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Old 07-25-2013, 07:32 PM
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Carb tuning- 400CI pontiac TA

Have a new motor in my grandsons Trans Am and have a holley 4150 750cfm that is running rich with very dark brown coloring. We're running 20deg advance initial and 36 total. Were running a 3:42 rear gear and 1800 stall converter also.

We're running headers and a comp XE 268-280 cm and 220-230deg dur at .050. The cranking compression is 155 lbs

Can someone give me some baseline jet , air bleed, etc suggestions so it can pick up some jets and start in the right direction?

Any help is appreciated.
Thanks

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Old 07-25-2013, 09:18 PM
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I've always started at like 74 primary 78 secondary but u can buy an assortment of Jegs brand jets cheap that's what I do there for u wont have to keep reordering. But be sure to use fresh plugs when tuning a carb so u get an accurate plug reading. I shoot for a light tan color.
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Old 07-25-2013, 10:47 PM
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I am by no means a carb expert but the basics is pretty fool proof.If you have done this already disregard my thoughts.I would first check the float level,and while the engine is running on level ground look through the site plugs(a slotted screw on the pass side of both fuel bowls)if fuel just trickles out when you nudge the car gently your gold(if the level is to high it will run rich).Next I would hook up a vaccuum gauge to full manifold vac port(usually front left bottom corner of the base plate) and adjust your mix screws one at a time very slowly until the highest vac is reached,then set your curb idle.
If you know the number of your carb it will be 4779-1(there are many variations of model number that differ in squirter size jets etc) you can look up your carbs specs and return it back to stock jetting and power valve( a good rule of thumb is to run a power valve of half of what you engine vac is(13inches of vac =6.5power valve)

I would give this stuff a try first before changing jets....

Look here to find your carbs baseline of jets squirters and powervalve http://www.allcarbs.com/tech.php?art=21
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Old 08-25-2013, 09:50 PM
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i did what you said and lowered the primary and raised the secondary float. Got the vacuum up from about 13.5 to 14.7-15-. I really woke up and runs strong.

I need to take it to the strip and find what it likes for total adv. I.m running 38-39 total and pugs color light tan from totp of plug to half way down electrode.

Any Pontiac guy s out there have suggestions<
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Old 08-26-2013, 06:44 AM
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Originally Posted by walt03 View Post
i did what you said and lowered the primary and raised the secondary float. Got the vacuum up from about 13.5 to 14.7-15-. I really woke up and runs strong.

I need to take it to the strip and find what it likes for total adv. I.m running 38-39 total and pugs color light tan from totp of plug to half way down electrode.

Any Pontiac guy s out there have suggestions<
Have you reset the idle mixture screws? You usually don't see that much improvement in idle vacuum by re-setting the float levels. It's usually a good idea to baseline the carb back to stock if it's been changed much. Holley has the stock settings listed here.

What heads? I'm guessing by the cranking compression that you have fairly large chambered heads (4X/6X, etc.) or dished pistons. You're going to find it likes no more than 36 degrees total. I use 34 degrees as my "go-to" setting and often it ends up right back there after testing. Swap the advance weight springs around to get the mechanical timing all in by no more than 2800-3000 rpm and earlier if possible.

Use a vacuum advance limited to give no more than 15 degrees. Stock vacuum advance cans give more advance than that so they need to be modified, or use an adjustable unit. There's no set way to supply vacuum to the vac. advance. Try both manifold (full time) and ported vacuum and see which it likes best. Usually the more initial you use, the more the chance that it'll want ported vacuum. But the only way to tell for sure is to try it both ways.

A page on setting up a performance advance curve is here. The page was written w/the GM HEI in mind and sometimes references the Chevy V8, but the advance info is valid for all makes except for possibly the amounts in degrees of timing. As an example, a Chevy may want as much as 38 degrees BTDC total, while your Pontiac needs less. The "all in by" numbers may be different for other makes as well. You get the idea, I'm sure.

You should be running a PCV system. Be sure the power valve is correct and not leaking (as in blown out) or wrong number. You're better off just replacing an old PV; old PVs tend to start leaking after being used for awhile and are almost always too stiff to work accurately.

