Need advice on Holley carb.

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LarryB340

LarryB340

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Gainesville Ga.
I have a 0.030 over 340, 10.5:1 compression, Edelbrock air gap intake, headers, big valve "J" heads with mild port work with a .513/.533 lift, 234/242 duration @ 0.050 Lunati Voodoo camshaft, backed by an A833 four speed and 4:10 8.75 Sure Grip. Topping it off is a Holley 750 double pumper with the HP center section and new baseplate. Looking for jetting advice. At the moment I have .72 main and .82 secondary, 28 main air bleeds and 57 idle air bleeds. I believe this carb was on a big block before I got it. It runs out good, but has always had a rich idle, only pulls about 9 inches of vacuum at idle, and I'm only about 10 degrees BTDC initial. I know it wants more timing. Can someone with a similar setup give me a good starting point on jetting, air bleeds and timing?
 
I wouldn't touch the carb until you get the ignition timing sorted out. No sense in tuning your carb twice.
 
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cawcislo said:
How much are your idle mixture screws turned in? Are they almost bottomed out?
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They were at around 1 turn out, then I blew a power valve and it got even worse. Its a four corner idle. I have the carb off and apart at the moment, just looking for a good suggestion on jetting.
 
mopowers said:
I wouldn't touch the carb until you get the ignition timing that sorted out. No sense in tuning your carb twice.
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Agree 100%. That cam is going to want in the 16-20 range at idle for timing. Set it there and see if you can get the vacuum reading up at the same RPM you currently have. Then work on the curve to get your total in range.

Stick car you can cheat a bit and have the idle in the 850 range. Don't try to lope it down a bunch.

Jetting, I'd start with whatever was stock in the carb and go from there.
 
crackedback said:
Agree 100%. That cam is going to want in the 16-20 range at idle for timing. Set it there and see if you can get the vacuum reading up at the same RPM you currently have. Then work on the curve to get your total in range.

Stick car you can cheat a bit and have the idle in the 850 range. Don't try to lope it down a bunch.

Jetting, I'd start with whatever was stock in the carb and go from there.
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Yep, I’m at 23 degrees with 251/258 with 593 lift.
 
I can tell you that .057 IAB is way too small unless the IFR is about .020 or something like that.

I’ve never seen a combo that wanted that small of a IAB. The MAB looks small too, but I have no idea what emulsion is in there.

4.10 gears will take an 8.5 power valve, and probably a 10.5 but you have to check vacuum to know that.
 
Go to Holley's website...Look up your carb...and reset it back to stock ....thats the beginning.....
 
70aarcuda said:
Go to Holley's website...Look up your carb...and reset it back to stock ....thats the beginning.....
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I'll ad to that 72/81 sounds right to me
for jetting with his 340. 'If it's just one PV in the primary side . iir 71 primary is stock on a 750 dbl .Must be an old carb to not have pv blowout protection.
 
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my new Holley 750 DP was supposed to have 70MJ and 80SJ, it had 73s in the main but did have the 80s on the secondary, PV is a 6.5
 
mopowers said:
I wouldn't touch the carb until you get the ignition timing that sorted out. No sense in tuning your carb twice.
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This right here^^^^^^ Timing is always first or else you'll be chasing your tail.
 
Hey guys, I appreciate all the responses, a few things about the carb, it is the way I bought it used on e-bay, I took the primary main jets out, 76 and put in the 72s. The main air bleeds were a suggestion from a friend, it has a new holley baseplate with a check ball in the power valve passage. I was using a 4.5 power valve. One in the primary, secondary blocked. Metering blocks are stock as far as I know. I have driven the car with this carb, after a warmup, it would idle, rich, but pull hard when I stood on the gas, no smoke out the tailpipe.
If 72/82 mains sound about right, I'll put it back together and try increasing the base timing. Just looking for a good starting point on jetting. This is only my second 340 build, the first one in my 30s. I'm 61 now, so its been a while.
 
RustyRatRod said:
This right here^^^^^^ Timing is always first or else you'll be chasing your tail.
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Didn't Smokey Yunick say that 90% of carburetor problems are in the ignition? :D
I'm running 27 idle advance (272@.050, .652 lift), idling at 1200 and 8" vacuum. Could probably use more but I drive on the street mostly.
 
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