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Unread 05-27-2010   #1 (permalink)
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Default Carbureted 302 Off-idle Stumble

Im not one to ask for help but i cant figure this problem out...

I have a 302 with an E303 cam, Trickflow Street Heat heads with 1.6 rockers, Edelbrock Air Gap Intake manifold, Holley 650cfm double pumper carb, MSD 6AL box, blaster 2 coil, and pro billet mechanical advance distributor.. That being said the car starts and idles just fine.. if i slowly roll into the throttle it will rev up just fine.. but if i open the primaries more than 45% open it stumbles and falls flat on its face until i let off.. when i first started it the timing was way off.. i have changed the timing to be at 15 degrees at idle and 36 total timing which comes in around 2600 - 2700 rpm.. that being fixed i moved to the carb.. i have an idle manifold vacuum of 7in Hg.. so i have a 6.5 power valve in the carb.. i changed the squiter nozzles from the 28 that it came with to a 31.. the car is acting like its not getting enough fuel when the vacuum through the venturi system is too low to draw fuel through them.. but ive done what i know to change that but its not helping hardly at all...

any help would be appreciated!
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Unread 05-27-2010   #2 (permalink)
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7"Hg at idle sounds really low to me, my setup is very much like yours (but with a longer cam), and I have 14-15"Hg at idle. Might want to check around for a leak somewhere.

What rear end gears are you running?

With that said, trying switching the little plastic accelerator pump cam. A 650 Double Pumper comes with the pink cam installed. For manual transmission Mustangs with low rear end gears (3.73+), I usually use the blue or white accelerator pump cam with a .031"-.037" squirter.

And by the way, vacuum has nothing to do with how well the accelerator pumps work, they're designed to work at 0 vacuum.
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Unread 05-27-2010   #3 (permalink)
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Oh, and if you have 7"Hg at idle and no vacuum leaks, the 6.5 power valve is probably spending way more time open than it should, which will throw off your accelerator pump tuning. Try a 4.5 or Maybe a 5.5 power valve.
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Unread 05-27-2010   #4 (permalink)
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I also thought 7in was low.. so i checked for leaks.. couldnt find anything.. i know the acc. pump is made to work when there is no vacuum.. thats why i changed the squirter nozzle to a bigger size because it seems like its not getting enough fuel when there is no vacuum.. It has the factory gears in the rear end.. but i have 4.11s waiting to go in.. but i havent even driven it since ive put it together... i have a 3.5 power valve that i was gonna put in it but just havent tried it yet..
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Unread 05-27-2010   #5 (permalink)
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Have you tried advancing the idle???

Usually stumbling like the is an indication of retarded timing.

I know with carb setups you can usually run quite a bit of timing. So you might try advancing it a little more and see if it helps.
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Unread 05-27-2010   #6 (permalink)
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The stock gears are killing you.

I re-read your original post, it sounds like the bog might be in the secondaries, which with stock 3.08 or 2.73 gears is gonna be pretty darn hard if not impossible to tune out with a 650 cfm carb on a 306.

Your timing curve seems pretty good to me, it's the same as what I'm currently running. You just need to get those gears installed.
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Unread 05-28-2010   #7 (permalink)
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I have thought about advancing the timing a little bit more.. but leary about going too much.. The gears i know will kill it.. but this problem is not even under load.. its while its in neutral.. i havent wanted to drive it while its doing this because the problem will be worse..
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Unread 05-28-2010   #8 (permalink)
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You are going to have to find the vacuum leak before you go any further. Pump in too much and you will plate the fuel up bad. I went power valve and accelerator adjustments to clean mine up once I was pulling 15 or so with a much bigger cam.
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Unread 05-28-2010   #9 (permalink)
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Ill look again for a leak..
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Unread 05-28-2010   #10 (permalink)
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If it's in neutral too, I'd say start back with the basics. Check your fuel pressure and make sure both the primary and secondary floats levels are set correctly.
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Unread 06-02-2010   #11 (permalink)
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I checked for leaks again.. none that i could find.. Fuel bowl levels are set correctly and i have 7 1/2 psi of fuel pressure..
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Did you buy the carb new from Holley? If not, take the air cleaner off, and move the throttle linkage just a tiny bit. Any movement of the throttle should get you a nice instant stream of fuel out of the accelerator pump squirters.

If the primary squirters are coming on fine, do the same thing for the secondary ones.

It's possible that there's a missing check ball or an incorrect fuel bowl or metering block gasket that would be causing the pumps not to work.
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