68 Q-jet, I just rebuilt, purchased the carb but never saw it run, running on new motor, pulling 5 to 10 lbs vacuum.o No: thanks Bruce
That is called nozzle drip and is a common problem when running a Q-jet on a cammed motor. The primary side of a Q-jet is very efficient, and it doesn't take too much to initiate fuel flow from the mains. Bottom line is the throttle blades are too far open at idle, and as a result, the engine isn't running on the idle system of the carburetor, but on the mains. A cammed engine will want a lot more initial timing. If you advance the timing to at least 12-14*, and close the throttle with the idle speed screw, it should help a lot. If you are trying to run a small block carburetor on a big block, it can happen also. The big block carburetors have passages that allow more air for the bigger engine, and different idle calibrations too.
Could also be excess fuel pressure, or trash in needle and seat. If not, and what Larry says is true, drill a small hole into each primary throttle blade so you can close the blades and still have a little additional airflow. Do yourself a favor and buy Cliff Ruggles' qjet book and read it cover to cover.
Thanks for both the reply's. A little history, new 350 rebuild, .030 over pistons , low compression, 1970 motor, ta 212 cam. The carb is a 70282441029, so 68, on the original manifold with a 1" carb spacer. First thing don't assume a new carb spacer is flat, this one was not. I could not get it to seal, just about went thru a can of Either finding the leaks. Finally took the spacer out and put the carb on the manifold Soon as I saw Larry's response I tried it with no avail. The only carb / manifold gasket I have is old and hard as rock, it still leaks, so first thing is to get a fresh gasket. Yes getting Cilff's book is on my list for tuning and rebuilding. thanks Bruce
Hi, I sanded the phenolic space for 1 1/2 hrs both sides on a sheet of glass, applied a thin layer of hi temp RTV to the mating surfaces. It no longer acts like a vacuum leak :beer I set the timing at 0, adjust the rpm to about 800, I get about 5" of vacuum. It's still running a bit on the main jets, it will idle in gear. If I put a screw driver down the vent hole and close off the power piston it will idle with the idle mixture screws turned out 3 turns. Now if I advance the timing will that help? I'm going to through a curve at you, because of the postal strike my rocker arm buttons got caught in the mail, I only had enough after breaking a few to do one side. One side I'm running the 70 set, the other side I'm running a used set of a 73 motor with 70 push rods. Is that going to effect the vacuum? thanks Bruce
5" of vacuum? What cam are you running? As you've discovered, that isn't enough vacuum to hold the power piston down. It would help a lot if you gave us a rundown on what's in this engine, and what you intend to do with it. Is this a street car or racecar?
OK, TA 212 cam. You have bigger problems than you think if it is only producing 5" of vacuum. It should make 3X that much vacuum. Either you have a massive leak somewhere, or that cam is not in right. Fix that first. There is likely nothing wrong with your carburetor.
Thanks Larry, I just had it for it's first drive, it had 8" of vacuum idling after the drive. Another member told me he gained 6" of vacuum after break-in. I have about 2 hours run time on the motor. I don't think I have a vacuum leak, I slowly cover the carb with my hand and it stalls out. Bruce
"I set the timing at 0, adjust the rpm to about 800, I get about 5" of vacuum." A bigger than stock cam is going to want, and need more initial timing. Might want to read the steps we took here a few weeks ago to get a 68 350 up to par after a cam swap. We ended up recurving the distributor and setting the initial timing at 12 degrees, and setting up the carb for more idle fuel to the mixture screws. Vacuum was "low" and irratic idle before we started, would NOT stay running when placed in gear, etc. After we were finished, near smooth idle, and very little drop in rpms when placed in gear, etc.....Cliff
When we try to use a factory Q-jet outside of the original application, more times than not, it will need more idle fuel available at the mixture screws. This simply happens because all factory q-jets are, or were, application specific. Increasing idle fuel becomes evident when there is little, if any sensitivity or control with the mixture screws, the idle speed screw is turned in far enough to cause fuel flow from the main system (nozzle drip), idle quality is poor, stalls when placed in gear, etc. The best advice I can give is to get our book and do some reading. How the system works, and how to improve it, is described with pictures. It would save me a LOT of typing here as well!......Cliff
thanks for the reply, where's the best place to purchase the book edit: I: found the web site http://www.cliffshighperformance.com/ Bruce
We ship same day ordered, and it's the only place to get a signed copy! Lots of folks get them at Amazon, a tad cheaper, and if you buy multiple items over $25 they do free shipping. You might grow a beard before it arrives, as they are "spotty" on shipping/delviery time(s)......Cliff
