acceleration issues

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jet_wrnch

jet_wrnch

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topeka, KS
Ok, here is what I got…I have one hell of a bogg when I’m cruising and lean into it. I’m running 36 degrees of timing; the carb is what I’m leaning towards. I’ve checked the power valve (4.5) and tried using different pump cams. went from 70/82 jets to 66/80 Plug gap @.45, getting 8-10 hg of vacuum at idle. It starts and idles fine. If I get after it right out the hole it does fine. Its only when I’m rolling at cruise and lean in to it falls on its face and then pulls out of it. It’s not backfiring just stuttering. Any Ideas before this car ends up in a dumpster????




408 with eddie heads, airgap intake, 750 aed carb, big cam,complete msd ignition. 4 spd, 323 gears
 
Your secondaries are kicking in to soon...
Mechanical and vacuum are adjustable...
 
Have you done anything to your squirter nozzles (drilled, replaced, etc)? Also, with changing pump cams did you make sure to adjust the pump arm for the correct clearance (I'd check that first if not)? If pump cams didn't work I'd try drilling the primary nozzle, or buy an assortment of new ones, to see what that does. I had the same problem once I was getting my carb close to dialed in and going from a .031 to a .037 primary nozzle solved my tip in stumble at light cruise.

Also, I'm thinking you run a mechanical secondary carb, am I correct?
 
Measure your vacuum at cruise rpm. I'll bet it's up in the 13-15" range. You need alot of throttle opening to get down to that 4.5 PV. I would suggest trying a 6.5/7.5 PV. Wont effect your idle and you'll have enrichment sooner when at cruise rpm.

The 1/2 idle vacuum power valve, that Holley recommends, works well in idle to full throttle situations but in real world street cruising you need a little higher # PV opening rate.
 
I run an AED 850 on my 408(410c.i)and run 79/87 jets.You should have 8 size difference with front and back,not 12 size difference.I,d start with 76/84 and work from there.good luck.
 
Lean into it? As in slowly rolling into the throttle or mashing it to the floor.

If it's a slow press to floor, PV opening too late may be your issue as Oldmanrick suggests. Might even want a 10.5 pv.

If it's mashing to floor, accelerator pump shot, especially if a DP. Vacuum secondary, then the spring is too light.

I'd put the jetting back to 70/80 range if it's a 750. 66 is REALLY lean, my 340 won't even run on that without surging badly.

What's the initial timing set at and idle speed in P/N and in gear? I'd want at least 14* initial and depending on cam anywhere from 800-1000 rpm in park.
 
I think Ive got a 6.5 pv laying around. If the temp drops below 100 tonight Ill get out there and throw that in. My distributor is mechanical so I know its not a vacuum problem. I got on Holley tech web site and they were saying to use a pv that is half of your vacuum at idle but Ill give it a try. Thanks for the input Ill keep ya posted.
 
jet_wrnch said:
I think Ive got a 6.5 pv laying around. If the temp drops below 100 tonight Ill get out there and throw that in. My distributor is mechanical so I know its not a vacuum problem. I got on Holley tech web site and they were saying to use a pv that is half of your vacuum at idle but Ill give it a try. Thanks for the input Ill keep ya posted.
Click to expand...

While your in there you may want to jet back up a bit. Regulation stock primary jetting on a OOTB Holley 750 are in the 72 range. Not sure if the AED requires less but i'd think about it.
 
you guys called it!!! jetted it back up and went with a bigger power valve and its like a new toy. Appreciate the help... I'm gonna try and hit the track next week so Ill let ya know what it does and where ya think I should go with it. Thanks
 
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