DZ 780 Holley Carb jetting help.

[camaroboy68ss]

Hey guys, wanted some input on some tuning issues I'm having with my Holley on my 68. Its a DZ 780cfm reproduction of a correct Z/28 carb. This thing is running super rich so I know I need to jet the thing down. Mainly rich in the secondary's, but it reeks of gas even at idle.  I've never messed with a Holley so I don't know how touchy they are with changes, meaning do I need to drop a few jet sizes to see small change or only a size or two? Here are the specs of my current setup in the car for further info:

68 327ci (not bored) L30 bottom end. factory crank, rods and flat tops. 68 camel humps, I cant recall if 1.94 or 2.02 been close to 10 years since the heads were done. Comp 30-30H cam (like a normal 30-30 but set up for hydraulic lifters). Winters 69-later over the counter Z/28 intake topped with the Holley. Stock exhaust manifolds and stock points ignition.

I was planning at seeing what is in it and then drop 2 sizes in both primary and secondary. Am I in the ball park for a first step or need to go less/more?

[Danzo]

Most 780's run well with 72 primary 76 secondary.

Doc.

[janobyte]

Type in 780 jetting or tuning in search. Several good threads, with explanation. X33 has a great thread, and utilized a wide band for tuning. Also check your vacume and what power valves you have in the carb. Doc is right, but always good to do a little review on tuning. IMO.

[L78 steve]

I would check the condition of the power valves before screwing with jets. 72-76 is standard and will run well if other factors check out.

[X33RS]

Thanks Janobyte.

  I'm curious about that hydraulic version of the 30-30.  I haven't seen a cam card on one of those.   Do they take into account the loss of duration and overlap that you end up with on a solid cam with the lash figured in?    You can figure about a ~10 degree difference between a hydraulic and solid cam with all else being equal.   So I would think that hydraulic would need to be ground with roughly 10 degrees less .050 duration than the original solid 30-30 to "act" about the same.

I only ask because it may have an effect on your idle vacuum.  Have you had a vacuum gauge on it?

If it's rich at idle and you've fiddled with mixture screws without success I'd be looking at the power valve circuit like Steve mentioned.  I'd also make sure your throttle blades aren't open too far exposing too much transfer slot making the mixture screws inop.

What I did with mine is block the rear PV.  I don't feel it's really necessary to have 2 of them.  I then jet up the back 4 sizes to compensate.   If you don't want to go that route then the suggested 72-76 jet setup works pretty nice on these engines. 

[X33RS]

Sorry I had to step away.   What I was getting at was to check your vacuum reading at idle.  You want to make sure it's not dipping below or even hovering around your PV rating.  This will cause a rich idle condition.

Stock PV for these things is 6.5 and should be fine with a stock camshaft.  Generally you should see 10-13 inches or so.  With your cam change it has me curious though.  Just something to check and be sure.

[camaroboy68ss]

Thanks for the info, ill run a vacuum gauge on it before i pull the carb off. Then check the power valves and what jets are currently in it. As for the cam specs here they are based off my cam card:

Duration - 286I/293E, Duration @ .050" - 239I/246E,  LSA is 112

[bcmiller]

Thanks for the info, ill run a vacuum gauge on it before i pull the carb off. Then check the power valves and what jets are currently in it. As for the cam specs here they are based off my cam card:

Duration - 286I/293E, Duration @ .050" - 239I/246E,  LSA is 112

How about the lift?

[X33RS]

Here ya go Bryon, I looked it up earlier.

http://www.compcams.com/Company/CC/cam-specs/Details.aspx?csid=111&sb=2   

It has a reverse split lift, and I noticed it's ground on a 112 LSA and 108 ICL.

it's going to act a little different than the original duntov 30-30 especially depending on where it's installed at.   So I'm curious how the vacuum is.

[camaroboy68ss]

well if the RS headlight doors are any indication on vacuum since i have not put a gauge on it, they operate just fine, exactly the same as when i was running a L79 spec cam in the motor.

[X33RS]

I'm not real sure.  They have a pretty large vacuum reservoir on them that I'm sure helps the situation.  I know on mine before I had the tune nailed down I had it idling slow and showing about 6 inches of vacuum and my headlights still worked fine.  Not sure at what point that becomes a problem other than just no vacuum at all, I know they don't work like that 

Switched my 56 over to electric wipers decades ago.  Quick throttle movements in the rain and the wipers stopping gets your attention.  Vacuum wipers weren't one of GM's best ideas, lol.

 

[camaroboy68ss]

yeah they are junk, my grandparents 55 was switched over years ago and our 56 chevy kustom show car just has them removed.

[bcmiller]

Here ya go Bryon, I looked it up earlier.

http://www.compcams.com/Company/CC/cam-specs/Details.aspx?csid=111&sb=2   

It has a reverse split lift, and I noticed it's ground on a 112 LSA and 108 ICL.

it's going to act a little different than the original duntov 30-30 especially depending on where it's installed at.   So I'm curious how the vacuum is.

OK, I just wondered how it compared to the 350/350 cam.

[X33RS]

The 69 L-46 350/350 cam I believe is 222/222 @ .050 and .450/.460 lift on a 114 lsa.   Same cam later used in the L-82's throughout the 70's. 

The L-79 327 cam is close at 221/221 @ .050 and .447 lift I/E.  Also ground on a 114 LSA.

[jdv69z]

If the cam is like the original 30/30 type, vacuum might only be 8-11 in-hg at idle. Stock power valve with the 780 is 65 ( 6.5 in-hg)  so that the power valve circuit is not active at idle.