SBC 350 Engine Backfire Through Exhaust and Carb

[SB'72]

I had the same issue last year when I freshened up my HEI, the Module on top doesn't come with the factory ground strip installed. It's in a small plastic bag, and has to be added. Not sure if you did anything with that, but problems sound the same as I had.

[Foosified]

I have the Street Fire Chevrolet V8 GM HEI Distributor. I read the install instructions, but it doesn't mention anything about a grounding strap? Is this typical for all HEI dist's?

Originally Posted by SB'72

I had the same issue last year when I freshened up my HEI, the Module on top doesn't come with the factory ground strip installed. It's in a small plastic bag, and has to be added. Not sure if you did anything with that, but problems sound the same as I had.

[rickpilgrim]

We had a very similiar problem with the fresh 355 we put in the 82. 190 psi cranking psi, .030 flat tops, 76cc 2.02 heads, arp bolts, Crane cam, rhoads lifters, full throttle shift@6500rpm, performer rpm intake with Carter 625 and MSD Street Fire Distributor. Day one gave us backfiring and popping mostly under load and idling when warmed up.
Turned out that the double platinum spark plugs and WELLS ign wires had to much resistance, and the MSD low buck distributor cap had sparks scatering everywhere. Of course we found that out after a spark hit the modual and fried that. If not that you have a cam/valve problem IMO.
I run 10w-30 Shell Rottela diesel oil in our flat tappet cam engines after break in with additive. Diesel oil still has the zinc in it you need for these old motors. Modern synthetic oils are tailored more for modern roller cam engines and really shouldn't be used in older motors, or so says my cousin who is an oil engineer at Exxon/Mobil.

[cableguy0]

Just for reference as of 2008 diesel oil no longer has the zinc in it either. It was removed when catalytic converters became mandatory on diesels. Yes it still has some zinc but not anywhere near what it used to. Theres a lot more info available on what oils are what. Heres a ton more info on the topic. http://67-72chevytrucks.com/vboard/s...d.php?t=381780

[SB'72]

I am not sure about the Street Fire, but I had picked up replacement cap/rotor/coil pack from Napa. I of course didn't bother with instructions and replaced everything as usual. Everything was fine at idle, but under accel. not so good. I retraced my steps until I discovered that problem, but if your info. doesn't mention it i would think it's a non issue.

[Foosified]

Checked my wires resistance. They are a universal kit from MSD super wires 8.5mm and they have the following resistance rating ;
#1 wire = 127.5ohms
#2 wire = 124.6ohms
#3 wire = 139.9ohms
#4 wire = 110.8ohms
#5 wire = 94.7ohms
#6 wire = 110.3ohms
#7 wire = 87.0ohms
#8 wire = 105.5ohms

These wires are advertised as being 40 - 50ohms WTF?? Brand new. I wonder if the 40 - 50 ohms takes into account the ends of the wires which are crimped on?

Do these numbers indicate an issue as far as plug wire resistance goes?

[rickpilgrim]

If I understand this right the mfg lists the resistance as 40-50 ohms, but is that per wire or per ft of wire?
Here in WI where its winter for 6 months I would replace those as it is very humid here,which can make for easy arcing.
As to the oil I last talked about this with Randy in 2006 so now when I get oil I have to find additive. Just what I wanted to not know. 28 flat tappet engines around here, from 5hp to 360hp.

[Foosified]

Not sure if it is per foot, but I checked my old wires, which were about a year old and they had quite a bit more resistance. The old wires are not high quality, so I think my msd wires are still good.

Originally Posted by rickpilgrim

If I understand this right the mfg lists the resistance as 40-50 ohms, but is that per wire or per ft of wire?
Here in WI where its winter for 6 months I would replace those as it is very humid here,which can make for easy arcing.
As to the oil I last talked about this with Randy in 2006 so now when I get oil I have to find additive. Just what I wanted to not know. 28 flat tappet engines around here, from 5hp to 360hp.

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[cableguy0]

Most claims are in resistance per foot. The longer the wire the more power lost through the wire itself. Thats why most modern cars have no ignition wires whatsoever but a coil pack directly on the spark plug itself. There will always be loss of power over a given distance.If you notice wires 5&7 have the least amount of resistance thats because they are the shortest wires on a sbc.

[stich626]

ok a few questions..
you have "new to you" vortec heads. correct..
that you had new springs put on.. and I'l guess that howard cam is taller than .480 lift..
did anyone cut the valve guides ??? if not I'll bet on bent pushrods.. and uncontroled valves that are bouncing off the seats..

[Foosified]

Originally Posted by stich626

ok a few questions..
you have "new to you" vortec heads. correct..
that you had new springs put on.. and I'l guess that howard cam is taller than .480 lift..
did anyone cut the valve guides ??? if not I'll bet on bent pushrods.. and uncontroled valves that are bouncing off the seats..

cam spec says its .470 lift and the springs I put on are supposed to be good to ~ .550 list. here's a link http://www.alexsparts.com/products/V...AT-TAPPET.html

[cableguy0]

Its not the springs that are the problem. Its the heads themselves. Vortecs need to be machined in most cases to take higher lift. Anytime springs are upgraded and aftermarket rockers are used the head studs should at least be pinned to prevent them from pulling out. Lay a straight edge across the top of all the rocker studs. look for one or a few that are higher than the rest. Screw in studs are the best way to go but pinned studs are sufficient.

[Foosified]

Well it was a good day today! I am a happy camper again. No more backfire and it runs beautifully. The problem is the carb, which is so out of tune??? I found this out by putting on my carb from my 68 bronco (edelbrock 1406). Also put new spark plugs in. Fired it up and no more backfire.
Now what to do with the other carb. Edelbrock 1406 bought it new 4 years ago and it never ran right. Of course I don't know how to tune it, but everything I ever did to it never made a difference.
Would a shop that builds engines be able to bench tune one of these? I need a carb for my Bronco now!


Thanks for the help boys!

[cableguy0]

Edelbrocks are fairly simple. Some are more sensitive to fuel pressure than others. I would take the top off and check the float. A kit is pretty cheap. Edelbrocks tech manual is fairly in depth and available online. At least it was a simple problem and a relatively easy fix.