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Old 09-24-2013, 09:41 PM
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Edelbrock Carb Secondaries Bog

I bought a remanufactured Edelbrock Thunder AVS 650 CFM carb from Summit Racing. The motor is a mildly built 350 with about 2000 miles. Runs great except any time I give enough throttle for the secondaries to kick in. The motor falls flat on its face-- it feels like I'm letting off the throttle. It will never recover unless I ease off the secondaries. It happens under any RPM, speed, and doesn't matter if I punch it or ease into it. It has been this way since day one.

I've tried richer jetting, leaner secondary jets, cleaning, adjusting air valve rate, checking for vacuum leaks, and checking float levels. Fuel pressure is at 6.5PSI.

Anyone heard of something like this?

Thanks.

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Last edited by SBCWes; 09-24-2013 at 10:10 PM.
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Old 09-24-2013, 09:53 PM
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Originally Posted by SBCWes View Post
I bought a remanufactured Edelbrock Thunder AVS 650 CFM carb from Summit Racing. The motor is a mildly built 350 with about 2000 miles. Runs great except any time I give enough throttle for the secondaries to kick in. The motor falls flat on its face-- it feels like I'm letting off the throttle. It will never recover unless I ease off the secondaries. It happens under any RPM, speed, and doesn't matter if I punch it or ease into it. It has been this way since day one.

I've tried richer jetting, leaner secondary jets, cleaning, adjusting air valves, checking for vacuum leaks, and checking float levels. Fuel pressure is at 6.5PSI.

Anyone heard of something like this?

Thanks.
Have you tried adjusting the secondary opening rate? It sounds like they are opening to early/fast.
Try going half a turn CC to slow down the opening and see if that's helps.
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Old 09-24-2013, 10:09 PM
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Originally Posted by bygddy View Post
Have you tried adjusting the secondary opening rate? It sounds like they are opening to early/fast.
Try going half a turn CC to slow down the opening and see if that's helps.
As in the air valves on the secondaries? I tried tightening them up even to a whole turn more and there was maybe a slight improvement but still backfiring/popping and no power.
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Old 09-25-2013, 12:11 AM
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Edelbrock Carbs always run best at 4.5-5 psi fuel pressure 6.5 is too much,it ain't a Holley.
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Old 09-25-2013, 04:42 AM
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I'm more familiar with the Edelbrock Performer than the Thunder, but on the Performer you can usually fix a bog like that by swapping step-up springs on the rods. The wrong spring brings the secondaries in too late and you get a bog. Switched to higher rated springs (6", 7", 8") usually helps a lot to get rid of the bog because they kick in faster as the vacuum drops.

Bruce
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Old 09-25-2013, 06:14 AM
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hi are you running an automatic? if you are the trans must down shift at the same time the carb is opened up or the motor will bog! it is a common problem with a turbo 350 trans and a miss adjusted kick down cable.
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Old 09-25-2013, 07:19 AM
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Originally Posted by carbman View Post
hi are you running an automatic? if you are the trans must down shift at the same time the carb is opened up or the motor will bog! it is a common problem with a turbo 350 trans and a miss adjusted kick down cable.
wth?


Anyway, try adjusting your accelerator pump, its a very simple design but many people overlook it on the edelbrock carbs. Also, don't lean out your secondaries.

If it really is a very mild build then I would just go one step thinner on the second step of the metering rods, run the stock jets, and run the stock springs. Make sure your accel pump is working properly and you should be good to go.
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Old 09-25-2013, 07:42 AM
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wth? when you kick the secondaries in if the the trans is not down shifting the motor will bog because it cant rev! he said it bogs when he kicks the secoundaries in! it is not the accelerator pump! it would stumble on off idle acceleration! i have 40 plus years of carb building and tune up exp.

Last edited by carbman; 09-25-2013 at 08:11 AM.
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Old 09-25-2013, 07:51 AM
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You're all beating on the wrong part (the carb).
Timing!!
Rule #1.
Timing first, carb second!!
Bump your initial (base) timing up to 14-16 degrees and see if the bog goes away.
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Old 09-25-2013, 08:33 AM
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your right rwenuts!
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Old 09-25-2013, 11:28 AM
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Does it bog momentarily and then recover and run hard? or just bog, and not recover until you get off the throttle?
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Old 09-27-2013, 09:21 PM
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Does it bog momentarily and then recover and run hard? or just bog, and not recover until you get off the throttle?
It bogs and doesn't recover until I get off the throttle.

I tried playing with timing but no change.

However, last night I had more time to fiddle with the air valve adjustment as well as secondary jets and I was able to get a small improvement. Maybe it is just fine tuning after all...

I had figured that such an issue as I'm experiencing could only be some sort of defect/broken part or installation mistake and not simply a matter of fine tuning.

I REALLY appreciate all the advice and replies. Thanks! I'll let you know when I get it straightened out.
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Old 09-27-2013, 10:31 PM
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I think you have a ignition problem.
Start by installing a ground wire from the engine to the car body. Then verify supply voltage.
both are critical on a GM HEI.
Then inspect the secondary side of the ignition system plugs,wires cap rotor etc. . New parts are highly suspect.
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Old 09-27-2013, 10:51 PM
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Quote:
Originally Posted by F-BIRD'88 View Post
I think you have a ignition problem.
Start by installing a ground wire from the engine to the car body. Then verify supply voltage.
both are critical on a GM HEI.
Then inspect the secondary side of the ignition system plugs,wires cap rotor etc. . New parts are highly suspect.
Distributor is new MSD pro-billet. One thing that comes to mind is the built-in rev limiter it has. The bog I'm experiencing isn't tied to a certain RPM but maybe its malfunctioning?

I'll check over that and let you know what I find.
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Old 10-17-2013, 09:25 PM
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Originally Posted by F-BIRD'88 View Post
I think you have a ignition problem.
Start by installing a ground wire from the engine to the car body. Then verify supply voltage.
both are critical on a GM HEI.
Then inspect the secondary side of the ignition system plugs,wires cap rotor etc. . New parts are highly suspect.
Hi again guys,

I still haven't fixed the bog issue. I developed a misfire as of late so my focus shifted. I noted that my tach would bounce every time it misfired so I scoped the ignition module. The pattern was definitely bad so I tested the p/u coil and power/ground to module. They were both good so I called summit and they are letting me trade it in for a new one. (very thankful for summit)

Maybe the bog is ignition related but I can't exactly reason why it would be as it is definitely tied to secondary venturi operation... I'll let you guys know.
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