Carb Questions

[Joep]

Hey guys ive got a 600 Holley that ive been trying to tune up and ive got some questions. When the engine warms up and i rev it up a little bit it is very slow to get back down to idle. Ive hooked up a vacuum gauge and it reads around 13-14 inches of vacuum but isn't steady. When i change the idle mixture screws nothing happens. Anyone got any ideas?

 

[krazykuda]

Have you checked your carb cable to make sure it's not holding the throttle open a little?

 

[Joep]

yes it brings it back just fine

 

[nm9stheham]

Can you be a bit more specific? Vacuum or mechanical secondaries? How is the idle timing? What type of cam? Etc.....

I'd remove the throttle cable and open/close the carb by hand to be sure. Beyond that, I would be checking to see if:
1) The primary throttle blades are not clsoing against the throat wall; there should be a tiny gap between the throttle blades and the throat wall when fully closed. If not, then the blades can gradually settle in against the throat wall and do as described.
2) In conjunction with the above, it sounds like the seconadary is open a bit.
3) If neither of these, then the primary throttle blades may be sticking open a bit for some reason; perhaps gum in the throttle shaft holes or a bent shaft; if so, then the idle mixure has less effect than desired.
4) Are there any holes in the primary throttle blades? Pix would be good...

 

[AJ/FormS]

Hanging up like that is usually caused by the transfers still feeding fuel or there is still a vacuum signal going to the distributor.. Primary t-blades may be way too far open.Remove and block the advance hose. If no change,take carb off engine. Make sure choke is off.Close t-blades all the way by backing out curb idle screw. Turn screw back in 1.5t or until transfer slots under the blades are square or slightly rectangular(remember this initial setting). set mixture screws to about 1/2 t from lightly seated.Make sure secondaries are closed but not sticking.Put carb back on.Fill it with fuel.If mechanical pump,I fill bowls through vent pipes. At least 50cc per bowl.Block vacuum advance. Put Fast-idle cam on 2nd highest step.Start her up.If she fails to lite off, squirt a few ccs of fuel down each primary and retry, again on 2nd highest step.Let her warm up.Try to keep rpm between 1500 and 2000. While warming up check fuel levels if you have sight-windows, or plugs. Once warmed up, see if she will idle at the initial setting.If she idles a little slow turn the curb idle screw in a bit,up to about an additional 1/2 turn. If she idles way too slow and its not flooding, here are the most likely reasons; A)Insufficient idle timing, B)Vacuum problems;maybe she wants bypass-air, C)incorrect float level, D)incorrect valve adjustment......An easy way to set Idle Timing is to check the cam card for the intake @.050 duration. Just divide that number by 15. eg 220/15=15*. No card;guess.Idle timing is not rocket science. You can work with almost anything between 10* to 20* at this point....... Bypass-air is just drilling the t-plates. If you suspect she wants some, prove it by introducing air at a vacuum port, such as the PVC port or the brake booster port. Just feed her a little bit at a time. If the rpm rises, there you go, bust out the drill bits........Once you have a satisfactory idle rpm, you can fine tune the mixture screws, and reattach the advance hose to a "ported" source. If the mixture screws still dont work, the circuit may be faulty. The above has worked for me with cams in the 200* to 240*range.If you get lost, just go back to the very 1st sentence.Finally dont forget to limit your maximum advance to something like 34*+/-2. I always run a vacuum advance on the street.It makes a ton of difference in driveability, esp with a stick car.