Arrowstorm

Hey guys. I'm at my wits end here. I have a 72 Vista cruiser that I bought a while back that has backfiring issues. Luckily, that's about it! It backfires through the exhaust when I rev it up at a standstill and also when I'm driving at normal speeds and let my foot off the pedal. A couple months ago, my buddy and I decided to upgrade it a little bit to fix this among other things and it now has the following upgrades:

Edelbrock 4bbl carb (up from 2bbl)
Edelbrock performer intake
Accel HEI distributer (former owner converted to HEI from points already)
Accel wires and plugs
Accel super coil
Headers
NOS lifters and rocker arms
Heads totally reworked

After the overhaul, things went pretty smooth and we ironed out all the quirks, but the backfire remains. I read on these forums that with an HEI distributor, it might a little advanced timing, so with a light, I advanced the timing to around 16 degrees and even though the idle sounds and feels good, alas, the backfire remains. The fuel/air mix also seems to be on point, and as I have heard that headers often have problems with leaks, I looked that over very well also and things look good there.

A little better description of when it occurs and what it sounds like. At idle, if I rev the car up, the rpms is constant and sounds good, but when I release the throttle, rpms start backing down as normal, but then pop! pop! pop! On the road, acceleration is smooth and noiseless with no stumble or hesitation, but after maintaining speed, when pulling off the pedal, there is backfire through the exhaust. It is far more pronounced when slowing down from highway speeds.

If any additional info is required, I will provide it to the best of my knowledge and any help or advice is tremendously appreciated because, to be honest, this is my absolute dream car and nothing would please me more than to have it running as well as it possibly can so I can keep it where it belongs; on the road.



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ent72olds

Sounds like a timing issue to me, or unburned fuel igniting in the exhaust....

Greg Rogers

Some amount of backfire upon deceleration is normal on a carburated car. If you have been to short oval track races you hear it and see blue flames out the exhaust. Really loud or excessive backfiring would seem to be too rich operation but since you had it with the old and also with the new carb... I don't know.. My beater Vista had a straight pipe all the way back when I got it and it would backfire thru exhaust on deceleration- after I put a muffler on it, no more backfire. No other changes. Maybe it is still doing it but you can't hear it. Do you have a loud exhaust system?

Arrowstorm

Ah yes, I forgot to mention that. Dual exhaust with glasspacks straight back and out to the side behind the rear wheels. The exhaust definitely isn't quiet...

And I guess I wouldn't call the backfire loud nor excessive, either. Just more like a popgrumblepoppop! which settles down when the rpms level out.

MDchanic

All engines will spit a little on deceleration, but, as Greg says, you only notice it with a loud exhaust.

An electronically managed car will completely cut off its fuel supply when you let off the pedal, and so will not do this, but any carbureted car will.

Ever have a sports car?

- Eric

Arrowstorm

Nope. Never had a sports car.

But I do also have an 85 Custom Cruiser with a 4bbl carb and dual exhaust with glasspacks out the back that does not have the same backfiring issue, nor has it ever. =/

It does have some electronic components to it's carb though......

Lady72nRob71

Nice looking wagon! What wheels are those? Look great.
If you could put up a short video so we can hear the noise, we could give better advice. However, with what is described, i am leaning towards a normal occurrence with loud pipes, and not really a true backfire. My 70 does a similar sound, but fairly muffled as she is rather quiet.
BTW, what fort is that in the background of your first picture? Looks familiar............

steverw

Im sure you have, but check to be sure you have all the plug wires in the correct order

Steve

olds 307 and 403

Find out what your timing is at idle with the vacuum advance connected to about 3000 rpm. Try it with the line plugged too. Most people run around 20 base with HEI.

Arrowstorm

Thanks for the replies!

The wheels are IROC rims. The previous owner put them on there, as he felt they gave the car a smoother ride I'm inclined to agree. He also took out the front bench seat and installed power bucket seats from a Cadillac. They're comfy as hell, so even though I do have the bench, I think I'll leave the bucket seats in. It IS a cruising car, after all.

The fort is Fort Pickens in Pensacola, Fl.

Firing order is definitely correct. Base timing at idle is 16 degrees. Chilton book says 12, but the engine really doesn't care or that. I tried to go as high as 20, but the engine didn't seem to like that too much either. Upon vacuum advance, it goes up to about 30 or so I guess. I can't really tell with the timing mark that far away from the bracket with the teeth on it.

The book also says to gap the plugs at .042, but I've seen people say .045-.050. Could that be a factor?

I will try to post a short video of the backfire this afternoon and again, all help and advice is appreciated!

seansolds

You can have a minor carbon build upon a single exhaust valve in combination with a weak exhaust spring... If its just on one side of the engine..

GAOldsman

When was the last time the timing gears and chain were replaced?

Kennybill

I would look at the distributer. Sounds like an advance issue.

Arrowstorm

The thing is... the heads were totally reworked, cleaned and machined when we overhauled the engine.

Timing chain was new and we reinstalled it with the Chilton Book; ok.

Distributor is a brand new Accel HEI w a brand new ignition coil.

billmerbach

Jw if you need an extra accel supercoil I have one but I'm not gonna use it for my application

seansolds

X2 on that sounds like your distributor is way advanced..they always sound real good far advanced.. Put it at factory specs..start from their... Did it make noise when you had timing set at 12 degrees before you moved it?

