SBC 350 Engine Backfire Through Exhaust and Carb
 

1979 350 block stock crank, pistons, rods. New rings and seals.

I added;
- Vortec heads stock other than new valve springs to allow for larger valve lift on vortec heads from Alex's Parts http://www.alexsparts.com/products/V...YD-ROLLER.html
- Howard's cam and liter kit 110991
.470 lift on int.and exh.
284 deg of duration
108 lsa
- MSD HEI ingnition
- new plugs and wires
- flotech headers
- Edlebrock Performer RPM intake
- Edelbrock 1406 carb

Let me know if you need to know any other info on the engine.

timing is at 12 deg adv.
compression is 155 - 165 psi (checked cold)

Backfiring happens on start up fast idle. Wife says it sounds like a popcorn machine. Pretty sure it is happening on both sides of the engine.

When I kick it down the popping stops, but when I rev up to 1700 - 2000 it comes back and if I hold it and then burb the throttle it sounds like I let off a round from a shotgun.

I have quadruple checked the lifter/rockers for overtightness. 100% sure that is not the problem. I have checked for vacuum leaks everywhere and cannot find any.

Not sure where else to look other than the POS carb, which is a couple years old, but has only seen maybe 5000km's. I had it on this same engine before all the add ons and while it was never tuned up as good as it could have been the previous set up never had a backfire through exhaust issue only the ocassional through the carb. It definatley could use some fine tuning, but I'm not sure if that is what would cause the backfire through the exhaust.

The second issue is the backfire through the carb. This I understand occurs beacuse of a lean condition, correct.

I'm at a loss. i have checked everything I could think of and researched. The only thing I have left to do is take it to a mechanic and bend over while I pull my pants down.

I'm about ready to put an LS motor in and be done with it, but knowing my luck I would have to show a whole bunch of loving too before it would run without issues:waah:

Any help is appreciated.
BG

Re: SBC 350 Engine Backfire Through Exhaust and Carb
 

Ok what happened???

it ran fine then one day it.......

Or you got done building it up and now it don't run?

new parts aren't always good parts, only time similar was half of the pickup coil in the HEI was bad it seemed. As with a timing light all cylinders would randomly miss. It happened after a intake swap.

Ended up having one built by Dave Ray / AKA IgnitionMan

Kinda like would you buy a Microsoft or Apple computer from a store or get one hand built, programmed and performance tested by Bill Gates or Steve Jobs themselves for about the same price?

http://www.davessmallbodyheis.com/

Re: SBC 350 Engine Backfire Through Exhaust and Carb
 

I recently built it up. I broke it in. I have't really driven because of the backfire issue and plus the fact I'm still finishing the body. I recall it backfired once in a while after the initial breakin out one side, but now it seems to have gotten worse and happens out both sides.
If it were a cam or lifter issue why would i have backfiring through the exhaust?

Re: SBC 350 Engine Backfire Through Exhaust and Carb
 

never broke in an car engine but wiping out a cam is the biggest worry, ive seen lots of post about to much spring pressure taking them out.
but reading and knowing are two different things.

I just know Ill dread my first cam break in when ever it happens ;)

Re: SBC 350 Engine Backfire Through Exhaust and Carb
 

Popping through the exhaust is generally a rich condition. Raw fuel in the hot exhaust ignites and causes the popping. Popping through the carb is usually a lean condition or improper timing. Did you run a zddp additive and the proper oil during break in? How exactly did you break it in? Did you let the engine idle at all? What oil are you running now?

Re: SBC 350 Engine Backfire Through Exhaust and Carb
 

Ignition issues? Module getting weak?
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Re: SBC 350 Engine Backfire Through Exhaust and Carb
 

I followed the cam breakin procedure provided and used comp cam breakin oil and howard's cam zddp additive. used the lube on the cam lifters etc. The oil in there now is meant for new engines, but I can't remember the brand.

Re: SBC 350 Engine Backfire Through Exhaust and Carb
 

Check your plugs an see if any of them are gas fouled or wet with gas. If so replace them. If you are having trouble with it starting it probably also has been getting to much fuel.

