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Offline kerryann

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Re: Flat spot with Holley 350 carb
« Reply #150 on: December 04, 2014, 01:44:42 PM »
Back again, haven't been on here in awhile.  I finally got around to putting on the efi intake on the 1980.  Bought the nice billet cnc machined adapter to make it easy.  Installation of the new intake was a challenge getting the lower bolts in and out without removing the distributor but i managed.  I didn't realize until I went to put it in but the upper most bolt towards the front of the car doesn't match up (newer intake on older head).  I'm sure the rest of you that have done this swap have run into this.  if i hadn't got so far i would have considered drilling a new hole in the intake but i left it as is.  I figured missing just one won't cause a leak.  Actually when i removed the old intake that bolt and the one next to it had fallen out long before and were gone.

The car fired up and has a slightly different idle tone too it now but the flat spot is back and is worse.  The car makes plenty of power and is actually pretty quick with the new intake on it.  It just seems to be a really bad transition flat spot from idle circuit to main circuit.  I'm convinced that something is wrong with the power valve circuit, maybe something i didn't get out after drilling for the racer walsh restrictors, maybe something wrong in idle circuit, the main circuit seems to be spot on.  The carb makes no response to power valve change.  It does get better when warm but now it doesn't go away when hot.  I'm going to look for a fresh 7448 from some of my circle track friends and start over.  already have the restrictors but I wan't to try one without them first and see what the difference is.  Then I will see what I can figure out.  I am convinced this is purely a carburetor issue at this point so I will see what I can find out once I can get my hands on another carb.  The one on it was an old swap meet carb so who knows what its history is.

-ian

Offline dick1172762

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Re: Flat spot with Holley 350 carb
« Reply #151 on: December 04, 2014, 02:02:36 PM »
I would try it out with no power valve. Take an old one and remove the rubber, then melt solder into it. I've done them both way. You may need to put bigger jets in.
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Offline kerryann

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Re: Flat spot with Holley 350 carb
« Reply #152 on: December 04, 2014, 03:40:21 PM »
up to 61s on the jets, that's stock jetting for  7448, i have a power valve blocker, i'll have to give it a try.  Yes it will probably need more jet to compensate, i think i have some low to mid 60s i can try.  also this intake is unported and i have the ACE adapter off ebay.  after reading through other posts some suggest porting with the short adapter.  This car is just a cruiser and were not looking to eek every bit of power out of it we can get, just smooth throttle response all through the range would be nice.  I plugged the injector holes with brass plugs.  My drilling and tapping job wasn't a precision job but they are tight and sealed up.

Offline amc49

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Re: Flat spot with Holley 350 carb
« Reply #153 on: December 05, 2014, 03:35:57 PM »
You block the power valve while checking the main jet and NOT UNDER HEAVY POWER, you can fry motor doing it. The valve should not be opening when main system (boosters) start up, rather, somewhat after that. If you are getting the flat spot while driving around like normal cruise then problem is NOT the valve. Could be trash in a main well from the drilling or the brass insert in too far to make a restriction. Going to the full size restriction of a dead stock carb should not work either, they restrict them for a reason, it being too big for a four cylinder. They are usually up around .040"-.055", these will like around .020"+.

You end up with two main jet choices with a blocked PV, one is good driveability at cruise and low demand throttle and the other is high demand full throttle power. There will be a separate main jet that motor likes for both, then you figure the difference in cross section of the holes and that ends up being the power valve restriction size. The power valve supplies the difference in fuel between low demand and high demand requirements. The jet size the motor likes for full power will be too big for creeping around in slow driving and the motor will be too rich then. Why the system was created to begin with.

Think kickdown on an automatic transmission, power valve comes on about then, the demand for extra load. Maybe a wee bit sooner but not until then, it needs to be closed for clean normal light load driving. Why blocking it to test drive at low load is useful.

If the idle is still set to make the throttle plates go higher in the transfer slots it never will run right, the transfers should be barely showing like maybe .020" of the slot and no more when carb is pulled to look at that after normal idling speed has been set. Too high in slot makes for too rich an idle, you cut back on the screws to cure that but then when you increase throttle no fuel left for transition load because the high slot condition has used it already. So, too rich, then instantly too lean right around normal cruising spot. The entire lower rpm/load workings of the carb are set up based on that idle transfer slot relationship and many mess up there.

If an old carb can always have some sort of blockage in a metering block well. A few of the carbs they put idle feed restrictions on the tip of a long tube that presses into the well, if those block up the tip almost impossible to clean.



Offline amc49

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Re: Flat spot with Holley 350 carb
« Reply #154 on: December 05, 2014, 03:48:04 PM »
My take on porting with the one inch adapter is that it is to help make up for the lack of depth, the short distance there makes for issues with fuel distribution, the throttle plate angles interfere with mixture getting to the other side of them since all fuel comes from one side of a 2 bbl. non-progressive carb until the boosters start up.

Just me though and I can't back it up............ ............LO L

Offline kerryann

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Re: Flat spot with Holley 350 carb
« Reply #155 on: December 07, 2014, 09:05:10 PM »
that method of figuring out the power valve restriction size makes sense.  as for the idle transfer slot setting, i'll check it again, but ive tried it from as low of an idle setting to stay running all the way up to too fast to comfortably stop the car while in gear.  no change in flat spot.  this is without adjusting the idle mixtures screws.  i tried adjusting them as well without changing the idle speed screw.  I tend to try and leave the idle mix screws at or close to the baseline 1 1/2 turns out.  guess i should try setting them with a vacuum gauge.

one of the racer walsh restrictors is pressed in farther than the other.  There could certainly be a blockage somehwere in the wells like you said.  Im afraid that may be the problem whether the drilling caused it or something else.  im going to see if i can remove the restrictors and find anything.

Offline amc49

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Re: Flat spot with Holley 350 carb
« Reply #156 on: December 07, 2014, 09:46:23 PM »
The only way you'll know is to set the butterflies dead correct on the slots and then put carb back on and see where you are idle speed wise then work from there. How I start with any carb. I use 1 1/2 as a starting point on idle screws too. Where they end up can tell you if idle feed restriction is close or not. Both need to have an effect, if one does and one doesn't then indicator of something wrong in that circuit.

What speed is engine comfortable at idle wise, i.e., what cam in car, affects that greatly. OEM around 700-800, longer cam will be higher (up to 1500) and how those slots begin to cause trouble, the idle has to be set too high and gets into them.

Power valve vacuum value is usually one half what idle vacuum is, a good starting point.

Assuming accelerator pump is working well.......... ..