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#1
03-25-2012, 11:23 AM
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Very rich at idle
I have a 1970 YH 455, bored .060, 6X heads ported professionally, comp cams XE284 with 1.65 Harland Sharp roller rockers. Factory 1969 RA exhaust manifolds, 2.5" SS Pypes w/ x-pipe through 18" race pro mufflers and tailpipes. Robbmc 1/2" sending unit, 1/2" SS fuel line and Robbmc 550HP 1/2" fuel pump feeding a 7040251 Quadrajet carb with 5psi. Stock HEI distributor, 8mm wires and R43TS plugs gapped at .046.
I have tried 2 different fuel pumps, 3 carbs and still have a very rich (eye burning) condition at idle. No change with any combo. Timing is at 18 degrees base and approx 36 at 3000 rpm. Makes alot of power off-idle. I have a 6AL box on the shelf. Can anybody tell me why this runs so rich at idle? I have been pulling my hair out. Any help or hint is appreciated. |
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#2
03-25-2012, 11:35 AM
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Too rich...........
Check the vacuum @ idle and adjust the power system to the vacuum produced. Most stock carbs and some modified carbs are still calibrated for oem vacuum of 17", or more. I'm guessing that you are around 11-13" vacuum, requiring modifying the carb's power circuit so that you are not idling/driving with an over rich condition.
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Les Iden ---------------------------- '65 Buick Sport Wagon Custom, 340, T350, 3:23 '66 GTO Post/468, 700R4, 3.31 (Mike's as of 9-16) '68 Grand Prix/455, dual AFBs, T400, 2:93 posi (sold) '72 TA tribute/461, T400, 3.08, (Russ's as of 9-16) '97 Mitsubishi Eclipse Spyder Turbo, Konis, 5 speed '09 Torrent GXP, nav, Sun & Sound pkg., Bilsteins |
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#3
03-25-2012, 11:36 AM
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Quote:
Karl 
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#4
03-25-2012, 11:46 AM
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If its a stock carb, the primary plates may be open too far. Will it run if the idle adjustment screws are in all the way? If it does, you are idling off the primaries not the idle circuit. Your primary plates are open too far. Other thing to check is if the power piston is seated at idle, you can do this with engine running by using a small screw driver through the vent tube on top. Been through this many times....get Cliff's book its been a big help for me.
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#5
03-25-2012, 01:25 PM
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Quote:
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#6
03-25-2012, 03:34 PM
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very rich at idle
Wow! Thanks for all of the prompt comments.
Made me think of a few more things I have done. Vacuum at idle is 5". Installed the yellow power piston spring so it stays seated. Works. Carb has #70 primary jets, 49 main rods, AU secondary rods. Purchased from Carbs Unlimited. I have epoxyed the plugs, trued up the mating surfaces where the top meets the body, set float at 3/8" down. I have tried to adjust the mixture screws in and out to get the idle down. I can get it to idle at around 800 rpm with the screws about 2 turns out. I don't think it quit when I turned them in all of the way. I know the throttle blades are above the transfer slots. The rpm's drop to about 500 or 550 when in gear. Constantly have to keep on the pedal to keep it from dying. Thanks again everybody. PY ROCKS! |
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#7
03-25-2012, 04:23 PM
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Quote:
Try 74's and a 39 to 43 rod to start Ste float to 1/4" with the Nitrophyl float Your bypass air should be about .109" Over all your too lean and you've already said your blades are above the transfer slot which should tell you your out of control. If you have the yellow Edelbrock spring that will open at 8"Hg of vacuum and if you have only 5"Hg of vac then the power piston is open all the time. The Gold spring is good for 4"hg vac but once you have the carb straightened out your most likely going to make more vacuum.
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Carburetor building & modification services Servicing the Pontiac community over 25 years |
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#8
03-25-2012, 06:42 PM
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Do not install larger jets in that particular carburetor, they use small main airbleeds and are fine with small jets in them. We usually set them up with .068-.070" main jets and .039-.043" metering rods, depending if we can get the APT system in the baseplate free'd up or not?
The light power piston spring has NOTHING to do with the problems you are experiencing. The vacuum at idle is too low for the carbs current idle set-up. Go after the idle system to increase fuel flow to the mixture screws, our book covers what to do there. Raise the float back up, 3/8" is WAY too low for one of those units. The 70 Old's carburetor uses the early float/fulcrum arrangement. They are not the best choice for what you are doing, (the float is too big and has poor leverage on the needle) but can be made to work OK with a superb fuel delivery system, regulator, and large inlet seat......Cliff
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If you can read this, thank a teacher. If you can read this in English, thank a Veteran! https://cliffshighperformance.com/ 73 Ventura, SOLD 455, 3740lbs, 11.30's at 120mph, 1977 Pontiac Q-jet, HO intake, HEI, 10" converter, 3.42 gears, DOT's, 7.20's at 96mph and still WAY under the roll bar rule. Best ET to date 7.18 at 97MPH (1/8th mile), |
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