What causes fouling spark plugs in a new motor?

[sdkar]

I have a brand new 502 GM crate motor in my 69 Camaro.  I have yet to drive the car other than in and out of my garage as I am rebuilding it.  It ran great at first but than started fouling the plugs.  I am on my second set of plugs already and they have less than 20 minutes on them.  I cleaned them but that only lasted about 2 minutes and they are fouled again.  This is the ZZ502 deluxe and came with a Holley 850 and I am running the fuel pressure at 7 lbs.  The timing is set to factory.  The engine starts running rough...then backfiring...then stalls and will not start until I clean or install new plugs, only to start the fouling all over again.  What do I need to do to stop this.  I am ready to drive the car to the paint shop, but at this rate I won't get 2 miles.  This is odd to me since this is a brand new motor.

Any and all help is greatly appreciated.

Steve

[JohnZ]

7# is pretty much the upper limit for fuel pressure before it overpowers the needles. Check the float levels, check the needles/seats for trash that could be holding them open, and make sure it's idling on the idle mixture orifices, not on the transfer slots - you should only have about .020" of the transfer slot showing below the throttle plate at idle so the idle mixture screws are effective (adjust them for highest steady vacuum).

[jdv69z]

Are the plugs black with carbon when you pull them. That's too rich

Jimmy V.

[hotrod68]

I'd check and make sure there is no fuel dripping from the boosters, too. Your floats could be too high. With the engine running, look at the boosters--they should be dry with the engine running from the idle circuit only. Like John Z said, 7 lbs is a Holley's limit for fuel pressure--too much will overpower the boosters, too. Good luck!

[sdkar]

When you say "dripping from the boosters" what do you mean and how do I check this?

[JohnZ]

See photo below - the boosters are the four circular devices in the middle of the airstream in each venturi - that's where the fuel is metered into the incoming air from the main metering system. At idle, there should be NO fuel coming out of the boosters at idle; idle fuel is metered from tiny round orifices BELOW the throttle plates.

[hotrod68]

If fuel is leaking past the needle & seat it'll drip from the underside of the boosters--I couldn't load the photo John Z posted--hope you did. With engine running, look for droplets of gas being sucked into the venturis out of the underside of the boosters. You can clearly see if they're leaking. The cure is to lower the floats--try turning the needle & seat screw down a 1/2-turn a a time until the leaking stops. Also, just after you cut the engine off, look for smoke coming from the carburetor venturis--if fuel is leaking from the boosters you'll see it clearly. It's actually raw gas that's atomizing from the manifold heat. Good luck!

[sdkar]

Solved the problem.  The front float was either stuck or set to high (or is it low?).  When I opened the sight plug on the bowl gas poured out.  I reset the float until fuel was just below the sight hole.  I then installed new plugs and it runs good again.  I have a holley regulator 803 but the lowest setting I can get out of it is 6-1/2.  I have turned the adjustment all the way out and it does not go any lower.  Anyway...it runs good again and does not seem to have a problem.  At least this is the longest it has run without fouling the plugs to the point of uselessness.

Thanks guys for all of the help....as always...it is greatly appreciated.

Steve