Hello, I'm trying to figure out my fast idle issue. I've read and followed Larry's timing post but I feel as though I'm starting down a road of tinkering. I did make it so I have 36* with initial and mechanical timing. It starts a little hard but starts. I tried it out without the vac. advance but just ended up plugging it it since it didn't matter. I forget what the vacuum advance adds but I have a fast idle condition where I have the idle screw backed all the way out and it still idles at 800-900. I believe this is a bit odd. I already made sure that the choke isn't hanging it up and it wasn't. I have a loud sucking noise that appears its just the carburetor (Qjet). I already checked for Vac. leaks and found the secondary throttle shaft leaking a little and the only other thing that makes a difference with the running is clamping the PCV hose. For my questions: 1. How much initial advance should I have before I make the rest up with mechanical? I ask this because the only way I can slow my idle up enough to get more adjustment from the idle screw is by retarding it. 2. Air/fuel mixture screws for the most vacuum are pretty far out and it almost seems as though all its doing is idling the engine up..... and not stumbling like I've experienced in the past. I just replaced cam, heads and intake -455ci .032 over -Cam is a Lunati Advertised Duration (Int/Exh): 262/268 Duration @ .050 (Int/Exh): 219/227 Gross Valve Lift (Int/Exh): .484/.505 LSA/ICL: 112/108 -Performer heads -B4B intake -Quadrajet carb -Unilite distributor -4 speed
Some Unilite distributors have an adjustable mechanical advance. Adjust in more mechanical advance so you can run less initial. I believe most Unilite distributors use 24* of mechanical advance. If the springs are too light, you will have some mechanical advance in at idle speeds. That is not desirable and will lead to an unstable idle. Use heavier springs. https://www.jegs.com/InstallationInstructions/600/650/650-29014.pdf
Ok thank you, i'll give that a try. I think I'll try to stay in 2-6* of initial and have the rest come in with the mechanical? I believe I already limited my vac. canister to 8-10*. I never had to deal with an issue of high idle with idle screw all the way out and being able to turn air/fuel screws pretty far out without stumbling. I do have a higher RPM (3800-4200)under heavy load , pop once through the carb. I will look into that issue when I make sure my timing is good.
If I remeber correctly I think I have 2 orange springs in the distributor and according to the Mallory curve kit will start at about 850-900 rpm and be all in 2k or so. Ill have to pull it out to confirm.
12* initial advance is a good number to shoot for. If it pops through the carburetor, it means it is lean. #1 cause of that is a vacuum leak. The idle mix needles should not be out that far.. I turn them out to highest RPM/vacuum, then turn them in a bit. That may be too light. You want the advance to start at 1000 RPM or so.
Ok will do. I will keep searching for a vacuum leak. If I don't find one Ill richen the secondaries a little with new rods.
Put a vacuum gauge on the fully warmed up engine. I'm thinking, with that cam, you should have around 14" of vacuum in Park.
With the current timing and tuning I currently have 18-19 of vacuum @ 800-900 rpm. I'm going to re-time it as you suggested and see what happens. Thank again for your help
That's way more than I expected from that cam. Vacuum leak seems unlikely. If it pops back through the carburetor, that is lean. Make sure the secondary rods are raising as the air valve rotates.
Ill check that, unfortunately I'm not very familiar with quadrajets and have a lot to learn about them. I think if the secondary rods are moving fine (if I can even figure that out haha!) then I will go with a smaller metering rod. I need to figure out what I currently have in it. I had the carb built for my last setup by Ken at Everyday performance So with changing cam, heads and intake I can only assume it needs some adjustment....
The air valves are the big flaps on the secondary side, up top. With the engine off, tip them in with your finger. As the air valve rotates, it lifts the secondary rods up via a small plastic cam and hanger. Easy to see. I think if Ken built it, it has to be pretty close.
Ok I'll check that out tomorrow night. My previous setup had a cam with a bit more duration and lift. I could never get it to run good and had a hell of a time trying to get the intake to seal on the heads and after many failed attempts at resealing and angle milling I gave up . So I chose to just replace the entire top end and change out the cam to something more streetable . Hope I can make this setup run good and enjoyable to drive.
Maybe the power piston spring is too weak and he isn't getting enrichment fast enough. He has a lot of idle vacuum. I bet the jetting is probably OK.
I don't know much about power piston springs but if they are dependent on vacuum then it could be a difference maker. My last setup produced 10" of vacuum. I would think it has a lighter spring in it?
Yes, that is my point. The spring is overcome by vacuum and holds the power piston down, holding the primary rods down in the jets. For a cam that produces lower vacuum, a weaker spring keeps the piston down at idle and low speed. Opening the throttle lowers vacuum and the spring pushes the power piston up taking the primary rods out of the jets enriching the mixture. Now you have a cam with a lot more vacuum. It still holds the piston/rods down, but it takes a lot more throttle opening to get the piston to rise for power enrichment. So it is leaner for longer.
These are from Quadrajetparts.com, I think other kits have different colors but just to get an idea I was thinking with my vacuum I should probably run black or even the blue with 18" of vacuum? Spring / Inches of Vac. / Length Yellow: / (2-7) / 0.950" Silver: / (4-9) / 1.020" Black: / (5-9) / 0.950" Blue: / (8-12) / 1.450" The only thing that leads me away from a lean condition is that it smells like raw fuel after a hard acceleration
Use the strongest spring that keeps the power piston down at idle. Again, if it pops back through the carburetor, it is lean regardless of what you smell afterwards. I would contact Ken and find out about recalibrating the carburetor for what you have now. Q-jets aren't cookie cutter carburetors, like a Holley DP, you can't expect them to run right if they are calibrated for one engine, then run on another that is much different.
I dont think I should start messing around with it, I'll take your suggestion and reach out to Ken for a re-calibration.
