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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 09-18-2000, 09:57 AM   #1
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Tuning and Jetting Suggestions for a 750 Holley on a 350. Need some help with it.

I posted earlier about getting a Holley 3310-6 (750cfm vac sec) carb for my car. Well, after a day or so I had to swap back to my 600 Edelbrock because I was running WAY too rich. It was running like crap popping out the exhaust when I punched it. I could smell gas. I had to change my plugs too, they were black. I fouled at least 1 of them too from being too rich.

I need some help with tuning. I know a little bit about Holley's. I'm going to buy a book about em, and see what more I can learn. I finally have a chance in the next few days to put the 750 back on and mess with it. I know it has #72 jets in the primaries, which I believe are too much. It has a metering plate in the secondaries, not jets and is a 134-21 I think. It has the stock power valve which is a 6.5. Before it started popping, it would sometimes bog about 4000 rpms. when I punched it. I think it was getting too much fuel when it bogged.

My engine specs are in the sig at the bottom. I really want to get the Holley swapped back on. Any help would be greatly appreciated.

Thanks
Steve

------------------
- the 92 RS is gone..
- 1984 Z28 with 89 bumpers and ground effects.
- 355 with ported World S/R Torquer Heads, 214*/224* .471/.496 lift cam, Crane 1.6 Full Roller Rockers, 2500 non-lockup stall converter, Holley 750 carb, Edelbrock Performer intake, Hedman headers with custom 3" Flowmaster exhaust, Lakewood LCAs, Powertrax Locker with 3.23 gears.
- http://s84z28.fbody.com

[This message has been edited by S92RS (edited September 18, 2000).]
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Old 09-18-2000, 10:17 AM   #2
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Hmm....well your jets and PV should be *about* right. you can try leaner jets..see if it helps, but it sounds like the problem is elsewhere. My first thought would be the wrong secondary spring to explain the 4000rpm bog. The stock spring should be close enough..check if it's in there, no blackages in the vacuum ports, etc.

Check all the basic adjustments, float level, idle mixture, pump arm, and make sure the PV is good.

By "when you punched it" do you mean right away when you punched it, a ways up in RPM, or what?

[This message has been edited by Jester (edited September 18, 2000).]
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Old 09-18-2000, 03:26 PM   #3
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What's the best way to check the idle mixture. It was idling like crap the day after I put the carb on it. It was rough. Was ok the first day but it was always rich.

Well, It was backfiring or popping out the exhaust on acceleration. Was doing it just driving normal(not punching it really hard but a little). It also felt like it was missing kinda.

Could the float levels cause this too?

Thanks
Steve

------------------
- the 92 RS is gone..
- 1984 Z28 with 89 bumpers and ground effects.
- 355 with ported World S/R Torquer Heads, 214*/224* .471/.496 lift cam, Crane 1.6 Full Roller Rockers, 2500 non-lockup stall converter, Holley 750 carb, Edelbrock Performer intake, Hedman headers with custom 3" Flowmaster exhaust, Lakewood LCAs, Powertrax Locker with 3.23 gears.
- http://s84z28.fbody.com
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Old 09-18-2000, 03:52 PM   #4
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Best way to check idle A/F mixture is to put a vaccum gauge on it. Lean it out until it suddenly loses vaccum, richen it by 1/4 of a turn. If it develops a lean misfire then richen it by another 1/4 of a turn.

Most other A/F measurements are done with an O2 sensor or an aftermarket A-F ratio. If you do use the "do-it-yourself" way, you want to shoot for about 400mv - 700 mv during crusing. No less than 400mv, or you're lean, a very, very bad thing.

If you change the secondary spring and it still has bog, the link could also be too long. Holley has used different sized links and some are so long they "bottom-out" even before the secondaries are fully open.

Don't forget to check for a blown PV too, that'll make it rich quick.

Once you get further you can look into switching out the secondary plate in exchange for jets, accel pump cams and nozzles, PV selection.

