Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor
idle help
73ss:
Wanted to give an update. I my above post I described how the #2 plug was turning black and was stumped as to why. I decided to pull the engine back out because I was never satisfied with the way it idled and wanted to degree and or replace the cam. I did a leak down test and all cylinders are 10~13% on the gauge with the exception on #5, It has a 35% leak past the intake valve, you can hear and feel the air escaping from the intake. #7 is the next to fire followed by #2. Since the #5 is still on it's intake stroke while #7 begins to open I'm thinking that when #5 starts the compression stroke, #2 is starting its intake stroke and receiving what is getting pushed past the #5 valve. # 5 & 2 do both share the same side of the intake, dual plane. I think this is why #2 is rich and turns the plug black. #2 also runs about 100 degrees colder than the other cylinders using a infrared heat gun.
What do you all think about this? That's the only thing I can come up with as to why it turns black. I second guessed myself and thought maybe I installed a ring upside down on that cylinder but the leak down and compression results are good.
I was quite surprised to find a leaky valve, When I got the heads we tested all the valves with a home-made tool to seal the ports and pull a vacuum on each valve to check for leaks. More than half of the valves were leaking. These were new heads, I called Brodix and they said to send them in for repair but I decided to lap all of the valves myself. Afterwards all valves held 20"'s vacuum for 15 mins or more and I called it good.
I also bought the tools to degree the cam and after checking I found it is 2 degrees retarded. I have it installed straight up. I have the specs above, open to cam suggestions, I don't think I made a good choice on the cam.
I did take the car to the track, Ran consistent low 12.30's @ 110, 1.7 60'. OK but was expecting more for a near 500 inch motor. 4100# chevelle, Through a full exhaust, 3.73, 2500 convertor. Q-jet ran flawless.
Cliff Ruggles:
Advancing the cam 2 degrees isn't going to do much for it and may even slow it down a bit. The cam is pretty small right to start with for 489cid with 10 to 1 squeeze and the head flow available. That's about how much cam I'd use in a 454 build with less compression.
Below is a dyno chart from a 455 cid engine with less than 9 to 1 compression using Pontiac Super Duty round port heads and a 230/236/112lsa HR cam. Idled with 16" vacuum at 800rpm's and I knew right then we needed a little more cam in it.
Low 12's at 110mph is showing about 460-480hp being used just doing some rough "math" in my head using the vehicle weight. Certainly some room to grow there for sure......IMHO......Cliff
old cars:
You didn't answer what intake manifold you are using, at least not that I could find
73ss:
--- Quote from: old cars on December 11, 2021, 03:53:39 AM ---You didn't answer what intake manifold you are using, at least not that I could find
--- End quote ---
Edelbrock performer RPM Q-jet. part # 7164
I believe my original poor idle problem was due to #5 intake leaking and also a poor cam choice.
233.6in 246.6ex @ .050, 110C/L, 114 lobe sep.
Too much exhaust duration for the amount of intake duration? This cam would be better suited for a blown application. Lift, duration & center line were all spot on with the degree wheel, other than everything being 2 degrees late. Cranking compression isn't that great either at 155 psi.
Cliff Ruggles:
I'm not sure exactly what's going on but something isn't quite right with the build.
489cid, 10 to 1 compression, 230/246/114LSA cam and you are only seeing 155psi cranking pressure and it takes 20 degrees initial timing to produce 10" vacuum at idle speed. It should EASILY make 13-14" vacuum at 700-750rpms with only 10-12 degrees initial timing and cranking pressure is down at least 20psi......IMHO
Hopefully you'll get it figured out at some point......Cliff
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