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Discussion Starter · #1 ·
I'm working with a holley 650 DP and I'm having some transitional throttle issues, so I'm pretty sure its something with the acclerator pump system. Stock jetting was 67/73, car felt kinda flat and down on power, plus I had just swapped to a better exhaust system, figuring it was lean I started jetting up. I worked up to 70/76 jets, with stock pump cams (yellow in rear I think), rear pump is 50cc, 28/31 shooters. Car ran well and picked up some good power, but was a little rich all the time and had a stumble(sounded like a 2 step when going down a hill) if I even thought about backing off the throttle while going down the road. Anyway, tried a few things, now I have 31/31 shooters, 65/73 jets, stock pump cams. The car has a slight bog before the secondaries open, then when they open it stumbles slightly, then takes off like an ape with rabies. Full throttle performance seems to be as good as it was with the 70/76 jetting, but this is just seat of the pants. I haven't had a chance to take it to the track with the smaller jets. Ideas? I have a pump cam assortment, jet assortment, a 28 shooter, 2 31 shooters, and a 35 shooter to tune with. Engine is a 10:1 306, stock 351 D0OE heads, crane energizer 284H (228 at .050, .512 lift, 112 CL), stealth intake, hooker comps, C4 w/ 3500 stall, 3.70 gears. Last time I went to the track I was running consistent 8.42's in the 1/8th, with the 70/76 jets and 28/31 shooters. So I think it was running pretty good, but then again, full throttle runs aren't much of an indicator of accelerator pump tuning. Where should I start with this? I'm thinking of going ahead and doing some pump cam swapping and adjusting the screw holes to see if that clears it up. Can anybody give me a little guidance here, or is it all just a matter of experimentation?
 

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Granade.......? Start with one thing at a time. Don't Change everything at once.

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1991 LX Mustang 347 C4 combo Mid 11's with the AC on.
1984 Mustang GT 460, Powerglide "Still putting it together" hoping for 9's !!

<font size=-1>[ This Message was edited by: dfree383 on 3/4/06 4:55am ]</font>
 

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When you let off the throttle going down a hill, I think most of your metered fuel is coming from the idle circuit... so you should be able to tune that out by leaning the idle mixture a little.

As far as your hesitation, does it happen during quick throttle opening or gradual? Quick throttle opening can be tuned with the accel pumps alone, but a hesitation during gradual transistion, that can be your power valve needing to be higher or lower numerically. If your hesitation is when you jump on the throttle, is it RIGHT when you jump on it, or slightly afterwards? If the hesitation is immediate, with black smoke out the tailpipe, that's too much accel pump or too large of shooter. If it starts to pick up, then bogs, then takes off again, that's too small accel pump or too small shooters.

You're best off to tune the primary jets and power valve first, for the absolute best cruise and medium load (primaries open, secondaries closed) performance. THEN leave the primaries alone and move to the secondary jets to tune for best WOT performance. And LAST, mess with the accelerator pumps for best transistion and to get rid of any hesitation.
 

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I would get the SA Holley super tuning book, believe it or not the CARB should be very close to as it was out of a box.I would leave cams alone and not go past 35 nozzles.Unhook and tie up the rear barrels to make sure where your problems are, you CAN get tricked.Remember theres a lot of so called Holley experts out there,thats why I suggest the book its number is isbn 0-931472-08-3 part no 08-0 from sa books
GOOD info.
Andreas.
 

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Discussion Starter · #7 ·
thanks for the leads. I'm pretty sure the jetting is right, because at cruise and WOT I have no problems. Might change jets around at my next track outting, just to be sure. PV should be ok, its a 6.5, car idles in gear with 15-16 vac. I'm still thinking I need to tinker with the pump cams and/or shooter sizing to get the transitional sutff right, its not very severe, just something I notice.
 
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