Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem
lean and variable A/F ratio
Shark Racer:
On that note - how much "wiggle" is normal on the APT screw in an M4M carb?
blarson:
Hooray! I finally solved the problem with being ultra lean all over except idle. I can't find reference to this part at all but a brass sleeve that was screwed into the airhorn was holding the power piston down all the way against the APT screw. I didn't notice before but the tab on the power piston actually left a little indent on the brass sleeve where they were touching. It was hollow and I was able to fit a screwdriver through it but the APT wouldn't turn because of the pressure against it. I omitted the part and surprise surprise I get 13s WOT. I easily turned the APT to 5 1/2 without the brass sleeve impairing it and now part throttle hovers around the 14s. Still no idea how high I can go with that thing but it looks like I need to turn it back down just a touch anyway. AFR does still fluctuate quite a bit with std dev of around .7 at all throttle opening. It was much worse before the throttle bushings but it's still happening. I hit the intake with carb cleaner and it doesn't appear to be leaking. I'm going to fix an exhaust leak and maybe block off the brake booster and see if it stabilizes. Should I be looking for something else as well? Now we are making some good progress!
As a side note so that the knowledge is out there on the internets: Doug Roe's book references 4psi as the minimum fuel pressure off idle. My fuel pump was indeed previously hosed.
TommyK:
On my carb the APT is about ready to fall out at about 7 turns out. This may vary from carb to carb. IMO if you have to turn it out more than 5 or so turns you probably have the wrong jet/rod combo.
blarson:
I discovered yesterday after driving it around now that I get a good mix off idle that sometimes if I come off full throttle the thing richens all the way out to 10:1 and start running rough when I come back to idle. The problem appears intermittent and appears to have no relation to how the idle screws are set. Turning the idle screws does nothing when it has gone full rich BUT does affect AFR during the times when it hasn't. If I give it some throttle it leans back out to where it should be off idle. If I let it idle or drive it around for a few minutes idle appears to lean back out to normal again. I never seem to have the problem with idle mix if I don't use WOT.
My first thought was that the power piston is hanging up but that didn't make sense because it shouldn't be pulling fuel off the booster at idle. Also, the mix is good everywhere off idle even if not completely stable. Choke is confirmed all the way open. It has a new float needle, seat, etc per Cliff. Perhaps that is getting stuck? Secondaries appear to be closing all the way and manually pulling on them to be sure they are closed doesn't appear to change anything. Any ideas?
TommyK:
Try slightly lowering float height?
Is the clip on the needle properly attached to the float arm? (on the back not through one of the holes)
Is there anyway you can monitor your fuel pressure while this is happening?
Richness immediately after overrun is not unusual but for it to last several minutes after returning to idle is.
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