That cam should be a good choice for the F-body. 3.73 gears and more stall is called for IMHO. Definitely needs headers.

If you don't have an accurate dial back timing light, you can make a timing tape (click on image for details):

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Old 08-26-2013, 02:46 PM
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I adjusted the idle mixture needles. Have headers
Total is in by 2900. Have degrees pulley.

If in need 34 total, can I run 29 initial? Lol. I won't know I guess if it won't turn over when hot.
I've got 14-15 centrifugal though. It runs strong but haven't pulled the jets out yet. Car runs so good now my grandson is Leary of more experimenting. You know how that goes.
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Old 08-26-2013, 03:09 PM
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Quote:
Originally Posted by walt03 View Post
I adjusted the idle mixture needles. Have headers
Total is in by 2900. Have degrees pulley.

If in need 34 total, can I run 29 initial? Lol. I won't know I guess if it won't turn over when hot.
I've got 14-15 centrifugal though. It runs strong but haven't pulled the jets out yet. Car runs so good now my grandson is Leary of more experimenting. You know how that goes.
If it likes 20 initial, no problem as long as it'll crank hot like you said. If the plugs look good, overall the carb can't be too far off...

The rest sounds like you all are on top of things. Good job.
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Old 08-26-2013, 08:57 PM
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I forgot to say that it has 6X heads.
Hoping to go to the drag strip Friday nite with my grandson and see what it can do in the 1/8th mile here at Music City Raceway.
I guess I'll get times at 39, 37,35,33 and see where it MPH's best.. Is that the best way without changing anything else?
thanks for your help
walt
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Old 08-26-2013, 09:05 PM
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Oh, another thing I forgot to mention, were running a 2000 Stall and 3:42 gears. Figure to run in 2nd gear through the eight, it shifts out second at 5,500 but don't know what it'll go through the traps at. What would it do with the headers uncorked. Thought I'd get a A/F meter , but what ratio is best to shoot for wound out?

I promised my grandson that he's driving the car at the strip and he's all in.
It's great to see this for sure. I'm more excited to see this thing run good than he is. Life is good!
thanks
Walt
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Old 08-26-2013, 10:01 PM
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Oh, another thing I forgot to mention, were running a 2000 Stall and 3:42 gears. Figure to run in 2nd gear through the eight, it shifts out second at 5,500 but don't know what it'll go through the traps at. What would it do with the headers uncorked. Thought I'd get a A/F meter , but what ratio is best to shoot for wound out?

I promised my grandson that he's driving the car at the strip and he's all in.
It's great to see this for sure. I'm more excited to see this thing run good than he is. Life is good!
thanks
Walt
LIG indeed! I don't know how it is at your track, but some tracks allow passengers on test and tune nights. It would be cool if you could ride along w/him as he makes his first pass... Either way, these will be cherished memories the rest of your days!

Uncorked or through the mufflers, around 12:1 to 13.5:1 air fuel ratio is often considered "ideal" for best power, but this can vary from engine to engine. That said, I'd tune it for best mph regardless of the meter's readings. I always ran my street cars through the mufflers, just the same as if I was driving on the street. I figured why tune for conditions I'll never see except at the track. But for S&G I can see uncorking it.

Your plan w/the total timing is a good one, but as the night progresses and the temps drop, most tracks tend to pick up speed/et, all else being equal. So try to be sure what you're seeing is an actual improvement and not being caused by the ambient conditions. This could mean a couple trips to the track. From experience I can tell you 6X heads don't need any more than 36 degrees, often you'll see best results at 34 degrees.

You're going to be holding second gear- like you thought- in the 1/8 mile. You can juggle the numbers to see about what to expect by using this calculator. Second gear in a TH350 is 1.52, the TH400 is 1.48, and you'll be somewhere around 80 MPH in the 1/8 mile. Other various calculators are here.
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Old 08-27-2013, 08:19 PM
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couldn't get to the strip tonite. grandson had tests tomorrow.
Will keep you posted cobalt and guys.
Walt
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