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Old 07-25-2008, 08:18 PM
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carb tuning help needed

After installing a 1/2" phenolic carb spacer to avoid percolation, my throttle response has been altered much more than I expected. The car nows bogs heavily upon full acceleration. I am running a 9.9 static/8.5 dynamic CR gen I 355, Brodix IK 180 heads, Perfromer RPM dual-plane intake, Holley 650 cfm double pumper (with choke tower cut and smoothed out), and Hedman 1.625" long tube headers which expel through 2.5" Dynomax mufflers. The cam is a Lunati Voodoo 227/233 and the carb had a set of 69s up front, 77s in the rear, and a 6.5 power valve. The squirters were 0.031"; switching to 0.035" and then 0.037" incrementally helped lessen the bog time. It seems a little too obvious to think that I should move up to 0.040" or larger, especially since the car accelerated so crisply without the carb spacer and using the 31 squirters. [The plugs look a little black and wet to me, before and after any changes.] My question is (finally) "am I tuning the carb incorrectly"?
Thanks in advance

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Old 07-25-2008, 08:39 PM
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1/2 inch isn't much, with your combo I wouldn't think you'd have to alter the shooter size that much, 31 should be plenty. I've got similar stuff on a 383 and the stock shooter works fine, what you probably need to play with is the pump cams, might need the shot to come in sooner, I know I had to play around with mine a bit. And the obvious, have you put a vacuum gauge on it to see if anything changed drastically? What's the timing? I think I'd put the 31 shooter back in and try a different position on the primary pump cam, try the second hole.
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Old 07-25-2008, 08:42 PM
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Thanks Greg, I was thinking I might be moving too far in one direction. BTW the vacuum taken at the manifold was about 14" in gear at 800 rpm.
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Old 07-26-2008, 05:08 AM
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carb

i would first change that 6.5 power valve to a 5.5 or a 3.5. in my 358 motor i run a 650 vac.sec carb that was built by circle track carbs out of ventura california. and i was running a 6.5 power valve and kept dealing with over rich conditions along with black plugs all the time. my cam is a elgin pro stock cam 230/230@50 480 gross lift and i wasnt pulling enough vacuum to use the usually stock 6.5 valves that comes with a holley trik-kit. so i ran a vacuum gauge and i run a 3.5 power valve with no problems. also get rid of that cheap spacer and spend the money and buy a hvh SUPER SUCKER spacer and you will be amazed and happier as well
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Old 07-26-2008, 05:37 AM
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you did use a 4 hole spacer right?
if you did then youll need to put the original squirter back on and flip the pink cam and adjust the pump arm for .015 at WOT.
I had same problem on my 406 after spacer install.
reason for the changes is because a 4 hole spacer increases the vacum signal
@ or near idle which will cause the richness and the cam and squirters need to be re tuned.
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Old 07-26-2008, 07:42 AM
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(the flip side of Dawg's post)

if you did install a open spacer without a center divider wall your dual plane intake is now operating as a "open plenum" intake....

the piston intake stroke is trying to pull a vacuum (to make a carb HG signal) over all 8 intake runners cubic inches worth of area with a open spacer instead of the cubic area of just 4 runners (with a 4 hole spacer)....
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Old 07-26-2008, 07:49 AM
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Thanks for all the replies. This forum is a great resource.
Malibu:
I purchased a 4.5 power valve thinking it could be a vacuum issue, but after driving around with a vacuum guage hooked up, I decided not to install it. The vacuum would stay between 12-15" (until I floored it of course). Do readings like this demand use of a lower power valve? BTW I am using 1.6 rockers which give me 0.522/0.538 lifts. I have read good things about the HVH spacers, but they don't sell the 1/2" sizes and I am hood space limited.

Dawg:
No, I did not choose the 4-hole spacer. In an overgeneralized way I am sure, I understood the 4-hole spacers to impart increases to the lower end torque while the open spacers helped out more at the top end. I felt as if the car already had all the low end torque I needed and wanted to see if I could affect my top end power. Although I don't believe the absolute numbers put out, the Desktop Dyno calculations show my combo produces much torque over a very flat range but is not too impressive with the HP. Does the open spacer change your comment about the squirter size? I too am inclined to think that I shouldn't need to go up that many sizes.
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Old 07-26-2008, 08:41 AM
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I have a 1 inch open spacer on mine and still have the stock squirter size. Your 6.5 power valve should be fine, I hvae 10 inches at idle and run a 5.5 valve. Try the pump shot (cam) and also some more timing, might need a faster timing curve.
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Old 07-26-2008, 01:00 PM
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Thanks Greg - I am about to take her out with the stock squirters back on and the pink pump cam in the number two hole. BTW my timing is at 19 degrees advance at idle and I get to 36 by about 2100 rpm.
If this doesn't help my plan is to move up the front jets to from 69 to 71. I had mentioned that the plugs were dark, but to be fair, I haven't been checking them immediately after a hot run.
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Old 07-26-2008, 01:07 PM
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You also might try running direct manifold vacuum to the can,
it seems to help when using an open plenum intake (open spacer) to have 35* of idle timing.
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Old 07-27-2008, 07:52 AM
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Scot - I can try hooking the vacuum advance up. I've been running without it. I assume that is what you mean by "the can". Another person suggest I bump my timing up even more, but they were insisting on doing it without the vacuum advance. Your thoughts?

As for the other changes: switching back to the 31 squirters along with moving the pump cam to the #2 slot helped remove most of the hesitation.
Moving from 69 to 71 jets in the front did not change things. Checking the plugs immediately after that run showed that I was not running rich.

My immediate plans are to make a run with the 35 squirters in the number two pump cam position and then fiddle with the timing.

Thanks for all the suggestions.
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Old 07-27-2008, 02:37 PM
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here's an excellent article explaination for what any spacer actually does do...

http://www.gnetworks.com/v4files/bar...h%20images.pdf

there is also an excellent spacer racing article in the Knowledge Base link at the top of the page....

it helps you tune the motor power band to work best with the car weight and tranny/rear gears.....
(it's not a major power adder at all)

Last edited by red65mustang; 07-27-2008 at 02:44 PM.
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Old 07-27-2008, 03:04 PM
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Red - Enjoyed reading that BG article by Sam Moore. Wish I had prior to buying the open spacer. The effect of just a 1/2" spacer has been more noticable than I anticipated, on both the bottom and top ends. With that said, I feel as though I am close to getting it tuned. Of course, I may think I am close for another ten attempts.

The original intent was to just stop the percolation and perhaps shift the powerband 400-500 rpm upwards. Are you thinking I would do better to just scrap the open spacer and get a four-hole?
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Old 07-27-2008, 03:58 PM
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There are 2-3 current threads happening regarding distributor curving and vacuum advance cans, Ill try and get the links and edit here.

Distributor Tuning and Theory - Part 1

Timing question

Here's a good write up by Steve Davis of Performance Distributors (DUI).
http://www.gofastnews.com/board/tech...implified.html

Last edited by ScoTFrenzel; 07-27-2008 at 04:05 PM.
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Old 07-28-2008, 12:30 AM
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First thing I would do would be get a 02 sensor and gauge so you have real numbers in real time. From there tuning is much easier. Otherwise your just shooting in the dark.

Jordon
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