Re: SBC 350 Engine Backfire Through Exhaust and Carb
 

Ignition is all new. Doesn`t mean there couldn`t be an issue with it, but it runs fine when idling. Seems to only pop and backfire when rpm`s are up around 1700 - 2000+ QUOTE=shortbed70;4630515]Ignition issues? Module getting weak?
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Re: SBC 350 Engine Backfire Through Exhaust and Carb
 

Are you using a zinc additive now? its just as important now as it was during break in.,

Re: SBC 350 Engine Backfire Through Exhaust and Carb
 

No trouble with starting and some plugs seem to be ok and others might have a bit of carbon built up (not a lot), so I clean and put back in

Re: SBC 350 Engine Backfire Through Exhaust and Carb
 

no zinc additive, but I`m pretty sure the oil I put in has some in it.

Re: SBC 350 Engine Backfire Through Exhaust and Carb
 

Most oil does not. What kind of oil is it? IM not condemning your cam or engine at this point by any means. Just want to make sure you avoid future issues. The first thing you need to start with is a compression check. Low compression can indicate a worn cam. As someone else said. Pull the plugs and make sure they arent black or wet. Also make sure the carb isnt just dumping fuel. Edelbrocks are extremely sensitive to fuel pressure. Anything over 5.5psi will cause them to run rich/flood.

Re: SBC 350 Engine Backfire Through Exhaust and Carb
 

I'll have to check on the type of oil. I believe it's a mobile 1 product? Compression cold is between 155 -165psi so I believe I'm good there. Fuel pressure gauge tells me I'm 5psi when running. I have to double check the plugs.
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Re: SBC 350 Engine Backfire Through Exhaust and Carb
 

Double check your firing order. But sounds like you are losing a couple cam lobes.
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Re: SBC 350 Engine Backfire Through Exhaust and Carb
 

Firing order is correct. Just double checked.Posted via Mobile Device

Re: SBC 350 Engine Backfire Through Exhaust and Carb
 

Oil doesn't have any zinc additive. Not sure why I thought it did?
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Re: SBC 350 Engine Backfire Through Exhaust and Carb
 

Next step. Pull the valve covers off. Roll the engine over by hand and watch the rockers move. Look for any rockers that arent moving as far as the rest of them. Have you checked fuel pressure by any chance? Get a bottle of zinc additive in that engine asap.

Re: SBC 350 Engine Backfire Through Exhaust and Carb
 

I will roll it over and watch the rockers again. Last time I did it everything looked ok. Fuel pressure is 5psi at the carb.
Are you thinking what I'm thinking... Valves are sticking in the valve guides?I will add zinc before I start the engine again and after I check rockers. If the rockers are moving fine I think my issue is probably in the heads and I'm thinking the valve guides. Thoughts?Posted via Mobile Device

Re: SBC 350 Engine Backfire Through Exhaust and Carb
 

very doubtful to have valves sticking on a fresh motor. Im thinking more along the lines of a cam starting to wipe out. Its really hard to sit here and guess. But to have a backfire both through the carb and out the exhaust its the most likely suspect. Just for giggles pull the distributor cap and make sure the mechanical advance is actually moving. I know you say your timing is 12 at idle but if you have a timing light capable of showing advance see what timing is at 3k rpm.

Re: SBC 350 Engine Backfire Through Exhaust and Carb
 

The heads I wouldn't say are fresh. They were bought used 15000km approx. on them. When I installed new springs I seem to remember some valves moved more freely than others, but I'm not engine builder, so never thought anything about it. I have a feeling they may be the problem or part of it, as that issue may have caused some detrimental wear on the cam and/or lifters/pushrods.

Re: SBC 350 Engine Backfire Through Exhaust and Carb
 

Turned the engine over and watched all rocker arms move. These all move the same distance visually, so I'm confident that rules out cam or lifter issues.

I'm going to run a vacuum gauge and see if there are any wierd symptoms happening when idling, and running at different RPM's.

Re: SBC 350 Engine Backfire Through Exhaust and Carb
 

mobile1 engine oil....as in synthetic? if it is synthetic, get it out of there. also, how did you adjust the valves. kinda sounds like you may have a few valves too tight?

Re: SBC 350 Engine Backfire Through Exhaust and Carb
 

Dino oil. Valves are adjusted with engine running. Quadruple checked!

Re: SBC 350 Engine Backfire Through Exhaust and Carb
 

I had the same back fireing issue under acceleration from the carb and it turned out that the cam was beating up 1 lifter and eventually made a small hole in the lifter. I would check the lifters and make sure they are ok and if you find one that is bad or getting worn down than pull the cam and replace it and all of the lifters...it only takes one. As far as the motor being a flat tappet motor its crucial to break in properly and keep a zddp (zinc phosphorate) additive in your oil. No synthetics. Just my 2cents from experiance.