Carb issues on 427/390
Thread Starter
Instructor
Joined: Feb 2010
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Carb issues on 427/390
All, I have a '69 L36 with a Holley 800 cfm Double Pumper and a quasi racing cam (that I don't know the specs on). When I punch the throttle between 2250-2750 RPM, I get a quick stumble, cough or hesitation. I've put a 750 power valve on both primary and secondary, and that doesn't work either, figuring I'd get more gas quicker. Changed accelerator pumps via kit, no change. On the high end, the engine roars, no issues at all. Upgrade accelerator pump?Carb too big? Go back to Rochester? If so, which one? Any other ideas? THANKS!!!
Le Mans Master
Joined: Jul 2005
Posts: 5,328
Likes: 405
All, I have a '69 L36 with a Holley 800 cfm Double Pumper and a quasi racing cam (that I don't know the specs on). When I punch the throttle between 2250-2750 RPM, I get a quick stumble, cough or hesitation. I've put a 750 power valve on both primary and secondary, and that doesn't work either, figuring I'd get more gas quicker. Changed accelerator pumps via kit, no change. On the high end, the engine roars, no issues at all. Upgrade accelerator pump?Carb too big? Go back to Rochester? If so, which one? Any other ideas? THANKS!!!
Without knowing the cam specs, it's hard to give a good recommendation. What RPM does it idle at and how rough is the idle? Those are usually good indicators to the general specs of the cam.
I don't think going back to a Rochester Q-Jet will work. The stock Q-Jet is good up to ~400 HP while a few simple mods will make it good up to ~500 HP. Again, without knowing the cam specs it's hard to determine how much HP the engine may be making. What RPM does it pull to and what kind of intake manifold do you have? That info will help determine the HP range of the engine.
Le Mans Master
Joined: Jul 2005
Posts: 5,328
Likes: 405
With the NOS dual plane intake, I doubt you'll be over 500 HP with the cam you have. If it was my car (and it isn't), I would go to Jet Performance (http://www.jetchip.com/JET_Carburetors.asp) and order a Rochester Q-Jet...the only problem is they will want the cam specs so you'll have to pull a valve cover to get intake duration/lift/CL and exhaust duration/lift/CL. A properly set-up Q-Jet will perform just as well as a Holley and get much better MPG...the best thing as far as you're concerned is the Q-Jet part throttle drivability will kick the crap out of the Holley, throttle response will be very sharp. Few people know the Q-Jet has an accelerator pump/"shot" for the secondaries. You'll see two little holes in the air horn just above the leading edge of the air valve...as the air valve starts to open, the air flowing across the opening "pulls" a shot of fuel out of the secondary fuel well. There's not really an accelerator pump, it acts on the same principle as a venturi to pull a shot of fuel into the airstream to cover the fuel demand until the main nozzle starts to flow...once the valve opens fully, there is no more venturi affect.
Le Mans Master
Joined: Oct 2006
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From: Gladstone MO
C3 of Year Finalist (appearance mods) 2019
Check the timing of the secondary pump shot. It should start to squirt as soon as the secondary throttle plates start to move.
Get a copy of Holley's installation instructions and set it up according to that. Assuming your power valve is good, it may be not quite correct for your engine based on the 1/2 engine idle vacuum plan. 800 CFM is not a bad choice for a 427- GM used an 850 on the L-88. Bottom end is a little soggy, but once it gets some RPM it's ready to go.
I'm running a cam with about 270* duration and .600 lift- my idle is around 900RPM.
Get a copy of Holley's installation instructions and set it up according to that. Assuming your power valve is good, it may be not quite correct for your engine based on the 1/2 engine idle vacuum plan. 800 CFM is not a bad choice for a 427- GM used an 850 on the L-88. Bottom end is a little soggy, but once it gets some RPM it's ready to go.
I'm running a cam with about 270* duration and .600 lift- my idle is around 900RPM.
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Advanced
Joined: Feb 2006
Posts: 54
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From: Reading Pa
There is a replaceable cam that actuates the secondary accel pump. You can buy an assortment of cams in a kit. Put a faster (richer) cam in and see if it makes it better or worse. It will probably make it worse because of the big cam on a duel plane intake, but you want to err rich not lean (lean could cause a backfire and blow the power valves out of the carb). If it gets worse then put a slower cam on to lean out the accel pump. If better fut the next faster cam in, you get the idea. A Qjet probably would have better drivability but it would have to be tuned correctly. I've had some pretty big cams under Qjets and holleys. If you can get it to idle at 800-900 you should be able to tune it for really good drivability. My recommendation is to change the secondary accel pump actuator cam to solve this issue.
Melting Slicks
Joined: May 2002
Posts: 2,717
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From: Sulphur LA
St. Jude Donor '05-'06,'11,'13-'14,'16,'18,'19,'24, '25
can you provide the list # for the carb?
At 2200 to 2700 rpm your still on the idle/transition circuit, the main circuit has not started flowing. You are more than likely at the 9 to 12% throttle position opening which you should still have primary pump shot left along with secondary pump shot.
What color pump cam are you using on the primaries and does the carb still have the 30cc pump on the primaries?
What shooter nozzle size are you using on the primary and secondary?
Have you checked the transition slot setting and if so is it adjusted correctly?
Did you put the carb on new?
Neal
At 2200 to 2700 rpm your still on the idle/transition circuit, the main circuit has not started flowing. You are more than likely at the 9 to 12% throttle position opening which you should still have primary pump shot left along with secondary pump shot.
What color pump cam are you using on the primaries and does the carb still have the 30cc pump on the primaries?
What shooter nozzle size are you using on the primary and secondary?
Have you checked the transition slot setting and if so is it adjusted correctly?
Did you put the carb on new?
Neal
Burning Brakes
Joined: Feb 2012
Posts: 925
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Is there slop in the accelerator pump linkage that sits on the plastic cam? It sounds like the engine needs fuel, you hit the throttle and it goes lean momentarily until the accelerator shot comes in. I think this is what most have posted more or less. The length of the vertical link with the spring wrapped around it affects both the amount of fuel and the timing of the shot. If the vertical link is too short, the lever that rides on the plastic cam is not in contact with it until the cam and throttle have moved through a sizable arc, by which time the engine goes lean and that's the bog you feel. Usually the cams are mounted to the throttle linkage with 1 screw, but there are 2 holes in the linkage, so you can advance the cam forward and it engages the follower sooner. Between the 2 adjustments, you should be able to tailor the pump shot and the timing of it.
Burning Brakes
Joined: Feb 2012
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