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Occasionally when Im at a 20-30mph roll and accelerate hard pressing the throttle past 1/2-3/4 (opening all 4 barrels), I get a big hesitation. What should I be looking for? Timing in need of adjustment? Carb adjustment? I have an Edelbrock Thunder Series AVS, so could it be the secondaries needing to be tuned? Could it be from running 93 octane with the 11.0:1 compression? Any other ideas?

Please let me know. Thanks!


P.S Ive probably only opened all 4 barrels 6-7 times max where I only have about 400-450 miles on the motor, so maybe it was just a fluke? Its happened 2 times out of the 6-7 I think....:huh:
 

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Occasionally when Im at a 20-30mph roll and accelerate hard pressing the throttle past 1/2-3/4 (opening all 4 barrels), I get a big hesitation. What should I be looking for? Timing in need of adjustment? Carb adjustment? I have an Edelbrock Thunder Series AVS, so could it be the secondaries needing to be tuned? Could it be from running 93 octane with the 11.0:1 compression? Any other ideas?

Please let me know. Thanks!


P.S Ive probably only opened all 4 barrels 6-7 times max where I only have about 400-450 miles on the motor, so maybe it was just a fluke? Its happened 2 times out of the 6-7 I think....:huh:
Sounds like it's going lean. You're opening the throttle faster than the fuel can keep up with the air. Maybe a slight increase in the accelerator pump shot will make it run right 7 out of 7 times. If it doesn't happen all of the time, it may just be those 2 times the rate at which you opened the throttle was faster than the times you didn't notice anything. Up the stroke of the accelerator pump a little, that would be my first move not knowing anything else about the car.

How is the timing set up?
 

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Sounds like it's going lean. You're opening the throttle faster than the fuel can keep up with the air. Maybe a slight increase in the accelerator pump shot will make it run right 7 out of 7 times. If it doesn't happen all of the time, it may just be those 2 times the rate at which you opened the throttle was faster than the times you didn't notice anything. Up the stroke of the accelerator pump a little, that would be my first move not knowing anything else about the car.

How is the timing set up?
Thats funny, I actually noticed that it seemed to be running a bit lean. Im willing to bet thats probably what it is. How do you adjust the accelerator pump? I know theres the two mixture screws at the front of the carb, for the left and right side....

Im honestly not sure what the timing is set at, the engine builder set it. I want to say its like 10*, but Im not sure...I dont really understand the whole timing thing. I read through this last night, http://www.digitalcorvettes.com/forums/showthread.php?t=75830

Once I adjust the carb I think Ill try adjusting the timing like he describes.
 

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Once I adjust the carb I think Ill try adjusting the timing like he describes.
you'll want to switch that around to avoid chasing your tail. Get the timing set per Lars' paper first, and then tweak the carb. Pick up a good dial-back timing light to make the process much easier.
 

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Thats funny, I actually noticed that it seemed to be running a bit lean. Im willing to bet thats probably what it is. How do you adjust the accelerator pump? I know theres the two mixture screws at the front of the carb, for the left and right side....

Im honestly not sure what the timing is set at, the engine builder set it. I want to say its like 10*, but Im not sure...I dont really understand the whole timing thing. I read through this last night, http://www.digitalcorvettes.com/forums/showthread.php?t=75830

Once I adjust the carb I think Ill try adjusting the timing like he describes.
Kevin,

I haven't seen a good photo of this carb, but the Aaccelerator pump appears to be at the left-front. You're going to see a lever riding on a cam profile on the throttle shaft most likely. You are looking to increase the stroke of the pump, which may be achieved by lengthening a rod, lever, or adjustment screw. Try to post a shot of the front corner of the carb and throttle linkage. Do you have any paperwork that came with the carb?

That's a great article on timing. Follow that when you get ready.

How's the Sweet Thunder department?
 

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Kevin,

I haven't seen a good photo of this carb, but the Aaccelerator pump appears to be at the left-front. You're going to see a lever riding on a cam profile on the throttle shaft most likely. You are looking to increase the stroke of the pump, which may be achieved by lengthening a rod, lever, or adjustment screw. Try to post a shot of the front corner of the carb and throttle linkage. Do you have any paperwork that came with the carb?

That's a great article on timing. Follow that when you get ready.

