VCCA Home Forums TECHNICAL - GENERAL Mechanical - Engine, Trans, etc Hesitates on acceleration

Hey Guys & Gals,

I recently acquired a 1970 Caprice with a 350/300 engine. It has 89K original miles. Upon taking delivery I had some routine maintenance done which included dropping and cleaning the fuel tank and rebuilding the carberator. That was about two weeks ago. Now, the Caprice hesitates when I accelerate from a stand-still. When I gently step on the gas, the rpm drops almost to the point of stalling and then comes back to life if I don't push the gas down further. If I continue gentle pressure beyond the initial rpm-drop, she stalls. This only happens when in gear and the engine is warm. Idling and continous running (once moving) is no problem. I'll have to go back and research my papers from the work I had done, but I believe the fuel filter was replaced. If it was, what else could be causing this?

Any insights would be greatly appreciated. Thanks in advance!

Take off the air cleaner and see if you get a squirt of gas when the throttle arm is opened. If none then the accelerator pump is the problem. Probably a piece of crud in the ball valve or jet.

Hey Chipper,
You were VERY close! It turns out the accelerator pump had a bad seal...even though it was replaced just a few weeks ago; it was defective. For some reason it "swelled up". Another new seal was put in and now all is good. Thanks for the lead! :-)

Hesitation on acceleration is often the accelerator pump.

GlideAways It's good when the accelerator pump "SWELLS UP".
That was your problem, when the pump was replaced "a few week's ago" whoever replaced it did not put some light oil on it to make it swell..SOOO... it took a while to "swell" by it's self when it started getting gas. Amen...Good Luck....ED

I always soak them in transmision fluid.

Great thoughts/ideas guys, thanks!!

New development: car behaves as if she's running out of fuel when just below half tank. I know the gauge is still right b/c when I fill up it takes about half the number of gallons that the tank holds. Any ideas??

Pickup tube in the tank is broken in half. Tank is partially collapsed.

(Sigh). I'm looking at dropping the tank again. Just did this about a month ago to install a new sending unit. At least now I know what I need to do and look for. Thanks again Chipper for your help.

The pickup may not be fully broken but have a crack at about the half way point. The other alternative is something floating in the fuel.
Tony

Before dropping the tank a couple of tests are warranted. Drain the tank through the gas line to the fuel pump. It can be done by a couple means. Put on a rubber hose on the gas line disconnected from pump into gas can(s) and siphon the gas out of the tank. Or use a slight amount of air pressure on the filler neck. I have used a rag to provide a partial seal on the tank. If held in place by hand you will not over pressure the tank. All you need is a couple psig.

Once the flow of gas stops, then check to be sure that the gas line is open and not plugged. If blowing air into tank have someone check for air flow out end of gas line. If siphoning then blow back toward tank on gas line and listen for bubbles or air flow. If bubbles then reduce the air flow to see if you still get bubbles. A crack in the pickup may not leak enough air to keep from bubbles if the flow rate is high.