Giftman23

I don't usually throw my hat in on tech questions, but if I recall from my high school days - We had a 69 Ford F250 4 spd that my Dad (much to my delight) decided to put a dual exhaust system on....Midas guy talked him into glass packs. Never forget the price quote at Midas - 75 bucks (lets say 1979 time frame). I was overjoyed as I soon would have my license. Dropped the truck off and picked it up the next day - the sound was beautiful, those glasspacks would really cackle. Point is, we used to run up the RPMS and leave it in gear and get off the gas just to listen to those mufflers and the backfire on the way down...they would pop and backfire and raise a ruckus - did not affect the way it ran in any way. Man, I can still hear that sound! May just be some worn out packs?

Olds442redberet

Recent rebuild?

Arrowstorm

Rebuild was this passed september.

No idea how old the glasspacks are. Maybe they need to be changed out? Do those even go bad?!

At 12 degrees, it runs pretty well, but it really likes 16 degrees. For academic purposes, I will retard the timing back to 12 and see what that does on the road!

MDchanic

Alright, folks, enough already.

"It could be this." "It could be that."

His symptom is NORMAL BEHAVIOR.

Just stop obsessing about it, go out, and drive your car.

- Eric

seansolds

Yeah I hear that!
His vista is bad ***..if I had it I would have a bug catcher sticking out the hood with a coat of rubber sealed to the bottom of the rear quarters..nice car looks like a lot of fun.

Olds442redberet

In my opinion, if the engine sounded normal, ArrowStorm wouldn't be asking questions.

I have had issues like his in the past, and unfortunately it wasn't normal and it didn't go away, it actually got worse. And both issues occured right after the engine was overhauled. In the end it was always a timing issue. The first time it was the cam, a couple of lobes got messed up. The second time, on a different motor, it was a bent push rod. Both of these things can be checked easily by removing the valve cover and spinning the push rods.First see if they spin freely when they are in the down position (valve closed). If you have a cam issue the reduction in surface material on the lobe will result in less tension on the push rod, thus allowing it to spin freely or worse, wiggle around. If there isn't any excessive play in the push rods you can then pull them to see if there are any bent rods. If both of these check out ok you may have a valve spring issue. Also, if you put new push rods into the car during the rebuild, measure the push rods to ensure you had the correct push rods installed in the car during your rebuild.

In the past I would time a car according to the specs, but I have found that technique doesn't really work well with modified cars. Cam upgrades require advancing the timing as do exhaust upgrades. (Exhaust upgrades such as headers or large diameter pipes cause something referred to as savaging). I usually time a car first with a timing light at factory specs, adjust the fuel mixture for proper vacuum, then I time it again by ear, adjust the fuel mixture again for proper vacuum, then I run the car up an incline such as a long hill in order to see how it performs under a load. More times than not it will require another slight adjustment in timing and then I am usually ready to go.

When I did these things with the engines that were failing, the drive up the hill made the symptoms more pronounced.

Arrowstorm

My buddy who helped me rebuild the engine spoke to an Olds guy who worked as a mechanic for the local dealer for 30 years and his belief was that it is bad fuel/air mix.

So, seeing as how the Edelbrock instructions are just a little vague on how to tune it, he told me to start (with engine warmed up, choke wide open of course) with the passenger f/a mix screw and unscrew it until I get the highest sustained vacuum, then tune it down to where it just starts backing down. Then, adjust the idle screw to desired speed; just before it bogs down, but not too low on vacuum/rpm. Then, adjust driver f/a mix screw until highest vacuum and back it off as before, then adjust the idle screw again. Rinse, repeat until "tuned".

I had a Christmas party to prepare for at my house, so I fiddled around with this procedure using a vacuum gauge just before my guests started to arrive saturdy and will experiment with this procedure after retarding the timing back to 12 degrees and see what that does.

As of Saturday after a quickie attempt at tuning the carb in this manner (Edelbrock's tuning procedure suggests turning the f/a screws out just enough to where the engine runs smooth, and doesn't really mention highest vacuum), upon reving the engine at a standstill, it no longer backfires, except for once, and that was just a quick "pop".

I will put more time and effort into this and post my results.

I feel I should also mention that the Olds mechanic said that in no way is a backfire "normal" under any circumstances. He said there's always a reason for it, and most likely can, and should be found an corrected.

oldcutlass

I'd leave your timing at 16 with the HEI. However the vacuum canister maybe bringing your vacuum advance higher because of that setting. I'm thinking the vacuum canister on your distributor is adjustable. You really need to get or borrow a dial back timing light. .042 gap is fine on your plugs, and most gap them @.045.


Please note if it's only backfiring at idle then your carb A/F mixture adjustment could help correct this, anything past idle these 2 screws have no effect. The 2 adjustments also have to be somewhat even in turns from gently screwed all the way in to their current position.


The rocker assy's have no adjustment, if the valve stem length is too long then 1 or more of your valves may not be closing all the way. It's something you may want to check as it can cause an intermittent backfire.


As Eric stated, burnt out glasspacks will backfire or pop.

Arrowstorm

No, there wasn't any backfiring at idle. Only when I revved the engine or decelerated.

Note: WASN'T a backfire.

I fine tuned the fuel/air mix with the choke wide open and disconnected (electronic) and at idle, the whole thing is pulling about 18in of vacuum. Kept the timing at 16 degrees.

That seems to have done the trick. There is no more backfire. Not even when coming off the interstate from going 80. When I would normally hear the rpms winding down and a few pop!grumble!pop!pop!s, there is only..... rpms winding down and the roar of the tires on the asphalt along with the grumble of the exhaust.

I find it strange though, that it had always done that. Even with the 2bbl Q-jet. I'm gonna have a closer look at the carb, as I don't recall seeing a f/a mix screw on it. Then again, that carb is 40 years old.

So, in conclusion, in this case, the backfire was surely bad fuel/air mixture.

Thanks for the suggestions and advice! As we all know, classic car ownership is a never ending joy and hardship of repairs and improvements! Lol