Thread: My 502 ain't got it....what am I doin wrong?...Please Help!

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I just fired up a 3-year build up...a 69 Impala, 502-4spd, 4.11 geared whale. I had the usual problems with a new ride, and I got most of em solved...but I still can't get the 502 to be all they say. Its the 8.75:1 street version rated at 450HP and 550 lbs.

I'm convinced the source of the trouble is the ignition. I'm running a full MSD set up, 8361 street billet dist., blaster coil, 6AL...you know the usual. Right now it is timed at 36 degrees total running at 2500 or so...which winds back to 17 degrees at idle. Thats with NO vacuum advance hose connected, and tiny advance springs. It fouls the plugs within an hour of street time, and starts dropping cylinders. I've tried several carbs, with some improvement...but the problem still exists.

So, here's where I need help. Does anyone know what curve I should be running on this? I do want to use the Vacuum advance...but I'm not sure where to connect it. Seems everyone has a different opinion on that one...whats yours? Finally...what total and initial settings are all you other BBC guys running? (this is my first rat). Keep in mind...this is a fairly mild motor. GM says to set it at 4 BTDC, which runs like crap. Perhaps I need more "travel" in my advace mechanism? Any help would be appreciated.

Thanks- CJ in Indiana

disconnct the vacuum and plug source., try setting it at about 10 deg. idle. , hook your vacuum back up and see what your total comes to at 2000 to 2500. what kind of induction are you using.

If you're using vacuum advance on a Chevy, you need to hook the vacuum hose to ported vacuum (above the throttle plates), NOT to manifold vacuum (below the throttle plates).

On most Holley carbs, there will be two ports on the base plate at the right front of the carb about a half inch apart. The front is prted, the back is manifold.

With the carb off the engine, you can tell by just looking at the passages in the baseplate under the carb.

By the way, were you checking timing with the vacuum hose removed AND plugged?

Check out this article:

http://www.chevyhiperformance.com/ho...438/index.html

And if you're fouling plugs with an MSD box you've got more than timing issues.

Thanks to all,

First...yes, I plug the vacuum when I disconnect the dist.
Second...I'm using a Barry Grant 850DP, (which is too big).
Third...I'll try using the variable vacuum.
Fourth...Amen to that last comment on the MSD! I agree.

Let me do some tuning, and I'll let you know what I find out.

CJ

A Barry Grant 850 Double Pumper may be a bit large for a 502, but not by much if it's set up properly. I ran a 780DP on a 400 inch GTO, and it ran . . . well, like a GTO. 11.80 on MT 8" wrinklewalls. The engine was pretty radical, though . . . 12:1, Ram Air V cam/valve train, 4:33's, Muncie, etc. I also ran an 850DP on a 454 in my 30 A-bone. 9:1 engine with Comp Cams Magnum 292, top-loader and 4:10s. It was a little over-carbed, but it tuned out fine.

Your BG is essentailly the same as my Speed Demon. The ported vacuum should be the small brass tube, furthest to the front, on the right side of the baseplate.

I've always been a fan of mechanical advance distributors. Both of those cars had mech advance. Get the right curve, wind the engine up to about 3000, set the top timing and let the initial go where it goes. Not much for mileage, but real easy to tune.