------------------
'84 Trans Am
Crate 350 - 4 bolt main, Holley 600 w/vac. secondaries now open element
T-5 - Stock with Zoom clutch and PP
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Old 09-18-2000, 08:45 PM   #5
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I'm gonna go get a vacuum gauge, slap it back on and screw around with the adjustments. I might do it tomorrow. I set the mixture screws 1 1/2 turns open from the fully closed position.

It's a brand new carb so I don't think it's the PV. I haven't backfired through the carb and it has the PV protection thing built in. I really want to get it set up right. I just spent $220 to get this carb and want to use it.

Maybe it's lean on the idle circuit, but it feels rich for everything else, especially with the missing, black spark plugs and backfiring through the exhaust.

I used to have a set of #70 jets somewhere in my garage, I need to check. Maybe they might help. I don't know.

Thanks
Steve

------------------
- the 92 RS is gone..
- 1984 Z28 with 89 bumpers and ground effects.
- 355 with ported World S/R Torquer Heads, 214*/224* .471/.496 lift cam, Crane 1.6 Full Roller Rockers, 2500 non-lockup stall converter, Holley 750 carb, Edelbrock Performer intake, Hedman headers with custom 3" Flowmaster exhaust, Lakewood LCAs, Powertrax Locker with 3.23 gears.
- http://s84z28.fbody.com
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Old 09-19-2000, 07:50 AM   #6
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My setup is VERY similar to yours and I do not run rich at all. I have the 3310 with no modifications. I switched my distributor advance springs to the lightest one although that probably wouldn't help your situation at least not the rich condition. I was planning on getting smaller primaries also but surprisingly I don't think I need to. My engine is even milder than yours and should run richer. It has about 1000 miles on it now and the plugs are spotless. I don't have a bog but my powerband seems to fall off above 4000-4500 which is not good. That is at or below where peak power should be.

I would try the leaner jets and vacuum spring as Jester suggested. I think it was HOT ROD about 2 months ago wrote an article called Goof Proofing the 3310. They explain several mods to adjust for proper(leaner) operation on a mild small block, such as enlarging the idle air bleeds. This leans out the idle and helps with transition and part throttle economy. The book is a good idea too, I'll probably get one myself.

350, forged flat-tops, stock Vortec heads, CompCams XE268 224/230 @0.50" .477/.480, Performer RPM intake, Heddman shorties, Holley 3310, HEI, TH350 w/Holeshot 2400 stall converter and transgo improver/rebuild kit, 3.23 posi
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Old 09-20-2000, 11:05 AM   #7
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Location: Atlanta, GA, US of A
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Car: 94 Z28
Engine: LT1 w/ headers, catback, CAI, tune
Transmission: 4L60E
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I'm also running a 3310 on my Vortec headed engine. Originally it was really rich on the jets, comes w/ what, 72 jets I believe? I'm currently running either 68 or 69 jets I believe. According to the plugs and my A/F ratio meter is about right. It initially had a # 25 accel pump shooter, which was too small and gave some bogging/backfiring through the carb. Ran a 28 w/ my previous cam and it was fine (218 duration single pattern cam). Apon swapping in my CompCams 262 (218/224) I needed a 31 shooter.

On the power valve side I wanna say it came w/ like a 8.5 or so, didn't think it had a 6.5 in there? Anyway, I messed around w/ it and ended up running the 6.5, maybe I accomplished nothing there...

The secondary spring in mine was wayyyy to strong, the secondaries didn't even open up worth a damn. The lightest spring in the little holley spring pack Jegs sells was pretty good, but I did some testing and discovered that the darn secondaries were still virtually not opening during first gear at all (test was done wiring the secondaries closed, NO difference felt in back to back accel runs in first gear. Only in second/third did you notice a drop in power). Anyway, I resolved this by removing the check ball in the vac secondary housing. That allows the secondaries to virtually flop open as soon as the spring starts to let them. IE it no longer opens slowly, which w/ my quick first gear was not allowing them to really open much at all. Anyway, w/ the check ball out it is noticeably faster in first gear. Need some more track times now. Anyway, you'll need to play around w/ this. In your case you may not need em to open as fast as my setup, you might be worse off like that, just experiment with it...

