ravenfan

I just had a guy adjust the timing, and vacuum advance. The car really goes, and shifts great. I still have a backfire when I stomp on it. I see they sell a power valve anti-blow kit on yearone.com. I've heard that one backfire on a Holley, will rupture the power valve diaphragm. Is the anti-blow kit a good idea, and I assume I will need a new power valve. Thanks, D.J.

sweeperking

First off, your right about the backfire rupturing the power valve diaphram, Second sorry I dont know much about the anti-blow kit. Last here, I would look into the reason for the back fire, by your description I would take a second look at the vacuum advance, if your sure that it's ok maybe your mechanical weights are frozen full advance.

Mopar_71

Do you have points or electronic ignition?
It the backfire through the carb or the exhaust?
.........

sweeperking

Good question, this would narrow it down

Barfly

Mine would backfire through the carb and it was exactly that, the vacuum advance diaphragm was shot. I ended up buying a rebuilt distributor for $58, $20 more than the vacuum advance unit would cost.

Guys here told me to just pull the vacuum line off the carb and suck on it, no resistance, bad diaphragm. I found that blowing on it is just as good of a test.

Joe.

ravenfan

I have electronic ignition. As mentioned, I had a pretty reputable guy fix the timing, and it now smokes the tires when it shifts in 2nd(auto trany)I was pretty happy with that, but now as I drive it, when I just stomp it, it makes a suction sound like the carb opens but no fuel. It's a Holley, and is the diaphragm just to the left? I was moving the linkage under the diaphragm, and it sounds kinda like the sound it makes when I step on it, only louder when I ask for fuel. I guess the word backfire is a little misleading, it does not make a big BANG! it's more like a sucking, or popping sound. Thanks, for all the responses. I'm a aircraft mechaninc/avionics tech. This is a whole new deal for me, fun, I'm hooked....

theomahamoparguy

the secondary diaphram on a vacuum secondary holley is the large thing on the passenger side of the carb. {not the choke ok} this actuates your secondary side of the carb.[when you "stomp" on it] The powervalve is located in the floatbowl of the primary float bowl. [this "enriches" the mixture during low vacuum operation when you "stomp" on it]. Each of these is tunable. The sucking noise is ok. when the secondaries open, usually it is accompanied by a sucking noise. especially if you try driving with the aircleaner off. slight popping may occur if the carb is on the verge of running lean mixture OR if mechanical valvetrain is not adjusted correctly. Even so, it is not abnormal to hear some very mild popping noises if driving with the aircleaner off. {try it on your slant six, very interesting}
Usually, the loud sucking noise means that your secondary is opening too slow. {think of the old Thermo quad equipped cop cars}. Usually, if the carb is too lean on the secondary side, it will back-fire. [through carb]
dont go more than 36 degrees total advance on a bigblock and usually 34 on the smallblock. {for mostly stock to somewhat hot street combo's}.
..........my 2 cents worth............

Sangetsu

Another thing to check for is exhaust leaks. A small leak will allow air into the exhaust system allowing unburnt fuel to ignite, causing a backfire. It only takes a slight leak for this to occur, and you may not notice it when the engine is running. This is especially common in cars running headers, but it can happen with cars running stock exhaust as well.

ravenfan

If it were an exhaust leak, would it have to be close to the headers to cause the problem? I did think I heard a small leak when I was under the car the other day. As I mentioned, the car runs great when I'm over 40mph, but under that, and out the gate, it just doesn't take off like it should. If I can figure out how to adjust the carb, I think I can make it right, or at least better.

Archer

raven -

I know I'll get some flack for this, but regarding the vac advance on the dist. Loose it, 440's with even mild cams don't need it. Initial advance should be about 10 deg. Block off/cap the PORTED VAC on the carb (the dist is connected to a ported vac, right?) and I cap the dist diaphragm, just for looks.

Give it a try and see what happens.

Archer

67 GTX

i would run it, but try it without like Archer said.
It doesn't hurt to try it.

Sangetsu

This is not recommended, and if your engine runs better with the vacuum advance not hooked up it means that the distributor may need work. The distributor has 3 basic advance systems. The first is the position of the distributor itself (you adjust it by turning it by hand), next is the mechanical advance, regulated by advance weights located inside the distributor, and last is the vacuum advance, regulated by engine vacuum.

The vacuum advance system keeps the distributor timing moved up when vacuum is present (such as when cruising on the highway or street). It is designed to increase engine efficiency and fuel mileage under normal driving conditions. When stepping on the gas and accelerating, vacuum drops, and the vacuum advance backs off, reducing the chance of detonation. It's really an ingenious system, and your engine is going to run a lot less efficiently without it.

Many people with high-performance engines will run a mechanical advance distributor because their engines generate little vacuum at lower engine speeds. But mechanical advance distributors have a greater mechanical advance range than vacuum advance distributors, meaning that if you try to use your vacuum advance distributor with the vacuum diaphragm unhooked, you are probably getting only 1/2 the ignition advance your engine needs.

Having tuned cars for many years, and having done much of that at the track, I've learned a lot. A vacuum advance distributor will outperform a mechanical advance system in most situations. If your car has enough vacuum to operate power brakes then you should be using a vacuum advance distributor.

Adjusting the carburetor is simple (depending on the carb). The first step to adjusting the carb is to do what Archer recommends, disconnecting the vacuum advance to the distributor and capping both ends. Then you set your idle speed to factory specs. Once the idle speed is set, you adjust your timing to factory specs. When you adjust the distributor the idle will change, so you'll probably have to adjust the timing and advance a couple of times until both are correct.

Once this is done, you'll need to get wide-range vacuum gauge (you can probably borrow or rent one from Napa or another large parts store). Hook it up to a full-time vacuum source, then start the engine and carefully note the position of the needle in the gauge. Adjust the 2 idle mixture screws equally (in or out) until the vacuum reading increases. Keep adjusting until you get the highest reading (your idle speed will move up as vacuum increases). Then set the idle speed back to factory specs using the curb idle screw on the throttle. Disconnect the gauge, and reconnect your vacuum advance distributor. Your engine should now be in-tune.

If you are using a Holley carb, there may be no change in vacuum when adjusting the metering screws. This happens when the fuel circuits get dirty, and/or the metering block gasket is swollen or crappy. In this case you'll need to rebuild your carburetor, or at least clean it out and replace the metering block gaskets. Some of the old gaskets are not compatible with modern fuels, and should probably be replaced with newer-style parts.

Archer

S -

Unfortunately, while that sounds great, it's is in direct contradiction to what we found 30 years ago, both on the street and the track. The engines, and Physics haven't haven't changed.

The simple fact is, as was stated, most mild to moderate 440s simply don't need (or benefit) the additional advance and since it's a readily reversible test that most people can perform in a matter of seconds, there's no harm in trying it to see if he has "too much" total advance.

Do I think that's raven's problem? No, sounds more like a transition circuit problem in the carb, but best to do the easy tests first.

Archer

dusterboy89

my 72 duster backfired real bad through exhaust at idle and when i left of gas when driven here it was just plugs and wires got some taylor wires through summit racing now she runs like a champ

1966sportfury

sangetsu,,got it!. you need vac advance..if you run your engine below 4000. no ands ifs or buts about it.