Thanks for the replies everyone.
I hate giving any advice on carb problems because there are so many variables. To answer your question you can be on the primary circuit, the throttle blades should show no more than .040 of the transfer slot in the closed position. Your idle screws should be 1 and a 1/2 turns out
I agree, trying to diagnose and solve carb problems over the internet can be nearly impossible at best, but I was hoping to get some insight and suggestions for where to start next.
Thanks for confirming that you can indeed run on the idle circuit, that sure seems to be the case in how it’s acting. I will definitely be double checking the # of turns on the idle screws and the position of the primary throttle blades. I’m fairly certain my idle screws are around 3-3 1/2 turns out at this point tho.
if it won't idle and it's running pig rich I would take the carburetor apart and start over making sure you are using the correct gaskets and make sure the float level is correct. If you changed the primary rods make sure you didn't mistakenly use a rods from a 74 and down version of the quadrajet.
Steve, I had already considered the possibility that there may have been the wrong gaskets or they were installed incorrectly, but that isn’t the case. All the holes for the passages line up correctly, and none are blocked off by the gaskets.
The primary rods have not been changed nor mixed up with others as I have no other spare QuadraJet parts around to confuse them with. The rebuild was a straight forward rebuild with all the original parts going back in it.
As said the rods aren't interchangeable between Qjet series. I am betting the carb is ridiculously lean at idle for his motor. Donovan, what do you think the compression ratio is approximately? Cliff Ruggles shows the modifying the idle tubes and drilling out and the passages at the mixture screws as well in his book. Do you remember what jets an rods were in it? A sinking float messes can cause all sorts of issues but that was tested and set.
I didn’t cc the heads or combustion chambers when I built the engine, but if I had to guess I would estimate the CR at around 9.5 to 9.8:1. I don’t recall offhand what the rods and jets in it were, but I did write it down so I can find out and report back on that later.
I agree, saturated floats can mess with things pretty badly, but mine is liquid tight and definitely not leaking. The float level was initially too high (I know this because fuel was leaking out of the throttle bores and other passages when it was shut off), but that has since been corrected and the level checked at least twice since. I’m confident the float level is perfect now and not too low either.
Donovan,
How did the car run with this carb before the rebuild?
Jared, that’s a bit of a difficult question to answer. When I bought the car the original 305 had a dead hole so the engine never ran properly to begin with.
After installing the 350 and breaking in the cam but before the carb rebuild, I had pretty much the same problem as I’m having now, just on a less pronounced scale. It ran rich at idle and I’d have to keep my foot on the gas to prevent it from stalling, but that was moreso on cold startup so I attributed it to the choke not working correctly.
Because it ran so rich at idle is why I figured it needed a rebuild.
I agree the carurators would not be the same but the problems Donovan posted about the quadrajet does not sound like they are caused by the cam. When I was in a big hurry I have grabbed the wrong gasket to use between the base and body that cause a bad vacuum leak that would not allow the car to idle. If the float is too high it could be flooding the carburator or the seat assembly inlet hole diameter is too large or the needle is stuck for some reason.
As I mentioned earlier, the float is good now after checking and readjusting it a couple times. I also considered that the needles could have been stuck in the bores too, but every time I check them they seem to move freely.
I agree you need another carb for test purposes. '67 to '75 would be best '76 to '80 are very lean but can be modified. If it works then yours is suspect. If it doesn't then your problem is elsewhere. I used a Crane cam with almost the same specs as yours on a Pontiac 400 and my stock Q-jet ran great with it. The post about emissions carbs being lean is correct. Doug Roe mentions drilling out the idle discharge ports (where the idle mixture screws are) to pre-emissions size. I have only needed to do that a couple of times and each time it was with a radical cam that refused to idle. Drilling them out cured the idle issue. I would try a different Q-jet first even if it is a cheapola Ebay find.
Thanks for this info Mark, I think this is the most useful so far. I wasn’t aware that the later ‘76-80 carbs were set so lean from the start, this may be the issue.
Unfortunately, at present the only other QuadraJet I have to test with is a 1980 442 carb from an Olds 350, and all else being equal, it probably won’t get me any closer to pinpointing the problem because it too is an emissions era carb.
I lent my Doug Roe book to my friend that rebuilt the carb to see if he could find anything I missed, do you happen to recall the pre-emissions size the idle discharge ports need to be drilled out to?
If everything thing else is good check and make sure you have used all the correct gaskets correct base and air horn if all is well opening up the idle restrictions,mix screw restrictions and adding bypass air work well all those mods are in cliff ruggles book and I did all of them to my olds 455 and the qjet works perfectly on it I don’t have super radical cam but is something in the 565 lift range and is noticeable at idle
Thanks for the info. I don’t currently own Cliff’s book, any chance you or someone else could post up the procedures that need to be done to add the additional bypass air work?
Thanks again for all the responses guys, you’ve given me a glimmer of hope that I may yet be able to overcome this persistent issue.
