Have a modified ZZ502 in my 66 Restomod.It came with 78/82 jets in the 850 cfm Holley.I ran way too rich so I stepped down to 76/78 and is still too rich.Any suggestions on what jet size is ideal?? This is a street car and not drag raced.

 

Your front/rear jetting may be still too rich. I'd try going down two more jet numbers front and rear to see if that helps. BTW, do you still have the original power valves in the front and rear? If so, they may be part of the problem is Holley typically has them open earlier (augmenting the jet fuel flow) than is needed on the street to protect against a leaning out the engine under heavy acceleration. I don't have a 502, but I've used 4.5 power valves in a 750 CFM Holley on a 427 BBC with good results. (It really depends upon your engine's vacuum profile from idle to WOT, so determine that first before replacing.)

Here a link to the last page of an article on tuning an 850 double pumper for a BBC. It gives a summary of the changes made. (After reading the summary, if you see something you might like to try, you can read the entire lengthy article.)

Hope this helps resolve the rich issue.

 

Thanks for the response,regarding the power valves the primary has a 12-65 (12-45 was stock) & the secondary has a 12-35 was was stock.I have 12.5 in of vaccum at 8-- rpm at idle if that helps.

 

That last two digits on the power valve indicate the initial opening point in inches of vacuum. In your case, the front power valve opens at 6.5 inches of vacuum and the rear opens at 3.5 inches of vacuum. I would replace the front power valve with the original unit that opens at 4.5 inches of vacuum to see if that helps your lean out your too rich condition. The vacuum at idle seems low (I would have expected around 15 inches for a hot GM crate engine); however, it may be cam dependent. As I said, my experience is with 427/435 engines.

You should have four corner idle screws on your Holley 850. Have you tuned these idle screws for best lean idle? (If they're running too rich, this might contribute to the overly rich condition. Try using the tuning tips posted on this Holley page.)

BTW, are you running way to rich throughout the spectrum (idle to WOT) or just at idle? Your original post lead me to assume it was throughout the spectrum, and that's what I was advising on.

 

First the carb that came with the 502 is not a double pumper.Its a single pumper so I don't have 4 idle mixture screws.These 2 screws have been adjusted correctly by a friend of mine that used to race Vettes.I don't feel comfortable using the tuning tips as they are for a different carb.I am running too rich at idle (it really stinks) and at WOT as it seems to be poring raw gas into the side pipes and I get explosions coming from the exhaust especially if I downshift to let the engine brake the car.

 

Okay, let's try this approach. Change the front jetting down another two numbers and replace the current front power valve with the stock 12-45 unit. (Leave the rear jetting where it is right now. That will put all jets front & rear down four numbers from stock.) See if that helps. (You'll know when you're close to the correct jetting when the exhaust fumes don't burn your eyes from the unburnt fuel.)

Other causes of rich conditions can be too high a fuel bowl level, inlet needle valve leaking fuel in the "closed" position, and incomplete combustion due to ignition problems, e.g., not enough timing lead cranked in, weak spark, etc. It wouldn't hurt to check out these variables as well. (Just for an example, too high a fuel bowl level will allow fuel to be sucked out of the bowl through the booster Venturis by the resulting high vacuum when the throttle is slammed shut. These raw gas droplets aren't atomized so they don't burn entirely inside the combustion chamber resulting in loud popping noises in the exhaust system as they continue burning.)

BTW, the tuning tips given on the Holley website will work for all Holley 4150 carbs like yours so don't be afraid to use their technical advice in tuning your carb. Is this carb new out of the box, or purchased previously used? If it's new, you can call the Holley Tech Line for additional support if the problem can't be resolved in this on-line forum.

 

Ok I will try your recommendations on the jet size.FYI I know the floats are set correctly as they are now set to just allow a little fuel out of the hole at idle,any lower the engine will stall out under hard braking.The timing is set at 39 degrees total.I don't believe a have a needle valve leaking but I can check that as well.Yes this was a new carb that came with the engine package back in 2000.I will also call the Holley tech line to see what they say but your suggestion on the jet size sounds like a possible simple fix.Thanks for the help!

 

this is good stuff..............

 

If its at idle jet changes wont help you youre running off the idle circuit or should be

Explosions out the pipe might be ignition related.

stinky could be rich, eyes burning quick probably a lean condition.