Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem
Can't get my rebuilt '78 400 to idle; runs great, but surges about 100 rpm
corellian corvette:
We used flat-top pistons so I believe I'm in the 8:1 ratio, with the XE262 cam which was recommended after we couldn't get the XE274 to idle AT ALL.
For giggles, I also measured the idle mixture screw hole, which is also VERY small, .055
My guess is to move up .010 on each hole - .065 on the idle mixture screw, and .56 on the idle down channel. Those numbers look more like what I see in some of the upgrades in the book.
I've also heard when drilling the idle mixture screw, you should look at the hole in the baseplate and make sure it's not also restricted.
Does that seem like a logical idea?
Shark Racer:
You should open it up a little bit, and you may well try that first; it's the easiest of the mods to do. I'm running mine at .090.
So my recommendation would be to start with the mixture screw holes, try that, then the DCR. I would definitely sneak up on the DCR, and not go straight to the 056, as your idle tubes are quite large.
As for the restriction - once you've drilled the hole out, it's pretty much guaranteed not to be blocked. :) Just clear any chips out of the way...
blazer74:
Just a note, are u sure your measuring all the way to the idle tube restriction. unless you have drill bits 2+ inches long u may not be getting accurate measurements like shark mentioned. U can get a set of small wire drill gauges from a decent tool store that are long enough to reach the restrictions that are like 1 21/32 down the idle tube. Taper length drill bits are longer but pricey, precision makes them.
corellian corvette:
WE HAVE SOME PROGRESS!
There has been some dialog on this on the PY forums, and I should first correct that I went back and re-took all my measurements by doing a more thorough tear-down. I'm still not 100% sure about the idle tube, but I practiced removing one from a junk q-jet, and used that to gage whether my bit was long enough. The idle tube in the carb was .032, and obviously I could not get my .040 bit down that tube. When I put the same bit in the tube in the carb, it went down MUCH further - almost flush enough I had to pick it out. So I'm still feeling like that tube is ~ .040.
Idle Tube: .040
Idle Down Channel: .046
Upper Idle Air Bleed: .067
Lower Idle Air Bleed: .070
Idle Bypass Air: None.
Mixture Screw Hole: .065"
I took a small step tonight and used a pin vice to open the idle mixture screws up to .082, and I used the same drill to put .082 bypass air holes. This felt like a good mid-step. I put the 74 jets back in, lowered the APT, and BOY did it make a difference. Timing is back down to 15 deg, idle speed screw has more adjustment, and the rhythmic vacuum and rpm drop is much reduced. I was able to get it to idle in drive, no MVA, pretty steady at 750 rpm. I still have to have the mixture screws about 5.5 turns out, and I still get a VERY mild RPM increase when I close the choke horn, but nothing like it was before. Lean idle burn smell is much reduced. Vacuum in drive is about 13hg.
New settings are
Idle Tube: .040
Idle Down Channel: .046
Upper Idle Air Bleed: .067
Lower Idle Air Bleed: .070
Idle Bypass Air: .082
Mixture Screw Hole: .082
If a little is good, more must be better, right?
I think I'm going to take ONE more stab at increasing the idle speed and bypass air holes, to bring them closer to the spec others have listed; maybe use the .89 bit on both idle mixture and bypass air to see if that gives me a little bit more mixture screw adjustment and a tad more enrichment.
Frankly I could live with it right now. Would be totally fine, but I just made a HUGE improvement so it seems like one more small step and it might get me there. I'll be very happy if I don't need to mess with the idle tubes and fuel restrictors.
Shark Racer:
I'd up the idle mixture screw size to the .089 bit and if you have a very slight up for the DCR I'd open it up just a bit.
I'd leave bypass air alone for now. How far are you opening the throttle blades to maintain the idle you want?
You probably could have gotten that XE274 cam to run at this point. :)
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