Edelbrock Carb Help - Idle Mixture Screws

Looked over some of other threads, but have not found any answers I am looking for.

I have an Edelbrock 600 cfm carb with electric choke (#1406). It is sitting on SBC, 9.5.1 compression with a comp cam (480 lift and 280 duration). When idling, the mixture screws have absolutely no effect. I have about 8'' of vaccum and the truck idles at 700 rpm (roughly). I do have the calibration kit for the carb.

I started about 1.5 turns out and after setting the initial rpm's I went to adjust the mixture screms. You can turn them all the way in and they do nothing. What am I missing here or what could be causing this?

 

Sounds like a vacuum leak, I had a similar issue with a Carter AFB (basically an edelbrock as they bought out carter). The mixture screws are just being overrun by the leak. What kind of intake manifold do you have? My issue was caused by trying to run my Carter on an open plenum intake without a spacer. While their are some differences, I believe both Carter's and Edelbrock's need to be run either with a divided plenum or a 4 hole style carb spacer, there are ports on the bottom of the carb that have to be covered or they cause a large vacuum leak.

Even if the manifold and carb are matched properly I'd still be looking for a vacuum leak with those symptoms.

 

At that vacuum your metering rod pistons are probably not seating, causing you to be overly rich at idle. That set up should make more vacuum than that though. Is the carb new?

 

Ignition malfunction will cause a rich mixture, but it sounds like a carb problem if there is absolutel no change in screwing them in.

go through the basic stuff, timing, points, testing wires and cap. Then if no change, try spraying carb cleaner in the mixture screw holes and blow them out with air. ...or take it apart.

I have had floats set WAY too high in those, and their specs are WRONG.

I have had one with a pinhole in one float, causing it to be heavy with gas and cause overloading and running black.

I am going back to Qjets myself

 

landseaandair almost hit the target! the problem you have is the cam is causing those metering rods not to seat properly.Edelbrock/Carter carbs have metering rod springs in different tensions to compensate for various cam durations which will affect vacuum,check your local Edelbrock dealer and get the two lightest available so you can try both.If you can't find them locally give these guys a call,they can give you good advice and sell you the parts(good prices too) Allstate Carburetor (631) 234-8327

 

That cam will be a little choppy at idle, but it should make more than 8". Ignition timing or a mechanical issue may be something to consider.

 

The carb is new out of the box. Has the yellow #4 step up spring in it. I do have the calibration kit with all the different springs. I am not familiar with how the spring numbers work. Should I be going up in numbers to like the pink #5 or down to the blue #3.

 

The igniton system is a Mallory Unilte with a 6AL. Other than the idle mixture screws, the truck runs great.

Performer EPS intake and no vac leaks to be found.

 

Blue should be the lightest, but I'd probably go with orange at 8". Also, is the cam in straight up or advanced/retarded at all?

 

Removed the idle mixture screws and sprayed some carb cleaner in the holes then blew it out with some air. Shot a little fuel out the top when I did this. re-Installed the screws and the #3 Blue springs. I am running 10 degrees advance on the timing. No change unfortunately.

 

GET the Fuel Regulated down to 4 1/2 psi!!!! It is severely rich due to high fuel pressure.Then return it back to 1 1/2 turns out and tune from there. Should have alot more vacume than 8" though. I had the same cam w/ 14" at idle @ 850 rpm
You need a Vacume gauge and a fuel pressure gauge at hand...
I have a carb tuning DVD sitting here from edelbrock. You can have it if you want.

 

Are the boosters dribbling at idle? If so, then it's probably from dirty needle and seat, incorrect float level or too high fuel pressure.

 

I have the fuel pressure at 5 and just read another post about keeping the edelbrock carbs around 4. I'll try this too. I'll check the boosters and floats next. Thanks for your help.

 

I would think that 10 degrees is not much initial timing for that engine. I would probably be at 16-18 initial with 36-38 full in. Any other thoughts on that guys??

 

thats what I run mine at..

 

Spoons, Which numbers??

 

I guess it depends on whether he has vacuum advance and what his mechanical advance limit is. If he does go by total, he'll need to get a dial light or a timing tape and make sure his advance springs are light enough to allow it.

 

If it is really a 280 duration cam (at .050), take a look at this link for some timing recommendations...

http://www.barrygrant.com/demon/default.aspx?page=5

 

I am only running about 30 total right now. I have mechanical advance on mallory Unilite and 6AL.

It's a 400 small block with 350 crank. Cast heads with 202/160 if it makes any difference

 

I run a slightly larger profile cam (560 lift) with a 358 (.040 over 350) with Dart Iron Eagle heads and a Mallory Unilite Mechanical Advance and a 6AL. Somewhat similar combo. I do run two 500 Edelbrocks on a cross ram. I have been running 20 initial and 40 total for the last 5 years with no problems. The extra fuel and larger cam requires that on mine.

