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Carburetors Carb discussion and questions. Upgrading your Third Gen's carburetor, swapping TBI to carburetor, or TPI to carburetor? Need LG4 or H.O. info? Post it here.

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Old 01-17-2001, 07:43 AM   #1
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Car: 2002 SOM z28
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Slight pinging after carb swap...

So now that I have the Holley 750vac on there, I have noticed an intermittantly occuring pinging (not exactly like 3 rocks in a can, but close). It doesn't happen every time I drive either, and only happens if I stomp on it when going uphill or just after it shifts into 2nd gear (700R4).

I jetted it from 72 down to 68, but now it is at 70. That seems to be a little rich for reference and I need to go back to 68 jets. I have new plugs and the MSD box is re-installed and didn't make a difference.

I have the blue medium springs in the HEI now and just put on an adjustible vac canister. It is out 3 full turns from zero (maybe about 15 degrees advance?) and may have helped a little. So I am seriously thinking this is a timing issue.

If it is a timing issue, what are my options besides reducing the advance (which is about 8 now) and going back to the heavy springs? Is this a fuel delivery issue?

------------------
1984 z28 w/ a 357 cu in. monster engine which is looking like the posterchild for Edelbrock with the exception of the Holley 750vac... all the suspension stuff... 9-bolt posi disk is in...

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Old 01-17-2001, 10:31 AM   #2
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A lean condition can do this, but from what you said I don't think that is the case. Your advance is coming in quicker than the engine needs. I would try to adjust your vacuum can and see if that will fix it. If not, back the base timing up 1 degree at a time until it stops pinging.
Purely as a test, you might try unhooking your vacuum can.
What is your total timing?


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82 camaro--original steering wheel, brake/gas pedals, seats--everything else modified
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Old 01-17-2001, 02:26 PM   #3
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Disconnect the vacuum advance, plug the port in the carb and drive it. The pinging will probably go away. If it does you still had too much vacuum advance coming in and will have to adjust(decrease) it some more. My car tolerates tons of base timing but adding very little of the vacuum advance makes it ping. It is okay to not use any vacuum advance but your gas mileage will suffer.

Sorry, 82 I didn't mean to repeat your reply. I didn't finish reading it the first time.

------------------
350 Vortec, forged flat-tops, CompCams XE268, Edlebrock RPM, Holley 3310, TH350 w/Holeshot 2400, 3.23 posi, Heddman shorties, HEI

[This message has been edited by ERIC'86IROC (edited January 17, 2001).]
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Old 01-17-2001, 03:01 PM   #4
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I use between 8 and 10 vac advance for most HO 350's, FYI. Good luck
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Old 01-17-2001, 03:19 PM   #5
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I was always under the impression that less base timeing and more vacuum and mechanical timing was better.

One source of my confusing is to what exactly the vac canister is doing (it's adjustable). Does it control the rate or the total amount of advance?

I assume the springs are involved in the time of advance, so switching back to the heavy springs would bump the time at which the advance starts up to a higher rpm. Or would it be wiser to only switch one of the springs to a heavy and leave the other at the medium?
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Old 01-17-2001, 03:28 PM   #6
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The vacuum can controls the rate of advance.

If you want to play with the mechanical advance too, disconnect and plug the vacuum advance line while you do it. Once you have the mechanical advance tuned correctly, reconnect the vacuum line and tune the vacuum advance.

------------------
1984 Camaro Z28 Astral Silver w/ 2 1/2" cowl induction hood
Engine: Decked 385 cid 4 bolt w/ oil cooler, 9.6:1 compression, ARP fasteners, 600 cfm Performer carburetor, Torker II intake, Performer RPM Heads, XE268H cam, Magnum roller tip rockers, MSD6AL ignition, Blaster2 coil, recurved HEI distributor
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Old 01-18-2001, 12:50 AM   #7
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just by reading the first part of your question i have a answer the secondaries aren't getting the enrichment for when you floor it if its vac secondaries it has a metering plate (maybe) instaed of a block. if so im not to good on them i do 4150 double pumpers.

------------------
1989 firebird formula

Mods: converted from T.B.I. to a carburator 305 to a 350. Flowmater exhaust,hedman shortie hedders,202 heads,350 horse cam,bored.40 over, Edlebrock torker2 intake.

Future mods performer rpm air gap intake (polished) and 600 edlebrock carb, comp roller cam, and way better headsa 400 defintely in the works!
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Old 01-19-2001, 11:11 AM   #8
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Good vacuum advance mechanisms can adjust rate of advance AND amount of total advance. You can minimize the vac advance by limiting the movement of the rod/diaphragm by welding a stop on it. Should be easy to understand when you look at it. Easier to buy an adjustable vac advance canister/setup, like Crane...

Base timing means nothing. That is just your idle timing. To set timing, you must set your high rpm centrifugal (or mechanical) advance (to say, 33 - 36 degrees at 3000rpm or so) and let the base advance fall where it falls. Disconnect the vacuum advance line and plug it during this procedure. Crank the motor to 3000rpm and set the timing light to show 33 degrees advance, as a starting point and add more if needed. Then reconnect vacuum advance line. You should do this whenever you change weights and/or advance cams and even springs.

Base advance will only depend on the advance cam(s) and weights and to a lesser extent to the advance springs, mainly because there should be (or usually isn't) no mech advance at idle...

[This message has been edited by FastBroker (edited January 19, 2001).]
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Old 01-19-2001, 03:53 PM   #9
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Well, I have stock weights and they are brand new. They came with the HEI for the swap. There is a chart that came with the spring kit that diagrams what the advance curve should look like with the blue springs (med). If I remember correctly, it started to come on aroung 2000 and was full in by 2800. I am thinking that is too much too soon.

As for the base timing, I have read somewhere that less base and more mech/vac will allow you to run lower octane gas without detonation. That could be BS though, I don't believe everything I read.

I have an adjustible vac canister too BTW. Thanks for the replies, I'll give these a shot this weekend hopefully.
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Old 01-22-2001, 08:31 AM   #10
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Mech advance should start coming in WAAAAAY before that, like around 1000-1500 rpm to START the advance curve, and ending at about 34 deg at around 2800-3600rpm, depending on your app.


FYI, MSD makes an adjustable vac canister tat adjusts the rate and total amount of vacuum advance. about $22 with allen wrench in Summit/Jeg's.

There are companies that will create an advance curve for you if you send them your distributor but I do my own. Search the web becasue I know that there was a pretty good sight that laid-out the methods and correct HO advance curve for HEI's.. saw it a year ago or so. Good luck
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Old 01-22-2001, 08:31 AM
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