Figure this one out - Team Camaro Tech
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post #1 of 7 (permalink) Old Jun 30th, 10, 12:03 PM Thread Starter
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Kelly
 
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Location: cary, NC
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Figure this one out

Alrighty, I've got a '67 SS with a 396BB. On top is a holley 750cfm (4160 model) w/vacuum sec. Carb is freshly rebuilt. Float bowls barely dribble out of sight plugs as prescribed. Idle mixture adjustment screws are set for max vac (non-ported of course) at idle, I get about 16"hg. Vac secondary is hooked up to ported vacuum, all other ports are capped. Timing is about 12*btdc and 36* total all in by about 3000rpm. Great fuel pressure (about 6.5 psi). Mild cam installed, otherwise stock.

Car runs like doo doo. Starts and idles fine, but hesitates and lacks power on acceleration

What is interesting is that when I lower the secondary float bowl to well below the sight plug hole, the car runs better. It accelerates better and hesitates less, not great but better. Bring it back up to the bottom of the sight plug, back to doo doo.

could it be that 750cfm is way too much for the car? I've used a few cfm calculators and they all target me around 650 cfm (actually they recomment 630 cfm, but I don't think those exist).

Thoughts?
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post #2 of 7 (permalink) Old Jun 30th, 10, 12:59 PM
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Jess
 
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Re: Figure this one out

I would double check your timing. Then I would check see if your distributor is up too snuff. Also, did you rebuild the carb or someone else do it. Mkae sure there is not any dirt or gunk blocking any ports inside the carb. Also check your jettiing.... Just a few things to check...

Jess67
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post #3 of 7 (permalink) Old Jun 30th, 10, 01:02 PM
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Harry
 
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Re: Figure this one out

well its plain to see that your carb is-

check accelerator pump shot linkage for proper settings


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post #4 of 7 (permalink) Old Jun 30th, 10, 01:14 PM
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Lane
 
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Re: Figure this one out

Hey Kelly

I've run 650 and 750 successfully on small blocks so would think you are ok on carb size...

I would make sure Power valve is good and sized properly and that acc pumps work and are set properly...make sure the choke allows the secondaries to open..

Cut and paste follow

After you've read the engine vacuum, install a power valve that is at least two or three full steps down from your reading. As an example, if your engine vacuum reading is 12", you should install at least a 9.5- or 8.5-power valve or even a 6.5 as supplied with most kits.

http://www.chevyhiperformance.com/te...ips/index.html

Seen this issue posted before but never played with it myself...(might try a camaros.net search)

http://www.dfwstangs.net/forums/showthread.php?t=25458

You really need to pickup a book on Holley tuning. That being said, take the Holley off and flip it over, at the butterfly bores you will find a small hole and a slot, the idle air setting on the butterfly’s should be at the beginning of the slot, not the end of it. Cams with lots of duration require more air than lots of carbs are designed to flow at idle. That's why small holes are sometimes drilled into the butterfly’s, to let in more air so you can close up the butterfly’s with the idle screw and close off some of the idle transfer slot. This slot feeds fuel to the motor when the butterfly’s open to get the motor past idle into the venturi's flowing fuel. If you have used up most of the slot just to get idle, it has nothing left to use to get the main system, (the venturi) flowing fuel. The Holley tuning book shows all of this and, for a racer is required basic knowledge.
The idle mixture screws should be adjusted to 1 1/2 turns out.
These screws add a varying amount of air to a fixed amount of fuel. The idle fuel jets are drilled into the metering block. It's hard to tell rich form lean sometimes but on a holley but this is easy because they put the accelerator pump so easy to get to, with the motor running at idle, just pump the squrter some and listen to the motor, if the idle is correct, the rpm won't change, if it's lean, it will speed up, if rich, it will try to stumble.
Sometimes, after you have drill holes in the butterfly’s, you may have to richen the idle feed jets, these are a drilled pressed in jet in the metering block, a finger miniature drill and a drill index that goes from 60 - 80 on drill sizes is perfect. You have to drill a tiny amount and test till the mixture screws are 1 1/2 out and the idle is slightly rich.
The correct fuel level on a Holley is with the fuel at the bottom of the threads of the level check screw hole, not coming out the hole. The most fuel pressure the Holley is designed for is 7.5 lbs.
Some will hold more but not reliably.
The correct setting of the accelerator pumps is done at the screw and nut with a spring on it at the end of the arm. At idle the arm should have no slack in it so that when you give the throttle gas, it makes the squrter pump gas out with no delay.
Squirter tuning is MUCH harder. It sometimes requires a plastic cam set and an assortment of Holley squrters. If you have done or checked all the above things and the carb still bogs some , then you have to fine tune the squrters. For me this is trial and error, when the motor bogs, it can be either too rich, or too lean, I change the squrter and see if the problem is better or worse, an example of this would be having a #25 squrter and a bog and going to a #30 and the bog got less, this would mean that it was too lean with a 25 and the larger 30 helped it. Keep going larger till the bog is gone. I have to change squrters for winter or summer because the air temp is so different. Colder is denser air and you may have to richen.
I hope this helps some one. let me know if you have easier ways to do all these things. Besides EFI.
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post #5 of 7 (permalink) Old Jun 30th, 10, 03:59 PM
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Jeff
 
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Re: Figure this one out

What jets are you running? 750cfm is a lot of carb unless that 396 is built to the hill
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post #6 of 7 (permalink) Old Jul 1st, 10, 12:11 PM Thread Starter
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Kelly
 
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Re: Figure this one out

Here's an update. I mentioned before that the car ran slightly better when the secondary float boal level was cranked way down. So anyways, I reset the float bowls, idle mixture, etc. all back to holley specs. Then I took of the dist cap and holy !#$! batman! The rotor contact was arced to hell with a slight layer of rust. So, not having a fresh replacement on hand I went ahead and sanded the rotor contact to clean it up as much as possible, took her for a drive, and got muuuuuuch better performance. I ordered a new cap/rotor kit from summit and it'll be here tommorow. Hopefully that was it.

I suppose all of that corrosion/shmutz on the rotor and dizzy contact points was really reducing the spark being sent to the plug.
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post #7 of 7 (permalink) Old Jul 1st, 10, 08:21 PM
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morris
 
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Re: Figure this one out

sounds like a jetting problem to me look at your plugs
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