I think 16-18 initial with 36-38 full in is pretty close to ideal for you to fire off the fuel with that duration, though I suspect that the 280 duration is not as measured at .050. It sounds more like advertised duration.

I know we are kind of getting off the subject of the carb situation a little, but I think the timing will need to be closer before you dial in the carbs.

Definitely interested in others opinions.

 

Another thought not mentioned.
Are the throttle plate shut at idle? Many times in the process off making a new engine carb idle with a cam etc the throttle gets adjusted up to make the engine idle. The throttle plates work a transfer slot and the plate ends up above the transfer slot ( not below) abd the idle screws have no effect.
If the metering rods are up you should be able to see fuel dripping in the venturis at idle. A cam usually causes the vac to drop allowing them to spring load upward causing the drip etc. Like the others said that requires a lighter spring.

 

36-38 also depends on his ignition curve on the dizzy...
But we're getting off track

 

I'm assuming you're talking about advertised duration when you noted the cam was 280*.

A cam this size, in a well sealed engine - good rings, valves, no vcuum leaks etc. should idle 10"-11" at 600 rpm.

If it's idling at 8" and steady it may be be off one tooth on the cam timing.

If you spin the engine rpm up to say 1500-1800 do the vacuum levels settle in steady at 17"-19"?
Assuming again, you're not too far above sea level.
Vacuum drops off about 1" per one thousand foot of altitude gain.

If the engine doesn't go to the 17"-19" level at a higher rpm that's another indication the cam timing is off.


As an example, my pal swapped for a nice 302 SBF powered 46 Ford sedan a while back.
It's a nice car, but came with a few problems.
Chief among them was low vacuum at idle and higher rpm.

Seems like the idle vacuum was around 8" and would go no higher than 10" when revved up a bit.
The cam, about an advertised duration of 270* was found to be off time.

Correcting that took it to 15" vacuum at idle and a touch more at higher rpm which is about right for his 3800' altitude.


Fwiw - you can get an assortment of five pairs of metering rod springs at Summitt's for about $7. - $8. plus shipping.
Try the pink springs once the cam - if that's the problem - is squared away.

Store any used springs in a marked baggy or pill bottle.
Once they're run the fuel washes the paint color off and you can no longer ID them.

10* BTDC is an ok initial timing figure, but 16*-18* strikes me as a bit much for a street runner.
You'll probably get some resistance to turning over with the starter with that much initial.


Check fuel pressure as well. 4 1/2# - 5# is sufficient.

Take a look at the carb manual on the Edelbrock site and tune accordingly.

When you're shooting the timing with a light, be sure to disconnect the vacuum advance line and plug it - golf tee's work great for this.

 

Some good info C9 and Bubba.

 

I bet you have that carb sitting on a edelbrock performer type intake? If so do you have the thin plate that seals off the secondaries? That intake is for a quadrajet and the secondaries will overlap the secondaries of the edelbrock and cause an internal vacuum leak. Edelbrock sells plate #2732 that seals it up.

 

Thanks for all the info. Here's where I am with it.

The cam is 280 advertised

At idle (Rooughly 700 - 800 rpm) I am getting a choppy 8'' to 9'' reading. When idled up it smooths out and tops 18''.

Fuel pressure is at 4lbs

I unhooked the throttle linkage and idle screw and turned it all the way out to make sure everything was closing up.

Motor is brand new and so are the heads. I have no concerns with quality of work. It is top notch. No vacuum leaks to be found.

I fired it up this morning and ran it to operating temp. I checked the step up springs and found that the rods were sucked all the way down. No movement. I tried three other springs (blue, orange, pink) and found the same thing. I ran the silver spring (8"") last. It was the only one that did not stay sucked all the way down and lifted slightly up and down (erratic & choppy).

No vacuum advance to unhook when timing, got a mechanical one. No vac lines hooked to the carb.

 

if this is idleing at 7-800 with the throttle closed all the way, the idle screw backed out so as not to move the linkage, you have a vac leak. At 10* advance and no vac dist, it should stall.

Do you have a PCV valve? it could be not sealing with that low of a vac signal and bleeding too much air into the intake.

You want the metering rod seated at idle, not floating with the vac signal.

 

Definitly advance the timing to what RT says, This is really important and his numbers are righ.t

Advancing the timing will bring the vac # up.

 

It is an Edelbrock performer EPS with the Performer 600 cfm carb. Will this set up require the plate you are mentioning? Thanks

No pvc valve. I was suprised to see it still running which I believe you are right that there is a leak somewhere. I have not found any on the intake or carb flange base. Could it be internal as mentioned above. Thanks

 

I advanced the timing with a total of 36 in total advance. The vacuum at idle has dropped to 6'' to 7'' now. Running the motor up it still gets around 18'' total vacuum.