Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor
Pontiac 400 - Idle bypass and Secondary rod hanger questions
Cliff Ruggles:
None of the symptoms you are seeing are typical for that camshaft if a 400 build at 9.5 to 1 compression.
I have tuned several of them, and they idled fine in and out of gear with less idle rpm, less idle fuel, and less idle bypass air. I also didn't run nearly the timing either, about 12 initial and used ported vacuum to the advance.
So why is your engine wanting all that bypass air with the timing at 24 degrees? It should idle fine in and out of gear with less timing.
I have ran into several engine combinations that hated a lot of timing at idle speed, so you may want to try less timing by using a ported source to the advance, and start the idle tuning process all over.
With the HUGE idle fuel delivery from the Edelbrock 1910, you should be fine without all that timing, and very little idle bypass air, at least from what I've seen here tuning similar set-ups with the same carburetor.....Cliff
77cruiser:
What does the exhaust smell like?
429bbf:
someone correct me if my understanding is wrong. the only way the idle screws get fuel if from the size of the idle tubes and and the size of the dcr.another small amount comes from the transition slots .if those 2 are to large you have to ether give it more bypass air or open the throttle plates . now you are not truly running off the idle circuit. my experience has been if the idle is to rich i have to smaller the idle tubes and the dcr so i can get control of the fuel with the idle screws and keep the throttle plates nearly closed so I'm not pulling a bunch of fuel from the transfer slots.this works for me. if my understanding is wrong please correct my way of thinking.fwiw
Cliff Ruggles:
The transfer slots are not in the exact same location in every carburetor, so some will provide more idle fuel to the engine with the mixture screws fully seated. This fact is not usually a big player, but once in a while one will run into a carb that's extremely rich at idle with no control of the mixture screws because it's getting plenty of fuel from the transfer slots.
I also see a very similar problem with some engine combinations where folks use manifold vacuum to the advance and put too much timing in at idle speed. Might be a player here, not sure. From what I'm reading the engine isn't very happy with the tune it's getting. Could be simply too much timing at idle combined with a lot of idle fuel available at the mixture screw holes and transfer slots.
It's also no big secret but I am NOT a big fan of Comp XE cam lobes, especially in Pontiac engines. I've tuned a number of them, and not in one single instance have I liked the end result(s). Those cams drop the valves too hard on the seats, and shove them up too fast. LSA is too tight, and intake valve closes too early. This makes for a "quirky" idle quality right to start with.
I actually built a 400 Pontiac WAY back when those cams first came out, with very well prepared #16 heads. I thought at that time I'd buy into the "new" technology and find some additional power. I couldn't get that cam out of that engine fast enough, very disappointed in it at every level......Cliff
73ss:
I agree on the comp XE cams. Had one in my 454 chevy. noisey valvetrain, stinky exhaust, poor vacuum... Swapped it out for a summit 1302 cam. Not for the price but for the specs. Did nothing else to the car. Now it has a very quiet valvetrain, Idles like a champ in & out of gear. Picked up 2 inches of vacuum. (The power brakes really improved) & picked up 10lb ft of tourqe on the dyno. Horsepower stayed the same. Came in 200 rpm sooner. Couldn't be happier.
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