'71 impala hesitation, backfire problem

I'd check all the tune up related items first, assuming the carb is good. Are the points OK ? Is the dwell correct ? Is the timing correct? Does the vacuum advance work ? Did it do this after you swapped the intake ? Was it a new carb or one some dude had lying around ? Whats the story?

I had a 73 Caprice Classic many years ago that had similar symptoms (454 but all stock). In my case, it ran fine, but the first time you jumped on it (didn't matter if it was cold or if the temperature was up to normal), it would cough, blow back through the carb, hesitate and then take off. Smoke the wheels off after that when you jumped on it, but let it cool off, it would cough, hesitate and blow back through the carb again the first time you hit it hard.. Never did anything about mine, but it never died.

The stalling when cold could be helped by adjusting the choke. Mark the current setting and adjust to increase the time it stays engauged (not sure exactly what it will look like on Edelbrock, but on my Holley, there is a black plastic round cap on the passenger side that I think would rotate clockwise. If the Edelbrock has this setup, mark it, loosen the screws and rotate the direction that will make the choke close tighter.) You can over do it and the choke never opens all the way, so take a look at the choke after the motor is warmed up to ensure it is wide open.

good luck and I hope someone posts a good answer as I always wondered what was wrong with mine and what I could have done to fix it.

bb

David Knaack wrote:

The accelerator pump isn't moving enough fuel into the venturi when you open the throttle. If you look down into the primary venturis of the carb with the air cleaner removed, and operate the throttle by hand, you should be able to see a small stream of fuel shoot into each one. If you see no fuel stream, or just a small dribble, that indicates an accerator pump problem.

The carb is a new Edelbrock 1407, should be good.

I had to fix the vacuum advance last year. The only way I found to do that involved pulling the whole distributor, punching out the pin holding the shaft in, pretty much disassembling the thing and then putting it back together. While it was out I cleaned everything. When I put it back I had to adjust the dwell and timing. I don't recall now what values I used, I presume I got them from the Chevy book I've got (or perhaps its on the sticker in the engine compartment). The advance does work now.

Before I swapped the intake the car would barely run and was missing bits and pieces of the original carb. I had the intake and carb unused from another project, and the mechanic I had do the work recommended I just junk the current carb and intake because of very badly pitted gasket surfaces and broken bolts and swap on the new equipment.

Unfortunately the new air cleaner does not have the thermostat and tube running down to the exhaust manifold as did the original equipment. I lived in south Texas at the time, and the guy that did the work did not think there would be any problem. Now I'm living in Omaha and its a bit cooler in the winter, so it might be an issue (someone pointed out the warm air stuff to me in an email reply to my original post, I'd forgotten all about it).

The original exhaust manifold was cracked due (I suspect) to a rear end impact of some sort. The muffler was buckled and the cast iron manifold was broken in half. I replaced it with the cheapest alternative I could find, $50 Dynomax headers from Autozone, plus dual exhaust (no crossover) from a local shop for $75.

I've been running the cheapest gas I can get, which around here is the ethylene. I just ran a tank of premium over the last week, and it seems to do much better on that, although it still has the same problem, just not as pronounced. I'm going to try the cheap stuff next, perhaps it just doesn't like the alcohol.

Anyway, whatever the issue is, I still need to find the cause.

Another symptom is that if I'm sitting still in drive, quickly give it about

3/4 to full throttle for half a second, then jump on the break, it dies every time.

Thanks DK

-- if logic and reality disagree, reality wins.

the fly" wrote

The accelerator pump seems to be fine. I've tried the various positions for the linkage, without a discernable difference in the problem. It gives a good strong stream when I operate the throttle. Right now I have it set to the highest setting.

DK

Here's my guesses.

Dwell, dwell, dwell. Use a meter, and don't trust it to be accurate. Try a little more and a little less, and set the timing every time you use it. By the way, only do this with NEW, and not cheap, points.

Wires, wires, wires. Make sure they are not touching each other or oily or hot metal wherever possible.

The above two things made the difference for me over many years.

Carb - see if you can find a Q-jet for it. Otherwise, look at ways to increase the accelerator pump squirt volume or decrease the amount or timing of secondary opening. I don't know anything at all about your carburetor but the engine is almost certainly over-carbed and that translates into bogging, backfiring and stalling. You might have to fiddle with jets, too. That engine should probably have a 600 cfm carb on it for street use.

I just looked up your carb, I believe you probably have the 1407, as I could not find a 1704. It is definitely too much carb for that engine. That looks like a square pattern carb, it comes with a bunch of jets and metering rods. To use one, you have to put in the right jets and metering rods, or the thing will bog. In fact, I have known lots of guys who put oversized aftermarket square pattern carbs on small block Chevies. None of them had what it took to become experts on carburetors and every single one of them had problems with bogging, crummy part throttle response, and terrible fuel economy. Meanwhile my Quadrajet eqiupped engine with stock intake ran circles around them with the same cam, headers, etc.

So, in a nutshell; Make sure the ignition is completely up to snuff first, then if you can't make that carb do what you want, go to the boneyard and get yourself a quadrajet. It looks like that manifold is designed for it, too.

Last -- if your timing mark jumps around while timing, save yourself a lot of headaches and replace the timing gears and chain. The engine will make gobs more power and you might save a lot of money by not having the thing slip on you and ruin a bunch of expensive stuff.

97T

had same problem on a 350 and had to get rid of the mechanical fuel pump and go for an electronic one to keep it from hesitating. the motor empties the carb faster than the mechanical fuel pump can supply gas. a electronic pump provides constant pressure regardless of RPMs.

Classic case of round cam syndrome. That vintage Chevy had a lot of problems with cam lobes wearing. When the valve does not open properly you get standing fuel at the carb on accel, and carb flash-back, along with a miss on accel. You will need a new cam and lifters to solve the problem. Almost 100%.

Sounds to me like a couple of possibilities:

1) Carburetor accelerator pump not working 2) Carb float level set too low

Have you verified that when you briskly move the accelerator linkage, a nice strong jet of fuel squirts into BOTH of the primary (front) carb venturis?

Its also possible that the Edelbrock carb is jetted too lean for your application. Read the spark plugs to get an idea if that's possible, but I'd be willing to bet its float level or accelerator pump.