Edelbrock 500cfm carb issue

[Mac]

I'm have a question about my Eddy 500 on the SBC 305. It has a condtion that I'm thinking could be internal vacuum leaking.

The symptoms are:
Idle stop screw backed all the way out and still 750-800 rpm
Stinky rich exhaust @ idle
I've had whistling from the carb but not recently since the idle is high
My "narrow" band A/F gauge reads good at cruise but pegged rich at idle

The Pro's are:
No linkage binding
Carb was kitted before install in spring of 2013
Plugs read nice and tan
Normal response on idle mix. screws
Vacuum runs steady 18-19 at idle (roller LT1 cam)
Engine runs pretty well - no bogs or surges
Spray test found no vac leaks on carb base, throttle shafts or mani
Holley fuel regulator set at 4.5 lbs
Floats set a tweak lower than spec

Would a warped carb top or blown gasket there  cause internal vac leak and these symptoms? I've read that whistling from the carb itself, not by vacuum leak at the manifold, carb base or otherwise is a sign of a warped  carb top.

TIA, Jack

[wayne petty]

see if you can bring the engine RPMs .. and them manually open the secondary linkage.. to verify that you don't have some debris trapped.. you might want to look with a flashlight and a mirror first..

i have a on the engine thing to try.. and there are things with the carb on the bench...

one the engine.. count the turns in to lightly seat the idle mixture screws on the front of the carb.   then take them all the way out..

use a spray nozzle and give the idle passage a shot of carb spray.. followed up by a quick blast of compressed air..  do each hole two or three times.. reinstall the idle mixture screws and reset to the former adjustment.

see if that cures the issues.  you may have to hold the throttle part way open the first time you try to start it after this. because of the extra spray that went into the engine..

lets look at the insides of an AFB/AVS carb idle circuit..

[wayne petty]

i should mention that there are a bunch of circuits in the carb..

idle..  

idle transition

primary mains..

secondary mains..

power enrichment.

there should be at least one more between B and C..




at idle.. all the fuel coming into the engine should be coming thru the idle feed passages..  in diagram A..

idle transition circuit starts feeding fuel thru the idle transition slot as you are opening the throttle exposing more and more vacuum to the idle transition slot.   this fuel is Limited by the idle feed restriction. but also the fuel curve response is controlled by the 2 idle air jets as shown above..

what i am having you clean out with the carb spray and the compressed air are these passages in the primary booster venturi's..  because they are air bleeds they are always pulling air in..

when you get deep into the idle transition slot.. the idle transition and the primary mains start flowing..  

when you get farther open.. you are just running on primary mains..

but low cruise is either idle and idle transition.. or idle transition and primary mains..

dirt in the air bleeds is a major issue..

i have always wanted to create a spare primary rod cover and install a tube on it.. so i could use a multi covered dip stick to watch when the primary metering rods come up.  so i know if the primary rod springs are too weak to overcome the slight reduction in vacuum as you open the primaries under load..

i hope this helps you.. feel free to ask questions..

and you can remove the primary and secondary boosters without taking the carb off. but its always a great idle to NOT have loose screws, gaskets and things dangling over the throttle bores..


what is not written down.. is that smaller engines may need to reduce the idle feed restriction sizing..   and that larger displacement engines may need to increase the idle feed restriction.

and if you are just upset  about all this.. and the 500 edelbrocks are a great carb..   the private label summit brand carbs that are really holley 4010 and 4011 models  that were remodeled from the autolite 4100 carbs.. but with the holley secondary  vacuum system.  why.. there are easily changed idle feed restrictions and easily changed accelerator pump discharge passages.
i don't think they have one in the 500 CFM range like your E version..

[wayne petty]

edit... OOPS..

i should have posted this before i typed everything in my brain out..

http://www.edelbrock.com/automotive/misc/tech-center/carb-faq.shtml


i am sorry.. i have a little more..

do you have Power brakes..  pinch off the hose for a quick test to verify the brake booster is NOT leaking vacuum ..

do you have a PCV valve..  with the engine warmed up.. if you have some kind of digital tachometer..  at idle..  blocking the PCV flow should drop the Idle RPMs between 50 and 70 if the PCV flow size calibration is correct..

if the RPMs drop more.. then you might need a smaller opening PCV valve..
if your RPMS drop LESS than 50. you need a larger opening PCV valve..

can you verify the fast idle screw that comes up and touches the fast idle cam is not engaging and holding the primary throttle open..  i know you checked it..  but i had to ask..  the choke should be wide open. if you are having problems.. loosen the three screws on the choke thermostat cap and loosen the adjustment so the choke does not close for right now..

disconnect the throttle linkage from the carb.. is the throttle linkage too sort and holding the throttle part way open.   this would not be the first time i have seen this..

if that is not the issue..    take the carb off.. turn it over and drain it of fuel..   look up thru the throttle blades from the bottom while holding it to the light .. are the throttle blades properly squared and sealing evenly all the way around..  

because of the throttle lever design.. i have found that carbs have been set down hard and the lever hit the surface and caused the throttle to open and the throttle blades edges hit the surface and drove them off center..  this prevents the airflow from being controlled properly..


if the primary throttle blades are not closing properly because they are off center or held open.. this will expose too much of the idle transition slots and it will run really rich at idle..

sorry for the landslide of info..







http://www.edelbrock.com/automotive/misc/tech-center/dl/carb-owners-manual.pdf

[Mac]

edit... OOPS..

sorry for the landslide of info..

