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Pops and Bangs....

antijam

CCCUK Member
My '71 base 350 runs well apart from a couple of frustrating problems. It pops and bangs on the overrun and it bogs down when full throttle is suddenly demanded.

I realise Popping and Banging are considered highly desirable by some modern boy racers but I'm and old fart and I prefer a quiet life. I thought possible causes could be ignition timing, fuel mixture, a vacuum leak or maybe a blowing exhaust.

Full ignition advance is a steady 36° and the car starts and runs happily; I've just fitted a new stainless exhaust which made no difference so problem is possibly a vacuum leak (which is going to be tricky to trace, given the comprehensive vacuum system on the car) or maybe fuel?

The bogging I guess is almost certainly fuel. I have a Rochester quadrajet overhaul kit, so a carb strip down, clean and rebuild is going to be the next job, but if anyone has any suggestions as to other possible causes of either problem I'd be very pleased to hear them.
 

Jimbo64

CCCUK Member
Certainly sounds like an air/fuel mix issue, so checking all associated ports, gaskets etc and carb rebuild could well assist. I had quite a lot of popping and banging from my Morris Minor Hot Rod with a Rover V8, fitted with an Edelbrock carb and discovered that a balance pipe fitted to a longer, straighter exhaust system (mine initially exited at each side before rear wheels, after a 90 deg turn) some better seals over the various unused ports, together with some fine tuning of the mixture and idle screws, sorted this all out.
My 72 had it's original Rochester fitted when I got it, but it failed on the long drive home and I was advised to bin it and stick on an Edelbrock 600, which I have done and not a single pop or bang.
 

teamzr1

Well-known user
One way to find some type of intake or exhaust leak is to buy, rent or even make a smoker ( which is what I did)
which connected to a air compressor puts out pressure into smoker tank and output hose into a vacuum hose, throttlebody or exhaust muffler tip
then pump smoke flow under pressure and will go out any type of leak
I use auto tranny fluid to make the smoke as to it's smell so if I do not see the smoke first I can smell the fluid coming out of leak and then
track that down to where the leak is.
Best rebuild the carb first, adjust the AFR and then determine if problem still exists to track for leaks

Does the car have catalytic converters, if so due to age they could be plugged up causing higher back pressure which
would effect the AFR in cylinders ?
 

antijam

CCCUK Member
Got the carb off for overhaul but it seems someone has been there before me - it's not the correct carb for the year.

P1320147.JPG

P1320149.JPG

My car is a '71 base four speed manual and the carb is from a '73 'high power' auto. Data sheets say the main jets, rods and springs are the same for the two carbs but the secondary rod is richer on the '73. I haven't stripped it down yet to see what jets and rods are actually fitted, but assuming they're stock I guess the newer carb should run OK on my car?
 

Jimbo64

CCCUK Member
At least the vacuum set up for secondaries looks standard on 2nd picture, mine was a complete mess, so yeh, hopefully if data is correct, a strip down/rebuild could well sort you out.
 

antijam

CCCUK Member
Well, three months on I finally have my pop and bang problem sorted! It turned out to be an object lesson in taking nothing for granted.:oops:
I stripped, cleaned and rebuilt my non-original carburettor and found it had jets and rods correct for the my year. Back on the car and nothing had changed - still pops and bangs and bogging down on demand from the secondaries. OK, it was the wrong carb for my year, but almost any properly jetted Quadrajet ought to run satisfactorally, shouldn't it?
Since the carb was incorrect for my otherwise number matching car I decided to bite the bullet and bought a properly rebuilt Quadrajet from a trusted source, correctly numbered and dated for the car....

s-l1600.jpg

Good news and bad news. Good - the bogging on acceleration had gone. Bad - it still popped and banged on the overrun.
So, I took it to my local garage for an emissions check and the car was definitely running very lean. They also confirmed no vacuum leaks. I couldn't figure out how an original standard car with a correct new carb would consistently run lean?

Then a lightbulb moment - my car wasn't actually standard! I had completely forgotten that the previous owner, in spite of telling me how original the car was, had replaced the exhaust manifolds with headers. This seemed a very likely cause of lean running, especially since my previous fitting of a free-flow exhaust in place of the original which, far from improving matters, had if anything , made the problem worse.

So, armed with a selection of jets and rods from Quadrajet Parts, I first upped the primary jet size from 74 to 76. A major improvement! Still some pops and bangs on overrun, but much reduced. Upped the jets one more size to 77's and she now runs almost perfectly. It certainly demonstrated the significance of improving gas flow on an engine. I suspect a little further tuning with jets and rods may be in order but at the moment the car now goes like the proverbial excrement off a shovel :D

Just shows - when trying to diagnose problems on an unknown car, take nothing for granted.........now, next job is to sort my leaky power steering.
 
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