Just put a new engine together and if you step into it at mid range it just lays down, if you let up and ease into the throttle it picks up speed. Here's the history, I assembled the original motor (a 327 SBC bored .010", 10.25 Speed Pro pistons, old used Crane Cam and 042 heads) 10 plus years ago, finally got it running last year and it had issues fouling number one plug and low compression -- it varied from from 115 pounds to 135 pounds. It would spin to 6 grand pretty easily but I knew something wasn't right. I pulled the engine, found minor scoring in all of the cylinders (I should have oiled the heck out of the engine before firing the first time after sitting for 10 years), took the heads to my machinist, he determined that the valves were shot, so he put in new valves, pushrod guides etc. I purchased a Howard Roller Cam, roller lifters, roller rockers, new pushrods etc. I bolted it all together using the same Edelbrock CB3X/Carter AFB carburetor and stock distributor with a Pertronix kit. Compression is 195-200. I've got a good neighbor (that means more skilled than me) and we've adjusted the timing, both by ear and with a light ending up with 36 degrees total advance but no matter what we have done with timing the flat spot in acceleration remains. I've also check the metering rods and springs in the carburetor and they are operational, at least I can move them. My suspicion is I have an ignition problem but.....I am open to suggestions.
Do you have vacuum advance and where is it connected to? Is your accelerator pump working properly , or providing enough fuel?
Very first thing is verify the marks are correct with a piston stop test or throw the timing light back in the box. The numbers and light are useless is the marks are off. Don't expect a 140 cranking compression 327 engine to have the very same needs as a 327 with a 200 cranking compression. It's going to need lots more fuel when you mash the loud pedal. From your first paragraph description id be looking at the accelerator pump. Rule it out first, it's easiest. Then be sure you're step up springs are working. They provide more fuel when the vacuum drops due to throttle opening. Loss of power tied to vacuum drop is generally a carb issue - should happen at any rpm the throttle is opened. but also drive with no loss of power at any rpm providing there's no drop in vacuum. You may have a lean miss. ( the power loss will be in lean condition because there's not enough fuel to burn.) An ignition related loss of power is usually accompanied by popping sputtering missing at a given rpm regardless of throttle position. It will be in rich condition because the present fuel isn't being burnt. If everything else is right- A few degrees of timing issue will not cause a car to lay down when the throttle is opened. You might be leaving a little power and efficiency on the table but not loosing. The vacuum advance pot and weight need to be working correctly. The amount of advance provided by the vacuum should drop off and rely on the mechanical advance weights. Sometimes this stuff gets sticky
Your carb is going lean. You'll need different springs for the metering rods or different metering rods.Edelbrock make a calibration kit that will have what you need and instructions to help.
Speaking of mech. advance weights, make sure they are moving freely so they can "advance" with rpm increases. Moroso has a spring kit that allows advance to come in quicker. One silver spring and one blue spring, I believe is used alot.
Wow....thanks for all of the input and good suggestions on possible solutions. I had ran the carburetor on BBC 20 years ago, it ran ok and pulled hard through the RPM range, but I ended up replacing it with an Edelbrock which left the Carter to be used on future projects, I will have to see if I have a carburetor on the "shelf" that I can do a replacement with, Thanks to all that have taken the time to help me resolve the problem.
20 year old carb? I'm amazed it runs at all. I'd say there is a High probability things like check balls are varnished up, and the accel pump (leather, in the old AFBs) are dried out. With the engine and air cleaner off I'd stroke the throttle linkage and watch for squirts from the accelerator pump. http://forums.aaca.org/topic/205174-afb-accelerator-pump-not-pumping-with-engine-runningargh/
I should have stated earlier that the accelerator pump squirts quite well, I've had the darn thing apart and cleaned it more than once, if I could figure out which calibration kit to use (I am not sure which Edelbrock kit fits this old Carter- it has 2639 9G 5S on the base plate) I'd probably take a shot at just trying to change rods/springs and jets. It may be that I have some mechanical advance problem (the vacuum can seems to be working) as well as a fuel mixture problem. I just hate to throw money buying a new carburetor and distributor when it may be a small issue or two that is creating the problem
You might contact "Bubba's" ignitions. He might be able to spin yours and set it up for your application. Check the hamb o dex for his contact info.
Also look at the accelerator pump timing. The pump stream should start as soon as the throttle is moved off-idle. And it shouldn't stop too soon on its way to wide open throttle. But I don't know how much you can do to adjust this on an AFB. I've had AVS carbs that had pump stream from idle all the way to wide open.
disconnect the vacuum advance and plug the pipe. Go for a drive. If the problem is still there,,it's not the vacuum advance. If the mechanical weights are stuck, the problem would be all the way through the rpm range,not just when you accelerate. Stay focused on the carb is my suggestion.
what condition is the inside of gas tank in - ever looked in it? if if fuel pick up has a sock on it could be clogging up on acceleration. if fuel pick up is on bottom of tank there may be debris loose on bottom that blocks tube on acceleration, then relaxes when you let off.
I'm in the carb camp also, having had the same problem you're describing. I had a .30 over 327 with cam and stock carb. It did the same thing until I had the carb rebuilt. It ran great after that.
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