Wayne, we can always count on you for good and lots of info. It's so much preferred to the random reply like "Ya I had that same problem once. It was a burnt valve."

Throttle linkage disconnected and choke wired open. Idle scew is backed completly off the cam. I belly surfed across the fender and peered down the primaries and the throttle blades look closed - down below the trans. slot.

I had seen your post a while back on the test for correct PCV valve, did it and it was fine.

I'll start with the non-invasive spray and air blast procedure, have a look at throttle blades in the bores for misalignment and go from there. I think the signs are pointing toward issues with the air bleeds.

I did look at the Holley 450 economizer as an alternative but parts availability is a problem and I have just about everything I need to upgrade to TBI FI waiting in the wings - the bigger goal.

Thanks, I'll post results.

[Mikej]

Have you checked the metering rods?
Stuck maybe.

[Mac]

Mike, I've had the metering rods out and swapped a few times while this rich idle problem has been constant from the beginning.

Wayne, no power brakes so none of those issues. I tried the spray and blow treatment with no luck. I'm digging deeper.

[wayne petty]

take the top off.. either on the car or off the car..

remove the primary boosters...

use some 0.023 mig wire or a bristle out of a wire brush to verify the passages are clean..  there can be so much crud in the 2 idle air bleeds and the idle transition restriction..

if you turn the carb  over... check to see where the throttle plates cover the idle transition slots.

and.. by the way..  do you have a 1/4" needle valve.. hook it up as a vacuum leak.. start with a lot of flow and back the idle speed down.. or is your idle speed screw already backed out.. if its already backed out. pull the carb and check the centering of the blades..




by the way.. what is the carb number you have.. its stamped into the front mounting foot of the carb.   1234s.   does your carb have a big screw between the idle mixture screws..

[wayne petty]

perhaps this will help...


http://www.championparts.com/carb_troubleshooting_guide.pdf

[Mikej]

If you turn the idle adjustment screws in will it kill the motor?

[Mac]

I didn't think so at first, after the carb cleaner spray and air blast treatment, but after driving the truck some I found that it's had some effect. The idle is back down where it's controlled by the stop screw, but the idle is still rich according to my nose and A/F gauge. So it looks like I'm digging around the right area but I have to put the job on hold while other duties call.
It's very drivable and yes the idle screws can be turned in to the point where it starts to run rough and a vacuum gauge shows it falling off. But the screws are only about 1 turn out from bottomed at the best vac reading.

Thanks Everybody

[Mac]

Wayne, the carb # is 1404 and it has no "big screw" between the idle screws.

I'm still fussing with it and now the idle speed screw can be backed off to the point where it will stall the engine. But now the idle mix screws have no effect, they can be turned all the way in with out stalling even when the idle is 600-700.

I've been reading about these carbs needing an adapter (squarebore to spreadbore  Edelbrock 2732) to mount on a Peformer intake (2101). Without it you can have internal vacuum leakage and symptoms like mine. It might be worth a try at under $10. It currently has a 1" phenolic spacer

[wayne petty]

if you can turn the screws all the way in.. and not stall the engine..


have you verified that the fast idle screw is not holding the carb primary open..

have you taken the carb off and verifies that the primary throttle blades are closed far enough to only barely expose the idle transition slot..

have you used a mirror and a bright flash light to verify the primary and secondary boosters are NOT dribbling fuel up and over at idle..

you should NOT be able to see any FUEL in the primary or secondary bore at idle.  sediment in the fuel line. or excessive pressure..  teflon tape has been a major issue as that stretches out and hangs into the needle and seat..

can you verify the throttle blades are centered in the bore properly.. all 4 of them..  this involves removing the carb..  backing off the idle and fast idle screws and holding to the light to site thru them...




this is NO WHERE NEAR PROPERLY ADJUSTED...

[Mac]

Wayne, the fast idle screw is backed completely off the cam, the choke is safety wired open.

I see no fuel in the primary or secondary bores at idle.

I had the top off and the bowls were half full and I rechecked the float level is just a little more than 7/16 spec. Clean fuel and no visible sediment in the bowls.

Haven't had the carb base off cause I thought I'd be adding that square-to-spreadbore adapter that people say this carb needs on the Edel Performer 2101 intake

[Mac]

Correction:
My intake is an Edel. Performer egr version (blocked off), probably model 3701. According to the Edel site it works for square and spreadbore and no mention of an adapter.