For the idle mixture you just hook up the vaccum guage and adjust the mixture screws until you reach the highest vaccum readings. Going back and forth from each side until you reach the absolute highest reading you can. That gets you right on. I believe that is the method reccomended on Holley's site. Double check if someone says otherwise. It seems to work the best for me. I think that is everything, the 3310 seems to work pretty good for me now, without the check ball removed it was screwing me though. I still have a urge to try a double pumper and see if I'm still lacking something w/ the vac secondary setup.

Good luck.

------------------
Ray87Z
-Vortec headed 350.
86 IROC w/ a cammed 305 TPI.
Formerly Ray86IROC.

[This message has been edited by Ray87Z (edited September 20, 2000).]
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Old 09-20-2000, 10:55 PM   #8
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Thanks for the info Ray. I haven't had a chance to try and tune it yet. But will be in a couple days. I'm definitely swapping jets though.

Steve
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Old 09-23-2000, 01:32 AM   #9
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I think I got it. I swapped in some #68 jets in place of the 72's. Also I put in a #28 squirter in place of the 31 that was in there. Then I checked all the adjustments and set the idle mixture with a vacuum gauge. It's running great now, feels like it's got more power than with the 600 Edelbrock. Also it pulls harder at higher RPMs.

Need to go to the track now and see what it'll run, if only it would stop raining.

Thanks for the help!
Steve

------------------
- the 92 RS is gone..
- 1984 Z28 with 89 bumpers and ground effects.
- 355 with ported World S/R Torquer Heads, 214*/224* .471/.496 lift cam, Crane 1.6 Full Roller Rockers, 2500 non-lockup stall converter, Holley 750 carb, Edelbrock Performer intake, Hedman headers with custom 3" Flowmaster exhaust, Lakewood LCAs, Powertrax Locker with 3.23 gears.
- http://s84z28.fbody.com
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Old 09-23-2000, 12:06 PM   #10
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Be careful with those 68 jets. I had those in my carb for a little while, and it ran too lean. Engine temp started climbing. It sounds like you're having the same problem I had when I converted over from TPI. The car would surge above 4000, and the best I could get on the G-Tech was a 13.70. If I removed the air cleaner, it stopped surging, and turned a 13.50. After weeks of screwing around with jets, secondary springs, and throttle cams, I finally got it dialed in. I replaced the secondary metering plate with the conversion kit, so I've got jets in the secondaries. I installed a quick change secondary spring cover. (because I was changing them so often) What I finally ended up with was 74s in the primaries, and 80s in the secondaries. The primary squirter is a 28, instead of the stock 31. The secondary spring is one step heavier than stock. The throttle cam is a flatter one so it doesn't hit quite as hard. The choke plate has been removed, and the choke horn has been completely cut off. The differences in the car are amazing. It pulls hard and smooth all the way to 6300. With, or without the air cleaner, the ETs are the same. Your engine specs aren't that different from mine, so this setup might help.

BTW, the carb is the latest mod to my car, so the ETs in my sig reflect the gains from the tuning.

------------------
91 Firebird Formula. Converted to carb. Holley 750. Forged Flat top pistons. 10.5 to 1 compression. ZZ-9 Cam. Ported L98 Heads.
150 shot NOS, 700r4 w/ 3000 rpm converter and 3.73s in the back. Headers w/ full dual exhaust. 27.5 inch McCrearys. (free slicks)
12.98 @ 106 Natural,
11.83 @ 113 NOS.
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Old 09-23-2000, 12:06 PM
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