Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem
Edel 1901, AMS out, Still Lean at Idle
Cliff Ruggles:
If you can't tune out the stumble with the tension spring you may need to tune the pull-off release time slightly.....Cliff
bry593:
Found a vacuum leak between the carb and manifold. I used a gasket left over from a previous kit. It looked right, 1/4" thick and open plenum, but it was cut short at the front near the PCV valve port. The leak wasn't too obvious until the air cleaner was installed, and then you could really hear the hiss.
After installing the correct gasket, I had to retune. Now my idle screws are out 1 turn (was 3.5) and the carb is loading up on fuel at an idle. I'm thinking I should probably add idle bypass. It originally had .098, which I blocked completely. I'm going to try .050 and see if this cures the idle. Will also check for nozzle drip as an additional verification.
On a side note, the vacuum is back up to 18.5 in-Hg from 16. I originally attributed the reduced vacuum to having the primary blades open slightly at idle.
I've driven it about 300 miles with the current tune and still have a hesitation going into the secondaries. Will wind the spring back to the original 7/8 when I drill the idle bypass.
Ironically, I'm almost back to the original Edelbrock tune. The primary difference being a reduction in the main air bleeds (body) and enlarging the .029" idle bleed tubes. At this point, I might even throw in the original primary and secondary metering rods and put the choke pulloff back to 3.5 s.
bry593:
Okay, back from the mod and tune. Turns out, I'm an idiot. The reason I was having a hiccup going into the secondaries is that I forgot to reinstall the link between air flap and choke pulloff.
In this round of mods, I went ahead and pulled the idle bypass plugs completely out. I also put back in the original primary and secondary metering rods. I left the airflap spring at 3/4 turn. I have a good idle, no primary hesitation and only a miniscule secondary hesitation which I can probably adjust out with the air flap screw.
Below is a list of the original Edelbrock 1901 tune, and final tune:
•Idle Mixture Holes: .077 ; .087 #43
•Idle Bypass: .098 ; .098 #39
•Idle Tube: .029 ; .036 #64
•Idle Down Channel/Restrictors: .046 ; .046 #56
•Lower Idle Air Bleed: .062 ; .070 #50
•Upper Idle Air Bleed (in body): .070 ; .070 #50
•Accelerator Pump Discharge: .026 ; .026 #71
•Main Air Bleed (in body): .120 ; .052 #55
•Main Air Bleed (in air horn): .050 ; .052 #55
•Primary Jet: .069 ; .071 #71
•Primary Metering Rod: .035 3EE; .035 3EE
•Fuel Inlet Seat: .110 ; .125
•Float Level: .420 ; .313
•Secondary POE Well Restriction (in body): .032 ; .038 #62
•Secondary POE Discharge (at flap): .052 ; .052 #55
•Secondary Tube Restriction (innermost two): .026 ; .036 #64
•Secondary Hanger: K ; K
•Secondary Metering Rod: DR; DR
•Air Flap Open Distance: 1.295; 1.295
•Secondary/Air Flap Spring: 7/8 ; 3/4 turns
•Choke Pull-off Release Time: 3.5s; 2s
Idle mixture screws are now at 2 turns out.
It seems the only things wrong with the original 1901 tune were the main body air bleeds and idle tubes.
Anyway, glad the carburetor project is finished. It was a lot of time and a great learning experience dialing it in.
Thanks to Cliff for helping me get it right!
bry593:
There is a typo in the bulleted list above:
•Primary Jet: .069 ; .071 #71
should read:
•Primary Jet: .069 ; .069 #69
This is actually a fairly rich primary, but it seems to be working. The other combinations I have available:
.071 & .045 = .0024 in^2 (Didn't get a chance to try this out and is what came stock on the 402 Chevy)
.069 & .045 = .0021 in^2 (Recommended by Cliff, haven't tried this with the idle by pass opened)
.069 & .035 = .0028 in^2 (What I'm currently running. Seems to be working okay in the 100 miles or so I've used it).
Eventually, I might try the smaller jet combo again. Lots of other projects going on.
Cliff Ruggles:
Are you using the APT system in the baseplate?
We install an external screw at that location so we can fine tune part throttle A/F without taking the carb apart and changing metering rods.
The .125" N/S assembly is too small for the power level and .420" is too low for the float setting unless you have a high pressure fuel pump w/o a regulator.....Cliff
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