Hi folks ed here: new guy
79 z28, 350, Hooker Headers,aluminum intake moderate lift comp cam,aluminum heads 2' intake valves. Stock ign. 600 cfm holley
vac sec. Problem i installed an msd6a ... bummer...backfires thru carb . checked for crossfires, changed cap, rotor, coil good. Installation double checked ok. Played with timing no better. Advised by holley and msd i'm too lean. Jetted up primary a ch68 Better no more back fires but still a stone boat on accel. There is an adaptor kit available for the rear bowl area which will allow installation of jets. It has been sugested that my carb is just too small and i should go to a holley 670 avenger.What say you oh learned ones.
Ps this vehicle ran quite nicely for 3 seansons prior to this installation.

 

Welcome...but LS1 tech is definitely the wrong place to ask this question.

Try Classic tech.

 

Well I will agree that the 600 maybe, and that's a big, MAYBE to small. But if it ran fine before, the carb didn't get any smaller after the MSD install.
Did you have to remove the distributor during the MSD install? Sounds like the classic, one tooth off, to me.

 

Don't know if 70 extra cfm is worth changing for. If you are going to go bigger, get a list 3310. It's a 780 vacuum secondary carb that was used (basic carb) by Chevy in everything from 302's to 454's.

But don't think your 600 is too small. You might want to buy a secondary spring kit to make sure the secondaries open up as quickly as possible without going too far and getting a bog.

On the subject of secondary opening: I once let my 71 sit for a while when it had original 350 and 600 carb. When I ran it again it seemed down on power. Tested to make sure secondaries were opening by cutting a thin piece of cardboard (like parts tag) about the size of a dime and making a small hole in center, slit to edge of tester. The hole would allow it to stay in place when I placed it on shaft that comes down from secondary cannister to rear throttle shaft.

So I slid tester all the way to the top and took it for a WOT run. Tester stayed at the top - secondaries weren't opening. Figured torn secondary diaphram. Nope, it was good. So I shot carb cleaner into the passage on side of carb that feeds vacuum to secondary cannister.

When it goes into carb body, this passage splits in two, one passage to front pass side bore, other to rear pass side bore. Well the carb cleaner was shooting out of the rear passage, but not the front. The little cork O ring that seals secondary housing to carb body was flaking and one of the flakes plugged primary bore vacuum feed, so secondaries couldn't open. Dug it out, engine ran like a champ.

Also used tester when I was experimenting with softer secondary springs. With the stock spring, the secondaries would not open all the way, maybe 80%. Ended up with the short yellow spring ( I think). Opened up more, sooner, and no bogs. When I went to a 780 on the 400 sbc, I ended up using the white (softest) spring.

One last thing (I'll try to keep this short ): whenever you're chasing tuning bugs, always make sure and check float levels first. Super easy on Holleys. Brass plugs on passenger side of float bowls. Check with vehicle on a level surface, engine idling. Fuel should just dribble out bottom of holes.

 

I'm going to get alot of flak on this, but a 600 is too small for a 350. I know, I've seen the neato airflow chart that says you only need 604 CFM at 6000 RPM for a 350, but it's all bunk IMHO.

I used to have a 350 in my Z28 with a stock '79 Z28 bottom end, L79 cam, Trick Flow Heads, Performer intake, 600 edelbrock carb, 2500 stall and 3.73 gears. It was my first year racing in the Pro Bracket (0-13.99), and my car was running 13.70's on cold nights, but it had problems running fast enough on hot days. I had tried everything, and I was pretty frustrated. The guy I was in points with, finally convinced me to try a reworked, Q-jet. The car ran 13.45 the very next run, and ran a best of 13.13 on a nice cool fall day that year. The carb GM put on my Z28 was a 750 CFM Q-jet. Why would GM put a 750 CFM carb on a 350, when 600's are cheaper? Leave the 600's and 650's for the 305's.

 

You might have taken flak in the past, but that formula has been pretty much discredited in this forum in the past. A lot depends on how built the 350 is. You won't lose anything if you use a 780 or Qjet on a 350 cause they are vacuum secondary. But if it's a mild 350, you won't gain much, if any, hp in going to a bigger carb. Eddore said he used a moderate lift cam, so tended to make his engine sound like a moderate build.

Once I tuned it, the 600 ran great on my original 350. It was a mild build and didn't gain anything when I popped a Qjet on the Performer intake. Probably cause the 350 was only demanding 500 CFM or so. I used the same 600 on the 400 when I first installed it and it definitely held the engine back, even thought the formula said it was right on the money CFM wise (I shift at 5,500). 780 chopped almost half a second off 1/4 mile time.

 

get ur self a nice holley street avenger 670cfm

 

I'm glad someone agrees with me. It's hard trying to convince someone not to believe everything they read.

The 600 Edelbrock was probably the most consistant carb I've ever had on a race car. Too bad it was acting like a restrictor plate.

 

That engine definately sounds like it is being choked off.

I know the chart shows 600ish cfm but once the rpms go over 4500, you might as well get the larger carb.

That 670 BG sounds like a nice fit. Or even a reworked QJet.

 

If it is backfiring, a bigger carb isn't gonna help. Sounds like you are experiencing a lean condition that can be corrected by going to larger jets in the primary. Maybe even check the accelerator pump. What do your plugs look like? Did you check the float levels? I'd keep going up in jet sizes and see how that effects the condition...while inspecting the plugs to see what they look like....rich or lean by the color of the porcelain.

Save your money and find a remedy for your current set-up. You can buy jets for under $10 and accelerator pump cams for uner $20. No need to spend $400 on a new carb. Atleast not without trying other routes first.

I suggest you toy around with what you got. A 600cfm carb is enough for a mild 350. True, you could get more potentially from a 650 or 670 but if you are unable to tune what you currently have, which by all means should provide enough for your current set-up, anything else is just throwing money away.

David

 

Just alittle off the subject but a friend and I are putting twin 750 Domanators on his 412 ci smallblock , had a 1150 single and ran a little lean on top end with lots of N20 so we upped the cfm to 1500 .. also i ran an 850 dp on my daily driven 350 in my old S10 mileage was terrible but it ran strong ! and i think I would return to your previous set up and see if any one thing is at fault by eliminating the MSD first and going back it will give you an idea if its at fault .. good luck .