Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor
Setting Up and Adjusting Idle
omaha:
If you have chenged the cam to one that is larger, you will need more initial advance. I would try 14 or 15 degrees initial advance. Also, the idles RPM speed ay end up a little bit higher. Since this cam is something that is new, it changes the parameters of the engine characteristics. The only other thing that I can think of would be the idle tubes not being large enough but it seems like this would be not the case.
What is the exact advertised duration of this cam? (the specs yu gave are duration at .050 lift I assume).
The 108 centerline is just a bit tighter than most of the cams out today that are made with 110. This small amount more overlap may be causing some of the issues with the idling. The vac reading sure seems a bit low. I would expect ~12-14 inches with a 217/224 @ .050 cam but what is the advertised duration? Bump the initial timing to 14 or 15 at Idle. try to stay below 1000 rpm when setting the initial timing. (although you can tell when it starts to advance with the timing light.)
Just trying to get an idea of what cam you have to figure out what you can expect with the idle quality. Some cam will make the engine shake a bit.
defiant1:
The camshaft is a custom grind. Erson Cam- 268/284 LSA: 108+2 Valve lift: intake-.494 exhaust-.504 Overlap: 60
Intake center line: 106 @ .050
I degreed the cam and verified all the specs.
When I get home tonight I will try the 14-15* at lets say 900 rpm and go from there.
Is there a correlation between how far the throttle plates are open and the Q-jet's idle circuit when the motor is at idle?
jamesF:
I meant the Secondary Flaps. On mine I had trouble initially getting the idle mixture to respond to the screws. THe flaps were not closing by the smallest bit. They needed some "working" open / close - open /close etc.
defiant1:
Ok- tonight I retorqued my intake manifold.
I plugged off both valve cover grommets and left the hose for the PCV attached to carb and left the other end open.
I started motor and it was pulling a steady 11" vacuum with hose open. When I put my finger over the hose and plugged it the vacuum jumped to a steady 15". Not sure what this exactly means but just an observation.
Ok hooked up everything else back up and ran motor. Set the timing at 14* at 850 rpm. The vacuum at this setting is a steady 12.5". I played with the timing up and down for awhile and this seemed to be the happy medium. The motor has an occasional slight even shudder at times but it was not abrupt. Although a shudder to me still screams vacuum leak...so still not sure.
It would run rougher when I turned in the idle mixture screw all the way in, but still would not kill the motor or make a substantial decrease in rpm.
It dieseled a little when I shut it down. But I guess this could be attributed to making multiple adjustments to the timing.
I might be gaining, but still not completely confident I got the idle figured out.
On another note: Does it matter what port on the carb (Chevy Q-jet) I hook the PCV hose to? I am using the one in the front on the baseplate, it that right? I plugged the rear port and am using the intake manifold for the power booster.
d1
defiant1:
I was referred to another carb builder, I explained the issues I was having and he suggested the throttle plates were open slightly interrupting the idle circuit.
He said this would contribute to the following issues:
Low vacuum
Idle mixture screws having no affect
He suggested I:
Replace the power piston spring with the lightest spring to help keep to ensure the needle stays seated
Ensure that the throttle plates are covering from the middle to lower third of the transition slots (vertical slots in bore).
Idle Bypass Air hole should be drilled a little larger. Mine is .081" He recommended .093" I will drill the hole incrementally until it runs the best.
And verify that fuel is not dripping from nozzle
I will make these mods and see what happens.
d1
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