Edel 1901, AMS out, Still Lean at Idle

[bry593]

I'm having an issue with a rebuilt Edelbrock 1901 quadrajet.  First off, here is my engine:

413 CI sbc, Crane .427/.427 lift cam w/110 center, Dart Iron heads w/2.02 valves, appx 8.8 to 9:1, iron quad, 2" rams into dual 2.25" pipes, GM HEI w/14 initial, 36 @ 2500 rpm and 50 at cruise.

I bought the rebuilt 1901 carb to replace a warped 4MV that was unresponsive to mixture screws.  After bolting it on, I proceeded to adjust the mix screws.  At 4.25 turns out, I achieved 18.5" Hg, but then could not reduce idle speed to 750 rpm.  A quick check showed that the fast idle was off and the primary throttle plates completely closed.

My first instinct was to check float level and it was found to be .420" (and the needle was clipped at the wrong side of the float bar).  Being that this is a 4MV carb, I set the float to somewhere between 5/16 and 3/8.  This didn't help.  Upon reassembly, and readjustment of AMS (again 4.25 turns), I placed my hand over the choke horn and the idle momentarily increased.  Still lean...  I then proceeded to check for vacuum leaks with a propane torch, but none were found.  Maybe I should use something with a bit more zip, like starting fluid?

My next attempt was to richen the main jets from the original 69 to 71.  This along with the original 36B needles.  Upon reassembly and placing a hand over the horn, the idle momentarily increased.  Still lean?

To maintain my initial timing and target idle rpm, I have connected the distributor advance to ported vacuum. 

My questions are:

• What is your recommendation for richening the idle?
• What can I do to reduce the idle to allow for 750 rpm with manifold vacuum on the advance?
• Is it possible the carburetor has a small internal vacuum leak?

Thank you for your consideration,
-Brian

[429bbf]

welcome to CHP you need to measure the idle tubes and down channel channel restrictorsand the idle air bleeds.  .they play a very important part of what you are doing. i would start with receipe 2  for my idle . if you throw some sizes of that stuff out here someone will gladly help you.jets and needle do very litttle to fix your problem . hth.

[bry593]

Thanks, I had heard the idle circuit is rich on the 1901 and was hoping it would be less involved.  But, this will be good experience.  Just a little worried I'll ruin a good carb in the process.

I currently don't have gauges small enough to measure the diameters.  I'll check with air/gas and see if a tip cleaner looks suitable.

[429bbf]

remember the idle tube restrictions are down about 2 plus inches. your standard drill bit will not reach. fwiw

[bry593]

I ordered the 61-80 bit set and pin vise along with cliff's book.  Hopefully these will be long enough.  Will get everything measured as soon as the items arrive.

I read somewhere that the 1901 is actually supposed to have a .420 float level.  Is this correct?

[blazer74]

Standard set will not reach the restriction for the idle tube. You will need to remove the idle tubes to use those drill bits.

Cliff can supply new tubes also.

You can go to amazon and do  a search for taper length drill bit # 65 for example and buy them individualy, they don't always have all the sizes tho.

[Cliff Ruggles]

Main jets and metering rods are not part of the idle system.  Idle tubes, DCR's, and idle airbleeds are the main players.

Having no control over idle speed indicates a vacuum leak someplace, provided the throttle plates are all completely closed. 

I'd remove the baseplate and verify fully seated throttle plates, and if you still don't have idle speed control you've got a vacuum leak someplace.

The 1901 doesn't have a generous idle fuel set-up, and if you don't have full control with the mixture screws after making sure everything is in good working order, then you'll have to go in and modify the idle system accordingly.

I'd also set the float at 1/4" and forget about it.  Float settings effect fuel delivery, too high or too low can have an impact on tuning......Cliff

[bry593]

Not many vacuum taps on this 1970 Chevy Coupe.  I'll cap the PCV and advance port, and also the booster and TH400 modulator on the manifold and try again.  If no change, then will remove unit for closer inspection.

Pulling the throttle body is probably a good idea since it will allow me to check the gasket hole pattern as well as torque on the screws.  I assume the gasket is probably correct since both mixture screws function.

That is not good news about my drill bit index.  Would a "jobber" set be long enough?

[bry593]

Off topic:  I was little surprised to see a '75 Forumula on Cliff's home page.  Very, very few of these cars around.  I have one and have owned it since 1987.

