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

[bry593]

Q. 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.
A. Yes, I got a new APT screw and spring from you.  I have tuned it with the tip in (+50ish rpm @ 2000), but I've noticed my gas mileage has dropped a bit.  I'm thinking about changing from the .035" to the .045" primary rods with the #69 jets per your original recommendation.  On a side note, the choke works great, starts right up and begins to roughen up after 30s, hit the pedal once to drop it to the next stop and it idles clears up.  At that point, I'm hitting the rouad and the choke is completely off.  Works better than any quad choke I've had.

Q.  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
A.  I drilled the seat out to .125" per recipe one.  You think I should go larger?  This isn't really a high-power car, just a large engine in a relatively heavy, 1970 Caprice.  I have the float set at .313", not .420".  I chose this as a compromise between your and Lars recommendations.  Seems to be working okay, and there are no gas fumes or soaked airhorn gasket issues.

At this point I have a few other projects to tidy up (motorcycle fuel system overhaul, front rotors for the Caprice, painting the house, etc.).  Might be awhile before I can provide further updates as I dial it in for peak efficiency.

[Cliff Ruggles]

Since you have APT install the larger metering rods, the smaller ones will not provide full control of A/F from rich to lean.

Install the high flow .135" N/S assembly and set the float to 1/4".

Fuel level effects the calibration, and the float level and fuel seat diameter effect the fuel level.  Unless the fuel pump is really high pressure, there is no need to run a low float setting......Cliff

[bry593]

Will do.  Won't take too much time to pull the carb off and drill and peen the needle jet, bend the the nitrophyl and swap out the primary rods.  I'm getting pretty speedy at this point.

Should I change out the DR secondary rods to DA?  The DA are a bit smaller, but have a longer tip.  If so, can you explain how this would benefit?  I'm not sure I understand the theory behind the medium and long tips.

[Cliff Ruggles]

They will richen up the A/F at full throttle, probably a good move, DR's have a much larger tip and leaner.  I always prefer a little extra fuel at full throttle, keeps the rod bearings out of the oil pan in most cases!.....Cliff

[bry593]

I swapped out the rods for the .045, reset the float to 1/4", but did not drill out the needle jet (was in a hurry).  Set the IMS to 2.25 turns out and readjusted the APT at 2000 rpm. 

Loaded up the family, filled up with gas and then drove 65 miles to another gas station.  Refilling the tank to the same level (all the way up the fill hose) took 2.4 gallons.  65/2.4 = 27 mpg at 70-80 mph.  That's with a TH400 and 2.56 gears.  I'm going to finish out this tank to see what my average is with in town driving.

Cliff, I'd say this carb seems dialed in.

[Cliff Ruggles]

Good news!......