Technical Revisiting exhaust backfire on decel issue

Following...but honestly, it's like enough already. 14 pages of life from two posts no one is getting back are they. It was if you look back, a whole lot of members who did bleed chips for you. One by one...turn the light out when you leave.

It's painful. We feel for you. But end the suffering. It's the USA for God's sake, someone must have a fancy electronic gizmo that has no place on or in a traditional hot rod but it might just tell you where and what is causing a problem.

Maybe post a picture, a video on you tube. It's like reading teeth being pulled.

Like a day time soap... Will the bigger jet help? Will the back fire go away? Will HAMBERS interest be rejuvenated by the addition of an old engine analyzer. Tune in again nest week for another exciting episode.

Pins and needles, pins and freakin needles.
But as we are all getting older...no one's getting younger, weren't you going to yank and regasket the manifold. Cause we are paying attention but wonder who the heck is listening some days.

And for the record, I'm waiting to mumble into my screen, it's about freakin time.
I think I speak for many. Me for sure. Good luck.

 

Come on Noel, many times putting things down in writing is a good way to sort your thoughts out. Maybe some have checked out, maybe some read the posts and move along.
But compare it to a build thread that goes on for years, the OP of the thread isn’t looking for approval, guidance, replies, etc. He/she is basically making a log of what he’s done and results he’s gotten. Sure, some additional input would be welcome, but now the OP can scan through his thread if he wonders “what did I do two months ago”.
But that’s just me

 

As Budget said this is a good rolling log for me to look back on and see what I've done so far and what results I had. Like the couple weeks I spent chasing distributor phasing that your brought up that unfortunately didn't pan out to be my issue.
Also thought someone may benefit down the road from the wideban tuning I just went through as a sidebar. Wasn't the cause of my backfire but definitely accounted for my terrible mileage and fairly sooty plugs.

As for regasketing the intake....that's about the last thing I have to try at this point. Haven't found anything yet that points to an internal vacuum leak which would warrant it. Kinda hoping the cylinder drop test points me in that direction. If and when I do pull the intake it won't be a 1 days job because the cheap spray paint the tripower supplier put on the intake and carbs isn't holding up so I'm going to have them cerakoted at that time which will likely take a few weeks if I have someone do it or a few days if I attempt it myself.

 

The skill of diagnosing a problem is what appears to be the problem in solving this painful wisdom tooth extraction.

Let's do the math...money spent. Money spent in replacement parts. I used to work as a Welding instructor and students would bring a weld out and say how's it look. Ya offer up advice and off he'd go, try something new maybe, but to try it again. That is what we have here.
NO improvement, or slight improvement, but he keeps coming out with the same bloody problem. 14 pages worth of trying to find solutions, trying different and the same things.

What do they say about insanity, what are the results here.

I mention that because frankly, I don't need to write it down, I remember it.
So I said end the pain call a professional, which would be me getting off my ass because he's not getting it, only to discover the problem isn't him, but the equipment that is the problem.

But yea, my memory say's this should be taken down to the timing chain cover coming off and rechecked completely. Now, that said, if you hooked up a modern computer function scan tool that has no place monitoring system functions on a hot rod you might be able to confirm or prove my suggestion in depth of tear down as unnecessary. Like a ultra sound or CT scan not just a gloved finger up the butt.

I think it's treating the symptom and not finding the source of the problem. So yea, I called it as I seen it, time for a second opinion. After all, it started as a gasket leak at a header and look where it ended up.

 

So far the only real money spent that was unnecessary was the MSD distributor and some extra spark plugs. All of the carb work needed to be done regardless to dial in my mixture. Time spent on the other hand has been excessive for sure. As for bringing in a professional I've already spoken to several and we've come up with nothing that hasn't already been posted and tested in here. As for degreeing the cam 2 engine builders told me if it was off I'd likely have more issues than a single cylinder misfire....but at this point I'm close to pulling it to check. I'd file a warranty claim with GM first before I open it up though.
The large portion of my automotive work is in restoration but I've been working on engines for over 25 years. I used to write tech articles for a magazine and have had my work featured in several over the years....let's not make it seem like I'm an amateur here.

 

I keep pondering on the fluctuating vacuum readings. Wonder if it's referencing to something besides the cam?
I've seen vacuum leaks from intakes not sealing at the bottom gasket surfaces, that weren't detectible with a spray test topside.

 

I'm still wondering if maybe the intake gasket is cracked between 2 cylinders but I feel like I'd have a backfire on both then. If it was internal I feel like I'd be pulling oil out of the valley and I'd have oil on a plug. The vacuum swing is only 1hg and doesn't seem to be consistent or rhythmic with what would seem to be the revolutions of the engine. Also why along with good leakdown and compression I don't think valve either.

Hoping next week to narrow it down for sure to a particular cylinder and try to diagnose from there again. With the low RPM tach I'm hoping I can do a cylinder drop test as well as try going around again with propane and maybe see a fluctuation that I haven't been able to hear.