[Cliff Ruggles]

No need to cap off the PCV, just remove the valve and put your finger over the end of it.  It adds a very small amount of air at idle when working correctly......Cliff

[bry593]

It will be a few days before I'm back on the carb.  I've got to get the dash back together.  Went in to fix the clock and blower switch, found some bad connectors and wiring. 

[bry593]

I am back on the carb and have drilled Recipe 1. 

First, I might say that it appears the Edelbrock carb is about as lean as possible.  The holes are much smaller in diameter than what Cliff recommends for a mild engine.  However, the body main air bleed is very large at .120".  I wonder if Edelbrock changes the size of the main airbed as a means to tune the entire system?

I haven't reassembled the carb at this point since I managed to lose my power piston spring, and I have a question for Cliff.  I had already drilled the airhorn main air bleeds before noticing the body bleeds were larger than Recipe 1.  What negative effects should I expect from a total main bleed of .07 + .12 over .07 + .07?  Should I try to plug the .120 and re-drill?  This is a large motor (413 CI) with a small cam, so maybe it's not an issue?

As I mentioned, I applied Recipe 1 and had to use both 0-60 and a 60-X drill sets.  I inserted drills into the holes until there was a no-go condition, and then assumed the previous size drill bit as the diameter.  A dial caliper was also used to record measurements before and after the holes were enlarged.  For the following data, the before diameters are listed first; followed by after and equivalent drill size:

• 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 ; .120 #31
• Main Air Bleed (in air horn): .050 ; .070 #50
• Primary Jet: .069 ; .071 #71
• Primary Metering Rod: .035 3EE; .045 45B
• 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; DA
• Air Flap Open Distance: 1.295; 1.295
• Secondary/Air Flap Spring: 7/8 ; 3/4 turns
• Choke Pull-off Release Time: 3s; 3s

[bry593]

I talked to Cliff and he said the .120" main air bleed in the body is going to cause issues with this large motor and small cam.  It will simply pull too much air.  I've ordered some brass plugs/restrictors and will install these to reduce body bleed to .050 and also change the airhorn bleed back to what it was originally, .050".  It was suggested that I return the lower idle air bleed (just above the throttle body plates) back to the original .050" as well.  The #71 primary jets might be replaced with #69, depending on how rich everything seems after the orifices are resized.  The APT screw is going to be made functional so I can adjust the power piston height.  As a final recommendation, the vac advance is being routed to ported vacuum.  This will allow me to open the throttle blades a bit at idle, making the idle circuit function better.

Hope to have the modifications made by end of next week.  With any luck, the 1970 Caprice will be on a roadtrip the following Saturday!   

[bry593]

Pulled the apt screw.  Was loc-tited in so had to heat the base with a torch for about 5 secs.  Came out easily while still hot.  Before removing the screw, I measured spring pin to base as a reference, was .118".  Will set it back to this height and later adjust to where it is just slightly lean around 2000 rpm using the hand over the venturis method.

Hope to see my parts by Wed.  Anxious to have the old Chevy going again.

[bry593]

I've made the recommended changes:

• Main air bleed (in body):  .050
• Main air bleed (in air horn):  .050
• Primary jet:  #69
• Idle bypass:  plugged
• Choke pull off rate:  2s

To change the choke pull off rate from 3.5s to 2s, the orifice was first drilled with the smallest bit in the set (.0135?).  This changed the rate to 1s, too fast.  I then silver soldered then port closed (metal housing) and then ground one side until I could just see a crack between the solder and the port.  This resulted in a 2s rate.

Main air bleed setscrew purchased from Cliff Ruggles:


Idle bypass plug, made from 1/4-20 UNC X 3/4 aluminum, pan head screw.  Purchased at ACE hardware, sectioned into two pieces, slotted with a Dremel and secured with red Loc-Tite.


Tuning the carburetor, I found max vacuum to be 16 in-Hg with idle screws backed out 3.5 turns.  Backing these out farther had only a slight effect on idle, however the vacuum began to drop.

Idle speed can now be controlled with the idle screw and it was set to 550 rpm in drive (about 650 in park).

I bought a new APT screw and spring from Cliff.  It is much longer than the original, so much easier to access with a screwdriver.  The screw was adjusted at 2000 rpm, and richened until a slight tilt of the choke flap caused no increase in rpm.  It was then leaned out until the tilt caused approximately a 100 rpm increase.  This was found to be 1/8 turn richer from the original setting.

A test drive revealed a noticeable improvement in primary circuit operation.  However, there is a slight hiccup (hesitation) when going into the secondaries.  Cliff, what would you recommend to address the secondary stumble?