fantogirl

Hello everyone, I am a new member to this site but have owned corvettes since 1990. My latest vette is a 1980 that is really driving me nuts with this acceleration problem.

I have an excellent condition 1980 corvette with a 305 motor, 59,000 original miles, and original carb (Rochester Quadrajet). It went from running perfect one day to not so good the next. The problem is that upon acceleration, regardless of how much I accelerate, the motor instantly misses and bogs. It has no power at all. It still idles fine though. I have checked all vacuum hoses - no cracks or disconnections, there are new plugs at the correct gap, new wires, distributor & rotor, HEI module, PCV valve, fuel filter is not clogged, carb is getting a good flow of fuel, fuel pump is operational, oil and filter changed, carb has been cleaned and all linkage is operational, fresh higher octane gas,... all of this was in place before the problem started a couple of weeks later. It sounds as though it is a fuel problem but I cannot figure out what it could be. It doesn't seem likely a vacuum issue but I could be wrong. I do all the work on this car and have been through it with a fine tooth comb. Someone suggested that it might be the carb floats sticking or too much fuel in the carb but I am not sure. I live in Fairbanks Alaska and my 80 is bagged up tight for the COLD winter but I am trying to figure this out before spring hits. Any helpers, I would appreciate it!

Raphiki

Well, you have lots of time to think about it! I had a similar problem and it turned out to be a problem with the vacuum advance. Make sure you initial timing is correct and that your vacuum advance is working freely. Check your vacuum pressure. You may also have the vacuum advance line hooked up to the wrong port. Mine runs best on the ported vacuum port on the carb. A lot of people say to connect to the full vacuum port off the manifold.

Hope this helps. Stay warm!

6550gate

In my experience as a mechanic, when an engine runs good one day and not the next you've broken something. The first thing I'd check is the cyclinder compression, you don't want to see more a 10% difference between any of the cylinders, if you have a larger drop you may have broken a valve spring or have a wornout or calapsed lifter,they don't have to tick to be wornout,a loss of hydraulic pressure inside the lifter can shorten the valve travel.
The second thing to check is the condition of the springs in the timing centrifical advance, springs can break and you'll know imediately. If they look ok, check the condition of the timing chain while you still have the cap off. First put a mark on the distributor housing rim to indicate the postion of #1 cyclinder.This is best done with a second set of eyes to help. Locate top dead center on cylinder 1 then slowly turn the crankshaft harmonic damper by the nut until the timing mark is visible and lined up with the 0 on the degree scale, look to see if the rotor is pointing to your mark,(this is also the best method for setting initial timing after engine assembly).Rotate the crank until the rotor pionts exactly towards the mark and then read the scale to see what degree of timing is indicated.If the rotor doesn't point to the mark you've made and your timing mark is nowhere close to the scale, your timing chain may have jumped a tooth. To test for chain slack, turn the crank back and forth and watch what the rotor is doing, the rotor should follow the movement of the crankshaft timing exactly, If there is play you can read the degree of chain slack on the timing scale. Plus or minus 1 degree is acceptable, any more and your timing will be off which can cause valve wear and poor perfomance. If all this checks out, run the engine and check the timing when engine is warmed up and the vacuum line is unplugged to the advance. Factory says set it to 6 degrees BTDC but 10 degrees works much better, more power and better fuel mileage, beleive me, my 81 Corvette does 0-150mph in under 40 seconds. Reconnect the vacuum line and check to see that you get 35 degrees of advance at 3000 rpm using a Digital Tach/Advance timing light. If all this checks out then you may have a plugged power valve inside the carb. This should keep you busy for a while.

OMF

Check to see if you have a broken air valve spring on the Qjet secondarys. With the engine off hold the throttle linkage wide open. Next check the air valve operation on the secondarys by pressing on the back of the air valve. If the spring is broken the air valve will not snap shut when released, or will be really lazy in closing

VegasJen

hmmm, KA only LG4 305. gee, i wonder what could possibly be weird about that set up?
my guess is going to be the MAP sensor. if it's where they put the sensor on the '81s, you'll find it under a stamped metal bracket mounted between the M/C and distributor. there's a small (1/8") hard emissions tube coming off the back of the carburator base that goes around to the sensor.
first check the tube to make sure it's not broken and the sensor is getting a good signal. then pull the plug and make sure it's not gummed up with a bunch of crap. spray some electrical contact cleaner in both sockets and apply a dab of dielectric grease.
it may not be the source of your problem but at least it's just good maintenance.
it's also possible, maybe more likely, that it's the TPS. These are not too hard to replace but if you can, just get a rebuild kit for that carb. there's a way to actually check it with a meter but i can't recall the procedure. at it's age, it's probably just as well to replace the TPS and MCS (both should be included in a 'good' rebuild kit).
the last thing i can think of would be the 7 pin module under the rotor. if you have an autozone or similar parts chain up there give them a call and ask if they can check that module. all the chains down here can.

Rayvan

I doubt it could be your carb float levels...I have an 80 also and had that issue only when I made sharp turns but your issue is indiscriminate bogging. My mechanic fixed it but I can still feel her stall for a millisecond when I turn and push it. Then again I have a new engine so we're in different worlds.

Hope this helps.

80 L48

Could be a clogged cataletic converter. This might explain the missing and bogging when accelerating, even though it idles fine.

boeing46

I doubt if this helps but i had an 84 Lg4 84 Z28 and it did the same thing. I found a good carb shop and they rebuilt it and changed the computer and it had never run so good when they were done.
A good mechanic makes all the difference in the world. I could easily break the tires in second gear with the auto trans. It was not the H.O. motor either.

DRIVESHAFT

Does it actually develop a miss [like you pulled off a spark plug wire], or is is more of just a bog where it is a smooth lack of power?
If it is just a bog, it could be a dirty/stuck power piston.
When that happens your metering rods usually get stuck in the down [lean] position, and you go very lean when you try to accelerate.
Of course I dont have any experience with the Ca. only 305 Vettes, so Im not sure if the computer was controlling the mixture yet on your car.