I have a 71 chevy c-10 pick up with a 307 v-8 with 38K on the odometer. I installed new wires, points, condenser, rotor, cap, and had the carb rebuilt. It runs, but there is a flat spot when you accelerate. The truck bucks and hesitates before the power picks up. This is actualy my wifes truck, it was her dads from new. This truck has a history of doing this from very early on. It has a divorce choke, and is three on the tree, and makes it very difficult for her to learn to drive. I can get around in it, but it can be anoying. Any ideas on how to rectify this problem would be greatly appreciated.
Check that the vacuum advance in the distributor is working.
And that the centrifugal advance is working, too.
Was this problem present before any of that work was done?
Retarded ignition timing can also cause this and so can a problem in the carburetor accelerator pump circuit.
You state the carb was rebuilt but since carburetors are dinosaurs many shops may know little about them.
This also gets into the area of how it was rebuilt. This means it should have been soaked for several hours in carburetor solvent, washed clean, and then all passages blown out with compressed air. The unknown, at least to me, is whether or not the shop even has a can of carb solvent sitting around.
You could remove the air cleaner, prop open the choke, and look down the carburetor throat while manually working the throttle. You should see an instant discharge of gasoline from the accelerator pump nozzle.
Check the vacuum hose at the distributor. There should be ZERO vacuum at idle, with vacuum gradually increasing as the throttle is opened. I believe that manifold vacuum at the advance diaphragm is causing the problem. If so, you must get the correct connection made and reset the timing.
The auto shop I took it to is very reputable, and known for working on older cars. I’m pretty sure he did a good job on it, but the problem has been there for a long time. I just took the truck out, and after about 20 min of continuous running, the problem seems to get much better. I put a timing light on it, and the mark on the wheel is about a half inch to the left of the indicator tab, which I believe means that it is advanced. When I look down the carb and open the throtle, I can see gas coming out, but it is not a stream, it just trickles out into the carb. I’ve read the book on how to adjust this, but it isn’t at all clear, but if you think thats the problem, I’m sure I can figure it out. Thanks so much for your help.
Concurr with all 4 posts! let us know!
Unfortunately, most of the people who really know how to set up this vehicle are in nursing homes, or worse!
Could be dumping a bit of fuel at low RPM acceleration too. Jets dirty (spray not mist)? Who rebuilt your carb?
I had a similar problem with a 1965 Rambler Classic with a 6 cylinder engine and the “three on the tree” manual transmission. I fought the car for about three years. The dealer tuned it up and it got worse. A local well known speed shop bored out the jets in the carburetor which cured the hesitation and stalling problem only to put the gas mileage in the cellar and cause the plugs to carbon up. I then had to replace the carburetor. Ultimately, a top notch mechanic found that the timing marks were way off. He timed it by feel and then put a mark as to where the timing should be set. You might try timing your engine with a vacuum gauge. Hook up the gauge and advance the timing. The point where advancing will not increase the vacuum, but reducing the timing a hair will show a drop in vacuum is the starting point. Take it out and road test. If flooring the accelerator in high gear at 30 mph produces just a slight spark knock, your timing is set correctly. Too much knock and you should retard the spark. No spark knock and you might want to advance the timing ever so slightly.
If there is only a dribble of gasoline from the accelerator pump discharge nozzle when the throttle is opened suddenly that points a problem in the pump circuit. There should be a steady stream as the throttle is suddenly opened.
Any problem could be due to a clog n the discharge tube, clog in the carb housing, or a fault with the pump discharge check ball. An insufficient stroke of the pump lever could also cause this but my fuzzy memory does not allow me to remember offhand if there is even any adjustment in the lever. The last one of these I ever messed with was 30 years ago on a brother in law’s car.
I’ll rummage in some of my old manuals this evening and get a quick refresher course…
In addition, they used to sell performance springs for the centrifugal advance. They may be weak or might have just been under rated trying to reduce emissions back then. Agree thats your options though, advance, timing, carb adjustment.
Yes, @OK, the accelerator pump should throw out a significant spray and the check balls or the pump diaphragm are likely failing although a low float setting might also be the problem. It is most likely a Rochester 2-G and the pump lever works through the bowl housing. It is a somewhat simple carburetor for those of us who were wrenching in the “good ole days” but carburetors are like astrolabes today.
Rod Knox is correct. A manual I found shows it’s likely the 2-G and as thought, the pump stroke is adjusted by bending a linkage rod.
Much could depend on how the overhaul was done.
- Soaking in solvent, washing, and compressed air.
2.On the bench with aerosol carb cleaner.
The second option can be kind of iffy and keep in mind that I have no idea how it was done.
Also, you might add an after market electronic ignition sytem. You will get better winter start up, improved mileage and power, and no points to fool around on. Did it to my 71 NOva, loved the results.
shade tree wrencher and avid fan…
So, if you have the original air cleaner setup, you can solve this problem as it only happens when the engine isn’t fully warmed up. When the choke is open on a partly warm engine,the engine needs warmed air.
The pipe from the exhaust manifold to the air cleaner is probably missing. If you have a slightly different setup, you can just buy the flex pipe (it’s like cardboard). If the choke stove is still on the exhaust manifold and it is the open hole type, you can get a short piece of exhaust pipe that barely fits in the hole and buy some flex pipe and connect it to the air cleaner housing.
If the vacuum hoses on the air cleaner are intact, you’re in business. Without the old type of air cleaner, you may never fix the problem. I fixed the problem on three Chevy engines.
You guys are great, thanks.