I have a 1971 estate wagon with a 455. The ignition is a crane XRi. New coil. new plugs. Recent cap, rotor and wires. Quadrajet carb. The car starts and idles great. Give it anything more than a touch of gas in gear and it bucks and hesitates. Will slowly accelerate to about 50 and no more. timing is set at 6 BTC. Fuel pressure is 6-7 psi- even drove around the block with pressure gauge sitting on windshield. New plugs are still very white suggesting lean. The engine has 1970 heads that were subject to a valve job and milled a few thousands to make them flat. I had this problem a year ago and it went away. No idea what the issue was. This time was cruising 80 mph on the expressway and the car started slowing down. Things I have considered: 1. internal carb leak. (Everything appears to be sealed- no signs of fuel leakage) 2. external vacuum leak. (can't find it) 3. fuel line blockage or sucking air. (fuel pressure seems fine) 2. slipped damper making timing off 3. Crane module is bad.-(will replace with points this weekend) The fuel tank is recent and is rust and junk free. The previous owner replaced it. I don't know the condition of the pick up or fuel sock(if there is one). I never see any junk in my carb filter. or in the carb. I'm at wits end. Any thoughts are appreciated. Mike
Accelerator pump. With the engine off, remove the air cleaner and work the throttle, does it spray fuel?
A fuel tank vent line obstruction will begin to interrupt fuel flow as a vacuum begins to developed at the top of the tank. Is the fuel filter in the neck of the Q jet is clean?
You need to monitor fuel pressure WHEN the problem occurs. The fuel pressure was 6-7 psi and it was bucking and hesitating? Sure sounds like fuel to me.
I don't disagree, but I have 5-7 psi on the gauge taped to my windshield as I drive down the street with the car hesitating. This is why I am stumped. I even replaced the fuel pump. Lines at either end of the metal line are new. I get real good flow in my face when I pull the lines off the fuel pump...
Ingition issues just don't fix themselves, my bet is the accelerator pump is non functional, or the power piston is hung up in the Carb. When a accelerator pump goes bad you can slip into the throttle little by little and not get a big stumble, but if the power piston is hanging up you can't!
X2, accelerator pumps can be affected by the ethanol in our gasoline. Cliff Ruggles has the most resistant ones. You can stick a small screwdriver down the carburetor vent with the engine off to check the power piston.
From the description, the problem seems fuel related. There is a big difference between ignition related symptoms and fuel related symptoms, at least to me. It can also be a float problem.
Fuel issue because it idles fine and the problem occurs under demand? My module died, but it wouldn't start? Can you replicate the problem without the load? Just in park, can you get it to stumble? Do you have a spare carb or distributor? Changing out the distributor revealed my module issue. Just a few thoughts from here.
I finally returned to this project. Reinstalled the points distributor no change. Built another carb- no change. Noticed a leaky exhaust manifold that was doing more than just ticking. Checked the y pipe to single exhaust junction. It to was leaking heavily. Separated the pipe and couldn't get a 1/2" borescope more than 12" into the pipe. My video won't upload but there are a series of buckles inside the pipe that close it off to almost nothing. I will be taking it to the muffler shop down the street that previously told me I couldn't have a blocked exhaust on a non catalytic converter car.
That must be a factory Y pipe. They were double walled and they collapse internally. It was a common issue back in the day. You don't really hear about it now because not too many people have OE exhaust on their cars. Your car must have low miles
I have 108000 miles on the car. Muffler and tail pipe were replaced about 3 years ago. I will have the shop inspect the y pipe too.
Mike, although you've found the problem. If you have a vacuum gauge can you simulate the readings in this video.
Yep, had that issue shortly after I bought my GS back in '85 still had original pipes, right side inner had collapsed, pipe was hot about 1/3 of the way back, then cool after that.
Mine is worse. I have an under Dash vacuum gauge. Give it any gas and vacuum goes to zero and car starts to sputter. Mechanic at muffler shop didn't believe me. I made him cut the pipe. Pic attached.
Pipe is replaced. Vacuum at idle went from 14 to 19. Cruise vacuum which was around 15 is now mid 20's. No more bog! Thank you everyone for your suggestions and support.