now I think it is YOU that missed the humor!
Of course I was describing port FI!