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Hey guys, something stumping me here.
Edelbrock carb idle mixture screws - turning them has no effect.
I know there is a LOT affecting this, done a lot of reading online, and can't crack it yet.
Brand new 650cfm manual choke carb
350 small block, 9.5:1 compression, bigger than stock cam, but not too much. 1,500 - 5,000 rpm range, slight lopey idle 218 intake/exhaust duration at .050" lift
Edelbrock performer EPS intake
Tried range of step up springs. weaker yellow in there now.
Vacuum guage reads 13.5 on manifold vacuum port side of carb, 6 on ported side idling at 800 rpm. Needle shakes slightly, but mostly a steady reading. so not suspecting a vacuum leak. (had one on old carb - through body of carburetor itself! throttle rods, etc.)
Tried disconnecting vacuum advance and capping the port. No change.
Played around with timing, no change. At 14 before tdc now
Idle is steady and actually sounds amazing. Were it not for the idle screws not doing anything, I would say it is running great, but I have no idea what is happening at idle.
Any tips would be appreciated. Beed a couple decades working on carbureted engines. Re-learning a lot here!
Not familiar with Eddy fuel makinger happeners,
but on Holleys, the mixture screws have a gasket on the threads to seal.
Those aftermarket seals come in black plastic, red nylon and cork.
I have found that the hard material seals don't stay at the carb body, but rather constantly work loose. Only the cork ones stay on the screw where it is supposed to.
Having said that, remove your mixture screws completely and see what gasket is on there (or missing
Usually a vacuum leak issue. It will overpower the ability of the idle mixture screws to make normal adjustments If there is already too much air in the mixture at idle, closing off the idle screws will have no effect at all.
Hey guys, something stumping me here.
Edelbrock carb idle mixture screws - turning them has no effect.
I know there is a LOT affecting this, done a lot of reading online, and can't crack it yet.
Brand new 650cfm manual choke carb
350 small block, 9.5:1 compression, bigger than stock cam, but not too much. 1,500 - 5,000 rpm range, slight lopey idle 218 intake/exhaust duration at .050" lift
Edelbrock performer EPS intake
Tried range of step up springs. weaker yellow in there now.
Vacuum guage reads 13.5 on manifold vacuum port side of carb, 6 on ported side idling at 800 rpm. Needle shakes slightly, but mostly a steady reading. so not suspecting a vacuum leak. (had one on old carb - through body of carburetor itself! throttle rods, etc.)
Tried disconnecting vacuum advance and capping the port. No change.
Played around with timing, no change. At 14 before tdc now
Idle is steady and actually sounds amazing. Were it not for the idle screws not doing anything, I would say it is running great, but I have no idea what is happening at idle.
Any tips would be appreciated. Beed a couple decades working on carbureted engines. Re-learning a lot here!
Have you checked to see if the needles are being held down when the engine is running? You can loosen the covering caps and slide them sideways slightly to observe that they are.
UTG put out a video a few weeks ago on this subject. I never thought about this and at 800 RPM idle maybe low enough not to cause this but it's worth taking about. He mention that if the primary throttle is open enough to start pulling fuel from the transfer slots that it no longer pulls fuel from the idle screws. The fix for this was to adjust the secondaries to allow more air so you can close the primaries more
I agree with the Above Post!!! Transfer slot issue is a great potential issue on a Holley, But what about the Edelbrock, I am sorry to say I don't have enough experience with them...
throttle blades are open too far and in the transfer slot if you have 6 in of vacuum on ported vacuum at idle along with 14 inches of vacuum normal vacuum causes non adjustment of the idle screw, most likely the timing is off or needs the vacuum advance hooked up to full vacuum port causing idle screw to be turned up to maintain idle, your cam isn't big enough to justify a spring change
Yes, Eric, it seemed to me my cam is not to far departed from what the carb is likely tuned for from the factory for a spring change. Don't think timing is the issue. I have had it back and forth between 12 and 16 deg btdc with no change in idle mixture screws. I will try hooking it up to manifold vacuum. I never liked doing that previously because all that advance is all there right at the start. Seemed like too much advance that low in rpm. Good to try for this, though. I will keep messing with it.
