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Old 03-28-2011, 07:01 AM
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holley running rich

I have a street avenger 670 on my 355 sbc. running stinky rich, fuel mixture screws don't seem to make a difference, the idle screws dont really change the vacumm reading much. vacuum at idle is about 8. the carb is a new rebuild. would a power valve thats to high make it run to rich at idle?

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Old 03-28-2011, 07:46 AM
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Quote:
Originally Posted by mainejohn
I have a street avenger 670 on my 355 sbc. running stinky rich, fuel mixture screws don't seem to make a difference, the idle screws dont really change the vacumm reading much. vacuum at idle is about 8. the carb is a new rebuild. would a power valve thats to high make it run to rich at idle?
If the PV will come in at or before the idle vacuum, it could be the problem. It could be leaking, too.

But in many cases like this, the problem relates to the transfer slot being over-exposed. If you have a cam w/much overlap, the idle mixture screws end up turned way out, and the curb idle screw is also out real far to try to get it to idle. If the idle mixture screws are unresponsive, that's a good indication of the slots being over exposed.

The solution for this comes from more initial timing, either by cranking it in by turning the distributor (be careful that the total timing doesn't go too high!), or by using a vacuum advance hooked to manifold vacuum.
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Old 03-28-2011, 08:04 AM
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i can get it to idle at 700 rpms with no problems at all, it does seem rough idle cold but not bad once warmed up. it just that the exhaust smells really strong. i did advance the timing a little bit but it started a little hard and had a delay to it so i turned it back to were it was. so should i start with a 35 power valve size and see from there or should i try something else?
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Old 03-28-2011, 08:14 AM
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Quote:
Originally Posted by mainejohn
i can get it to idle at 700 rpms with no problems at all, it does seem rough idle cold but not bad once warmed up. it just that the exhaust smells really strong. i did advance the timing a little bit but it started a little hard and had a delay to it so i turned it back to were it was. so should i start with a 35 power valve size and see from there or should i try something else?
Re read what I posted above about using the vacuum advance to your advantage.

Once you connect the vacuum advance to manifold vacuum, the idle speed will increase. By adjusting the idle speed back down using the curb idle screw, the transfer slot will be less exposed.

If this isn't the problem, the IFR could be too large, the idle air bleeds too small, or a bad or wrong PV. About 1/2 the idle vacuum in drive (neutral if a stick) will be an estimate of the PV needed. But you want to maximise the idle vacuum. The vacuum at idle is dependant on the cam timing. What are the cam specs?
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Old 03-28-2011, 08:21 AM
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all i know the cam is that its a howard cam, 224 duration 488 lift. so could i unhook my brake booster from the manifold and hook it to the booster on the carb and then hook the dizzy vacuum to the port were the booster was hooked to the manifold(in know i have to put a smaller fitting for the vaccum advance hose)? is that right? this is all new to me!
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Old 03-28-2011, 08:36 AM
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Quote:
Originally Posted by mainejohn
all i know the cam is that its a howard cam, 224 duration 488 lift. so could i unhook my brake booster from the manifold and hook it to the booster on the carb and then hook the dizzy vacuum to the port were the booster was hooked to the manifold(in know i have to put a smaller fitting for the vaccum advance hose)? is that right? this is all new to me!
That's a healthy cam, but I believe it should get better vacuum than 8 in/Hg.

In any event, refer to the image below and attach the vacuum hose to the vacuum advance to the arrowed manifold vacuum port.

If the port is already being used for the trans vacuum modulator (for example), use a "T" in the line to run them both from the one port.

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Old 03-28-2011, 08:44 AM
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okay i see, that is were it is hooked it up. so what do you think i should next? mess with the PV? the vacuum does seem to bounce around a bit, is that normal? i have done the propane gas check for leaks!
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Old 03-28-2011, 09:15 AM
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any ideas?
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Old 03-28-2011, 09:21 AM
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Is the PCV hooked up? If not, it should be.

It won't hurt to replace the PV if you suspect the old one is leaking or is the wrong number.

But I'm wondering why the idle mixture screws are unresponsive. Is this a new carb? Has it been apart or had any mods done to it that you know of?
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Old 03-28-2011, 09:26 AM
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its a rebuilt carb from Allstate Cabbs in new york. they claim they fix the off ilde stumble. so iam not sure what they do to them when they are rebuilt.
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Old 03-28-2011, 09:58 AM
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Quote:
Originally Posted by mainejohn
its a rebuilt carb from Allstate Cabbs in new york. they claim they fix the off ilde stumble. so iam not sure what they do to them when they are rebuilt.
My mistake- I see the carb having been rebuilt was in your first post.

You can (provided the transfer slots are about 0.030" 0.040" exposed as you look at the carb from the bottom side) open the secondary butterflies using the adjustment screw provided for this, about 3/4 of a turn and see if this improves things. The thumbnail shows a Holley that is at the limit- you want the slot to look like a square hole, not a rectangle.

You might need to readjust the idle mixture screws and the curb idle speed, but it should show an improvement if the PV isn't leaking.
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Old 03-28-2011, 10:02 AM
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On the picture you posted what screw do i adjsut for the seconday butterflies? thanks for your help. i would assume the power valve is fine but might be to large. if its bigger then 4.5 do you think i should change it?
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Old 03-28-2011, 10:07 AM
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Quote:
Originally Posted by mainejohn
On the picture you posted what screw do i adjsut for the seconday butterflies? thanks for your help. i would assume the power valve is fine but might be to large. if its bigger then 4.5 do you think i should change it?
If you cannot get the vacuum any higher than 9 in/Hg, then use a 4.5. Are you sure the vacuum advance can is working? The engine speed should drop considerably when the line to it is disconnected and plugged off.

The arrow points to the secondary adjustment screw. It's a narrow screw and almost always recessed, so you may need to 'make' a screwdriver to fit it.

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Old 03-28-2011, 10:13 AM
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the dizzy is new from jegs, so whn its ideling and i unhook the vaccum line and plug it the rpm should pick up? should all my fuel mixture adjustments be done with vacuum plugged or all hooked up?
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Old 03-28-2011, 10:33 AM
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Quote:
Originally Posted by mainejohn
the dizzy is new from jegs, so whn its ideling and i unhook the vaccum line and plug it the rpm should pick up? should all my fuel mixture adjustments be done with vacuum plugged or all hooked up?
All hooked up just like you run it on the steet. 2 places to hook up the vacuum gauge either manifold fitting behind the carb on the manifold or on the carb in front where Cobalt pointed out the full vacuum port.
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