 

Well interesting find tonight. After moving my test pipe with the wideband to the driver side this week I only had a chance to run it at idle. Tonight I finally took it for a ride. The AFR numbers look pretty much the same as the passenger side and guess what.....NO BACKFIRE with that test pipe attached to the header pipe! The pipe is about 18-20" long tops. Mind you I had the backfire with and without the baffles inside the lake headers.
So the obvious statement is I should put a full exhaust on the truck and be done with it and happy. But in the interest of knowing.....why is this happening? There are probably tens of thousands of guys running lake headers with or without baffles on a SBC and they're not getting a violent backfire. I have to assume it's scavenging fresh air back into the exhaust....is it due to cam profile? And I know I was overly rich before all the carb tuning but I've definitely leaned it out to what are generally considered the appropriate target AFR numbers for idle, acceleration, and cruise.

 

Finally! I first mentioned the exhaust in post #16.
I hope you get it performing in a way that makes you happy.

 

A few of you mentioned it earlier. It just never made sense to me that it was so violent on one side and not the other. I'd still like to completely understand why. Is it a combo of cam profile and the 5&7 cylinders firing back to back?
The test pipe is coming off today because I can't open the driver door with it on so I'll be back to backfiring. At least now I can start to design an under vehicle system while still driving it. I'm thinking of even sandwiching the end caps between the headers and turnouts to keep the look of them despite routing all the gasses out of the bottom takeoffs.
As for the performance....I'm much happier with the mixture now. Very curious to calculate my mileage after a long highway trip soon. If I hadn't been chasing this backfire I would have probably never put a wideband on it and really set my mixture this well throughout the curve.

 

Sometimes weird problems with an engine can be explained easily and sometimes they aren't worth trying to explain. Like a june bug stuck in holley dominator vent tube at the dragstrip, (ask me) . I am sure there is a very scientific and rational reason and explanation for why your test tube cured your problem, but unless your the type of guy who just has to know where that june bug came from and what speed he approached that dominator and from what angle, Well, I'm just glad you fixed your problem and have some idea how to correct it now. And yes sometimes I overthink things but not this time.

 

Lol I may be that guy who needs to know why. You know....for science.

 

Any update?

 

Looks like you tried everything but swapping the manifolds side to side .

Kenny

 

I’ve had many T buckets with short headers , race cars on the street , etc , most had a decelerating backfire . It was part of the price running no exhaust .
Made lots of people look , after all thats why we have a hot rod .
I use to live in N jersey and know that cops dont like loud cars , at least they didnt .

 

here's what I say is going on....you have valves sticking....what I would do is this... put one or two quarts of marvil mystery oil in the pan... that'll break the motor in nice ,... when you let off the gas the valve is still open ... but after a few secounds of sitting it closes... thats why you get a passing compression test.. and also why that cly is running cooler...this was a problem with ol ford Y blocks... run kinda good then when you stop at a light they would die...one minute shooting ducks,... next rabbits.... I would also run lucas in the fuel... go twice what they say on the bottle... you need to lube up the upper cylinders....this trick also works on the triton tic problems on newer ford f 150s new fuel have nothing to lube...

oh and I ran this same set up on my woody gilmore digger... it did the same thing when I broke an inner valve spring...

 

wish I had a better pic... when I built the lake headers I swooped the cones up like a super comp digger...

 

My Dad use to say if it pops out the exhaust when you let off of it it's running good.

 

also, adding full exhaust make sure you don't use turbo mufflers...it'll blow the ends right out of them....

 

Exhaust has been leak tested with a shop vac blowing air in as well as smoke tested...no leaks at the flange or in the welds. ???????????????????????????????????????
how the hell would blowing into an exhaust pipe find the leak??? or even doing a smoke test????
i've owned a performace muffler shop for 40 years and never heard or tryed that cause I know it would never work... here's how you it,..... get about a 2' piece of brake line tubing and add about 16'' of rubber hose... put the rubber hose end up close to your ear and run the steel end around the gaskets... you'll find leaks you didn't know you had..

 

another thing... are you sure the lifters are pumped up???


warm the engine up and adjust the valves with it running.... I only go 1/2 a turn.... tight past clatter.... some go a full turn...


the problem you have is unburnt fuel... not exhaust... and JMO.... one 2 barrel is not enough carb... and if the runners on the manifold are designed for 3, the little carb is getting used up in the middle so the 1/7 &2/8 will lean out..

 

since their are people on here helping me,... I'm gonna do my best to help you...

don't go drilling holes in your carb till you know why thats done... and their is no reason with your set up... drilling holes in the throttle plate is for motors with big cams that have to idle so fast that idle screw is turned in so far that the plates/butterflys are open past the idle circuit so you can't adjust the idle mixture....

 

such a perfect build!!!!




can I own it????

 

Did I read Vaccuum was changing while idling? If not misfiring. I agree with a possible lifter issue, Might have one bleeding down excessively. Popping in (70s) 305 soft cam days was a bad lobe, one valve Ex not opening properly. Other issue, might be a valve seat, or bent valve. Slightly bent... or out of round seat. Had a set of heads updated with hardened seats, one was loose, would get cocked.. never do that again. Snap on made a nice cyl cut out tool, to isolate cyl perf issues, find a buddy with one.