390 Carb tuning, timing help

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Jan 26, 2002 | 12:55 AM

phaffy12

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Joined: Mar 2001

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From: Granger, IA

390 Carb tuning, timing help

I recently rebuilt a 390, +.030, RPM manifold, crane energizer cam. I've got the idle RPM set, but when I set the distributor things start going bad. If I get it to accelerate without any hesitating, it'll stumble all over itself when I put the pedal to the floor from idle. When I get it to spin the tires without effort, the darn thing hesitates and backfires under normal low RPM acceleration/cruising. Any leads on this problem?

Jan 26, 2002 | 01:40 AM

Post Fiend

Joined: Feb 2000

Posts: 5,130

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From: Brea CA

390 Carb tuning, timing help

How long did your carb. sit while the engine was being rebuilt? They don't like to dry out and get wet again. Sounds like accelerator shot is weak. Start your timing at 10BTDC and go up from there.

Jan 26, 2002 | 08:49 AM

gtex

Lead Driver

Joined: Apr 2001

Posts: 6,175

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From: Texas

390 Carb tuning, timing help

I'm having similar problems, and I installed a new Ed Performer Carb. So, in my case, I don't suspect the carb. I went with a MSD billet dizzy, just to simplify the timing adjustments. That vacuum advance seems like voodoo. If I get anywhere near the 12 degrees reccomended at Idle, it runs like crap. With a real bog just off idle. If I crank the timimg up at idle, the idle RPM goes up (I compensate with backing out the idle set screw on the carb to get around 700 RPM in drive), and the bog just off idle get's better. Unfortunately, that would put my 3000 rpm advance timing advance too high with the old vacuum advance dizzy. I can change the stop bushings in the MSD dizzy to limit the advance and I seem to have found a decent combination. It's running reasonably well.

However, I'm starting to think that my real problem, is that I did not do a precise job of setting the degree for the cam when I installed it. I just got the alignment marks in the ballpark and off I went.

FE gurus: Are these symptons one might see if the cam to crank timing was off a couple of degrees?

Thanks,
Greg

Jan 26, 2002 | 04:59 PM

phaffy12

Thread Starter

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Joined: Mar 2001

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From: Granger, IA

390 Carb tuning, timing help

I have a hard time believing that its the accelerator pump, b/c I bought the carb brand new. Its a Holley 750 with vacuum secondaries. The only thing i've messed with on it is the idle mixture screws and the fast idle screw to adjust the RPMs. I also lightened up the vacuum secondary spring and that seemed to help a little, but its the second lightest spring Holley sells. The accelerator pump is definitely doing its job, in my opinion. Don't they sell a pump with more flow? Should I try that next, or how about jet size? When it comes to jets, squirters, and accelerator pumps; i'm still low on the learning curve.