I have a 402+CID BBC with a high lift long duration cam that is extremely vacuum challenged. It has approx 5-6 vacuum at idle. I used to run a2.5 power valve in my 750 Holley. I purchased a brand new Edelbrock 750 and installed it. As usual even with my Holley there was no change when adjusting the Idle mixture screws. It is running very rich at idle. Any suggestions on what tuning methods I can use and parts I need to lean this out at idle? I have already maxed out the timing at idle.
Thanks, Pat

What does "maxed out timing" mean. Your cam is going to require timing in the range of 16-18 degrees, plus vac advance connected to manifold source. That will get your engine RPM up and allow you to tune the carb to run on it's idle circuit properly.

Pat, I also have a 402 BB with a rather long duration cam, I have been engaged in this battle for months now, I have two 750 Edelbrocks and a 3310 750 Holley, I change them around like I change my socks, your idle mixture screws will not most likely ever respond, with only 5-6" of vacumn you are operating out of the idle circut at idle, be glad to share my experience with you later, must run now. Feel free to e-mail me. As Zeke mentioned the key in this situation is to crank in as much idle advance timimg as the engine will stand. Mine is set at about 18* now.

Jesse

What does "maxed out timing" mean. Your cam is going to require timing in the range of 16-18 degrees, plus vac advance connected to manifold source. That will get your engine RPM up and allow you to tune the carb to run on it's idle circuit properly.
I mean I have my timing set for full advance of 36 degrees at about 2500rpm and at idle it is now at about 24 degrees @ 1100 rpm. It is a mechanical advance dist. I spoke to Edelbrock Tech today and he also suggested 16-18 degrees @ idle and to change the metering rod springs to the blue springs. Will pick them up asap and try that setup. I'll let you know how it works

At 1100 RPM you are most likely into the mechanical advance. The 24 degrees is thus a mix of initial timing plus whatever mechanical advance is in play at 1100 RPM. So we still don't know if you have the 16-18 initial suggested here and also by Edelbrock. Plus vac advance connected to manifold.

What size cam are you running? I am running a Comp CB 286H-10, @ .050 duration is I-236, E-236 with a .556 lift across the board on a 110 LSA and my idle is at 950 confirmed and not lugging either, 1100 like Zeke mentioned will most likely put you into the mechanical curve. Also I am at last able to run my vacumn adv off unported vac. Ported does not work well with my engine. What is your vacumn reading at idle on your unported (passanger side) port. It is almost the same reading as the ported side? You want to obtain a zero reading, that was a real challenge for me to do so. You may want to confirm that your balancer has not slipped also , consider use a pistion stop and confirm true TDC and go from there. Keep us informed. I also have the Edelbrock Cal Kit for my 750 and I hope the blue springs help but I would almost bet not, I went all over the chart in the Edelbrock Book and I am currently back with stock out of the box rods, jets and orange springs. The best to you.

Jesse

Pat,if you want to save time do a search here in t/c titles "running rich" * also "timing 101" and you will get a ton of info /solutions to your problem becasue this is a very common issue with perf cams esp hotter ones.

at least 1/2 the times itvery retarded base timing issues with larg cam on street and and no vac adv just makes that worse with hot cam in street motor and sometimes p-valve issues too.

Large cams on street often need 18-20 deg base timing woth only 18 deg mech adv in dist oib by 2600-2800rpms for 36-38 total by 2600-2800rpms along with vac adv hooked to ful int vacuum.

In some cases running timing/dist lockewd out at 36 deg is required due to not enough idle vacuum to full enguage a vac adv for stable idle.

but if you base timing isnt at lewast 18-20 deg with a hot cam that reduces idle vacuum duel to retartded base timing which also forces you to adjust idle speed screw to far in to obtain decent idle spped that the prim throttle blades are adj completely out of the idle circut now idling on main circut.

Then on top of that if you dont have proper power valve or a blown one that can also be open at idle dueo to imprper match of p-valve vacuum lvl/rating for motors idle vac,example,idle vacuum in gear with auto trans is approx 5.5-6.o and someone is running a 6.5 p-vlave. In that case the p-valve would be open at idle with the motor idling in the main circut & witu p-valve open get salmed with a lot ore fuelthen it needs at the taime,way over rich.

so do a search on the 2 titels i suggested and you will find all the info yu need to get squared away.

But always get the timing esp base/total along with proper p-valve for idle vacuum (auto trans in gear) diald in correctly 1st,then if still running rich after getting all that correct them go after carb issues that can cause rich issues or have it complete rblt andthen place jetting back to stock and start over from there etc.

Scott

Thanks to everyone for your responses and info. Will start working through it as soon as I get a chance. Thanks again, Pat

You are most welcome.

Jesse