How's the Sweet Thunder department?
Fred,

Heres the owners manual for my carb (Thunder Series AVS 650cfm #1806)

http://www.edelbrock.com/automotive_new/mc/carbs_acc/pdf/carb_owners_manual.pdf

Page 20-21 have a diagram of the carb and list each part. There is a Pump mentioned on these two pages, Im not sure if its the one youre refering too. Now that I think of it though I think I may know what youre talking about.... looking at the diagram on page 20 it would be around parts 19 and 20 right?

I had the car out all day today doing some cruise hoping. It seemed to run great and didnt have any hesitation issues. I plan on getting it to a local performance shop once they get there new dyno in. The guy there is supposedly the best around for tuning carbs. I may just leave it until then unless the issue returns.

The Sweet Thunders are sitting in my living room lol. I cant get the old glasspacks out so Im going to have to send the pipes back to Josh and have him try to get them out. This is probably another reason to hold off on adjusting the timing and carb....it would probably be better to do once I get the new mufflers in right?

EDIT: Just came across this on page 8:

"CALIBRATING THE PUMP
If you encounter any hesitations or stumbles that do not seem to be related to the basic metering or have not responded to changes in the basic metering, move the
pump drive link to one of the holes closer to the carburetor body. This will increase the stroke length of the plunger and result in more pump delivery."

So it sounds like adjusting the pump is pretty easy... but, it says to adjust it only if changing the basic metering did not resolve the issue. Is that something I should try first, or should I just try moving the pump drive link and see what happens?

you'll want to switch that around to avoid chasing your tail. Get the timing set per Lars' paper first, and then tweak the carb. Pick up a good dial-back timing light to make the process much easier.
Thanks Jason. I was going to actually ask that because I figured there was an order it should be done in, and that I probably had it wrong :smack
 

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Fred,

Heres the owners manual for my carb (Thunder Series AVS 650cfm #1806)

http://www.edelbrock.com/automotive_new/mc/carbs_acc/pdf/carb_owners_manual.pdf

Page 20-21 have a diagram of the carb and list each part. There is a Pump mentioned on these two pages, Im not sure if its the one youre refering too. Now that I think of it though I think I may know what youre talking about.... looking at the diagram on page 20 it would be around parts 19 and 20 right?

I had the car out all day today doing some cruise hoping. It seemed to run great and didnt have any hesitation issues. I plan on getting it to a local performance shop once they get there new dyno in. The guy there is supposedly the best around for tuning carbs. I may just leave it until then unless the issue returns.

The Sweet Thunders are sitting in my living room lol. I cant get the old glasspacks out so Im going to have to send the pipes back to Josh and have him try to get them out. This is probably another reason to hold off on adjusting the timing and carb....it would probably be better to do once I get the new mufflers in right?

EDIT: Just came across this on page 8:

"CALIBRATING THE PUMP
If you encounter any hesitations or stumbles that do not seem to be related to the basic metering or have not responded to changes in the basic metering, move the
pump drive link to one of the holes closer to the carburetor body. This will increase the stroke length of the plunger and result in more pump delivery."

So it sounds like adjusting the pump is pretty easy... but, it says to adjust it only if changing the basic metering did not resolve the issue. Is that something I should try first, or should I just try moving the pump drive link and see what happens?



Thanks Jason. I was going to actually ask that because I figured there was an order it should be done in, and that I probably had it wrong :smack

Kevin,

The parts associated with the accelerator pump are 15,20,21 on page 20 and 29,30 on page 21.

This problem didn't happen all of the time and the variable there is how fast you put your foot down each time. When you make a sudden throttle position change, the metering system can't respond instantly, so all of a sudden there is a ton of air, but you don't have the fuel to keep up with it. That's where the accelerator pump comes in to supply a shot of fuel until the metering system catches up. The basic set-up is probably fine, as your saw in your last cruise. If you put your foot down gradually, you probably won't see a problem again. Being a hot rodder, I would change the pump stroke at the linkage as they describe. If you don't see a change or you see a load of black exhaust when you get on it, you can always change it right back.

This leaning out will happen whether you re-set the timing or not. If you plan on tackling the timing, Jason is right and do it first. If changing the muffler inserts results in a big reduction in backpressure, that will also lean out the carb, then you're into the metering rods to set it up right.
 

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If I remember right I had one of those carbs on my 71 Challenger. You should be able to buy a jetting kit that will increase fuel flow. It's simple to do. There should be 2 small access flaps on top of the carb. The kit had a variety of jet sizes. Sounds like you need more fuel flowing. Timing with a combination of re-jetting the carb should get you where you need to be.
 
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