Technical Revisiting exhaust backfire on decel issue

Ok everyone I just went out there for an hour or so before I have to go to work. Low and behold when I pulled all 3 air cleaners and checked again the front carb was drawing a substantial amount of air when it should be closed. Threw the blockoff plate back onto that carb and readjusted the idle screws....still backfiring and I can see black smoke now when I snap the throttle in the garage. Maybe I over jetted based on that leaking vac from the front? I'm going to block the rear carb back off again tomorrow as well and I guess try jetting down a bit. I'll run it just on the center carb for a while til I dial it in more.

 

Try removing the spark plug wire on the cylinder you suspect and run on 7 cylinders and see what happens. Do each plug that way on the side you get the backfire out of.

 

Did that already. The #1 plug wire makes little or no change in idle...rest cause a notable stumble. Plug is new as of yesterday. I've swapped wires side to side no change, leakdown, compression, springs, pushrods all good on that cylinder. Wondering if the fresh air from that front carb is causing it to lean misfire?

 

Watchin this I have same problem with mine but from Passenger side?
Same set up lake headers/baffles.

 

Interesting. What engine? Custom build, crate, original? What have you checked/tried so far?

 

Was a stock 1980 350, put in a basic thumper cam, and intake with the 3 large base 2G's

I am running a little fat. I believe I have pretty large jets that I started with 62's or 65's on the primary if I recall correctly.
Havnt played with jetting much since it runs/drives as is. Same as you I just get off the throttle easy most times.
The intake is a old skool Offy, Also do get the #2 cylinder running cooler than the other 3 on that side Especially at a idle also. Running R45TS for plugs myself and they are a bit dark in colour. 43's/44's got banged FAST.
Ignition Is electronic.

 

Certainly sounds similar. Have you checked if your end carbs are sealed at idle? Maybe we're both over jetted.

 

I really have not gone in depth But I presume so, Butterflies "look" shut at idle.

But as everyone knows LKS are deceiving.

 

Yeah mine look shut too...but put your hand over it while it's running and see if it pulls

 

Maybe a bit to do, Have to blow the dust off and fire it up.. Havnt drove in a couple few months. Work/and now neck surgery I am heeling from. Being a chopped channeled 34 truck its a tight squueeeezzze to fit in LOL.

It has been on my mind to at least fire it off to keep from screwing up the carbs from sitting tho.

Guess I can crank it from outside the cab and warm it up a bit.

 

Hahah same boat...chopped and channeled 34 with a flat firewall as well.

Going to get my other blockoff plate on tomorrow and try to go for a spin. If it's still backfiring I'll see what smaller jets I have in my stash and see if they help.

 

I wonder if you might find someone with one of those old giant test machines that would hook your engine up and see if it spots anything unusual happening. I know by now you must be pulling your hair out, but I applaud your abilty to keep working at it.

 

Well was COLD in the garage this morning.. 47 when I decided to come out here After doing my weekly shopping.

Cold blooded beast no choke had a heck of a time lighting off and took a while for fuel to get up to the carbs.

Finally after warm up and temp to where it belongs I checked front and back carbs they are not sucking air at a idle, So we know its good there, In my case Me thinks the manifold is half the problem or more. I am sure there was a reason this one wasnt made for to long.

 

Funny because my Edelbrock isn't available anymore either

 

If you decide you don't want the intake I'll buy it.. I have read all the way through all the posts but to tell you the truth so much has been posted I can't remember. Did you check for an intake leak? Lippy

 

I just spent some time on mine again....hot the 2nd end carb blocked off and reset the idle. Still backfiring just in the garage if I give it throttle and let it snap shut. Just for the hell of it I threw a set of 58 jets into the carb...down from 63s to see what it would do. Reset idle again and seems to have more backlash and popping now.

Tired of chasing my tail with fuel so I'm grabbing a wideband O2 sensor and gauge tonight from someone and I'll make a test pipe that slides right through my baffle and bolts to the header flange to keep it in place. This way I can just start tuning fuel to the correct mixture. Maybe the long test pipe will be enough to stop the backfire too

 

Odd how a stock sbc will run fine with one rochester 2 bbl on a stock manifold, (no pop) but yours won't with a 2bbl rochester on an aluminum intake. Hence I mentioned the intake leak.

 

Yes I have

 

Agreed. I'm wondering at this point if part of it is the cam profile. GM even states it gives an "aggressive unique" idle or something like that. Aka runs rough at idle haha. Maybe idle with that cam just dumps fuel and is overloading the exhaust? I don't know anymore. I think the wideband will be the biggest help at this point.