I will try. I looked in there with engine running and no fuel dripping from primary venturies. Never the less, idle circuit is being bypassed for some reason.
Looks like I'm a bit slow on the uptake here. I now get the timing connection. Because guess what? Checked things, moved vacuum hose to distributor to manifold vacuum, and timing did not change at all from ported side. Looks to me like the vacuum canister is bad. Does look old. Having to adjust idle too high to get to appropriate idle speed when timing should have been bringing it up a bit. Like I said, been while! You'd think at my age (50's) I would feel a little more seasoned. But no, still feel like a kid who is still trying to learn everything.
You don't want the advance can hooked to timed/ported vacuum fitting. It should be connected to manifold vacuum. From the factory it was connected to ported vacuum; but that was for EPA/emissions reasons. It retarded timing so that combustion chambers would run HOT and burn excess hydrocarbons. No really something the engine likes, but it kept the Feds off GM's back...for a while.
You should be able to see the timing move on your timing light when you hook up the vacuum can to manifold vac.
And the rpm should increase.
Forcing you to back down the idle screw to get back to your 850 idle.
Backing off the idle screw should cover more of the transition slot, helping the idle mixture screws to work.
The reduced throttle blade opening will increase your vacuum level at idle and slow cruise.
That increases your MPG, and both the engine and exhaust manifolds run cooler.
It's a win-win-win!
20-28* timing at idle, with the vac can on, is to be expected and not too much.
Looks like I'm a bit slow on the uptake here. I now get the timing connection. Because guess what? Checked things, moved vacuum hose to distributor to manifold vacuum, and timing did not change at all from ported side. Looks to me like the vacuum canister is bad. Does look old. Having to adjust idle too high to get to appropriate idle speed when timing should have been bringing it up a bit. Like I said, been while! You'd think at my age (50's) I would feel a little more seasoned. But no, still feel like a kid who is still trying to learn everything.
install a new vacuum advance and hook it up to full vacuum , this alone should raise your idle high enough that you should need to back down the rpm to 800 rpm in order to get the throttle blades into the idle portion so you can adjust the mixture screw and not be in Idle tranfer slot , transfer slot also doesn't come out of the booster so you can't see the fuel and to actually see where the throttle blades are at is to unbolt it and look underneath at the throttle blades
You should buy a mity vac pump and test your vacuum can.
It is critical for decent performance.
13.5" vac at idle is not too big for an AVS, but it is close.
They do have transition slots on the primary side, but not on the secondary side. #7 is the primary transition slots, which completely over power the idle mixture screws once the throttle is open enough.
There are no transition slots on the secondaries on an AVS, and no way to adjust idle air, or secondary throttle opening.
Because of the lack of a secondary throttle blade adjustment screw on an AVS, it is going to be very sensitive to the primary throttle blade angle.
It seems you have a broken vacuum can. Without the extra advance from that, you are forced to open up the idle screw too far, opening the throttle blades more, and into the transition slots.
Fix the vacuum can and then you can lower the idle screw back down. covering up a lot of the transition slot, and your mixture screws will work again.
Above is what it took me what feels like too long to get through my head. Makes perfect sense now. I ordered a new vacuum can. Yes, opening idle screw to compensate for inadequate advance from lack of vacuum help. Can't wait. I have been working on this thing for so long, after a rebuild to get things right. Things went wrong that I have never seen go bad before. Like leaking through the throttle rod in the side of the carb for instance.
Thanks for all the input guys, really helped clear things up.
Well, got the new vacuum canister installed. Things change now when connected to manifold vacuum, not ported. Still, Idle mixture screws do not adjust. New carb, so did not bother pulling idle screws to check for blockage. So for kicks, I just did. Guess what I found? The idle mixture screws ARE JUST SCREWS! No needle there at all. Nothing. So nothing making its way down to adjust fuel. Everything just pouring in as if I pulled the needle screws and put some random screw in from the hardware store. Idiots. Summit and Edelbrock are going to hear about this.
idle mixture screws