 

I harkin back to what was posted about the mufflers. Possibly it needs some backpressure. Cam profile could possibly be an issue, but I always remember the lake style headers on the sprint cars and how when the mixture was rich on a cold night and you would snap off the throttle going into a turn and pow! Just thinking out loud. Cold air entering a big diameter hot exhaust may have something to do with it somehow. Sounds like a header leak . A header tube cracked or leaking will do that. Dunno, still weird about that #1 cylinder deal, having little effect when the wire is pulled. Lippy

 

I agree....I'm wondering if it's poor fuel distribution with this manifold. Like I said I'm picking up the wideband setup later and I'll make a sturdy test pipe for it tomorrow. While I have the turnout off I'll yank the baffle quickly and try running it just to see if the baffle isn't letting the unburnt fuel escape. I'm going to up the jets back up first too because aside from looking a little rich part throttle all last season felt good at that jetting.

 

I feel like I need to look into this potential cylinder 1 misfire more as the cause of my backfire too. Aside from fuel distribution inside the intake what could it be?
I've had multiple plugs in that cylinder, swapped the wire with the #2 on the other side, 2 different distributors, caps, and rotors, leakdown and compression on that cylinder is the same as the other 7. Pushrods aren't bent and I've checked valve lash and opening multiple times.

I'll check the header again tomorrow for leaks as well as the intake. But these have been done before. Only thing I haven't done is check for internal vacuum leak from the intake. Just read up on hooking a vacuum gauge to the dipstick tube and plugging the holes in both valve covers as well as the other vac ports on the intake.

Beyond that what could be causing a misfire??

 

Low ignition voltage or insufficient engine grounding. Might verify proper and consistent voltage from ign. switch to the coil. Verify solid ground connection to the engine.
Just more thoughts

 

I'll check those both but not sure that would kill just 1 cylinder. Pretty sure it's getting good spark anyway...zapped me good the other day when I was pulling wires.

 

Try locking out the advance on the distrib. Zip tie the weights to give full ignition advance al the time.
(Good battery should spin it and crank.)
Set the timing to 36-40 degrees. The more advance you can run ...the crisper it will rev.
If nothing else it will eliminate any spark advance questions.
6sally6

 

Ran a bore scope through the intake earlier and don't see any restrictions...the intake valves all already have quite a bit of carbon on them though after 1,000 miles. Leads me to think I'm crazy rich despite the truck feeling like it pulls well. Saw it inside more than 1 intake port.

I pulled the exhaust baffles and it may have helped the backfire a little but still loading up and seeing flames when I really revit it in neutral and let off. I'm just about done making a test pipe that bolts into the header with a wideband sensor in it. It's raining so I want get any drive testing today but I'm curious to see what the a/f is at idle.

 

Maybe this has been mentioned before, but I don't remember.

Does the problem seem worse, or exist only when the engine is warmed up? Solid or hydraulic lifters? If solids, I'd try adding another .005" to .010" lash to the exhaust valves only. If hydraulics I'd back the lash out until it just starts to make an audible ticking and then maybe go another 1/8 of a turn out, again on the exhaust valves only. See if it makes any change to your vacuum readings while you're at it.

Either way, this will alter the cam timing by opening the valves slightly later and closing them slightly sooner. If you see any improvement at least it might point you more towards an issue with the valve train or cam shaft timing. And you won't have to stand back and just throw money at it.

 

They're hydraulic rollers currently set to 1/2 past zero lash. I could go 1/4 turn back on the exhausts I suppose.

 

I played with the engine the last hour or two with my makeshift test pipe and O2 sensor. I also maybe found my misfire cause. I pulled the #1 plug wire again with it running and with insulated pliers got zapped and saw sparks from the boot to the frame and everywhere else. When I pull any other plug wire I don't get that. Threw a new boot on the #1 wire and doesn't seem to be doing it anymore. Still backfires when I rev it up good and let it shut.
So here's where I'm at....baffles are now out of the pipes. I have to sorta cover the end of the test pipe to get a good idle reading I think because of the pulsing of the engine. When I do that my idle is now around 13-13.5 a/f ratio....this is probably slightly rich at this point and I may be able to turn the screws back in some.
In neutral if I hold it part throttle I'm at like a 12 a/f and goes as rich as into the 10.5's if I snap the throttle a bit. I know readings will be different under load but I'm thinking a bit rich on jets. I have 62s in it and may go to 60s and try to get a cruise in later this week now with the test pipe in. Maybe the rich cruise mixture is causing the backfire when the throttle snaps shut now and the unburnt fuel hits the exhaust.

Attached is a pic of my test pipe setup.

 

Weird thing is eventhough the old #1 plug boot was zapping me the last few times I pulled it while running I can't find anything wrong with it. No splits that I see. With the new boot no more shock though. Of course with the baffles out now and test pipe in I can't hear if there's a misfire at idle still or not