: fuel pressure is eratic 0n my 70 coupe

11-26-10, 05:45 PM
i have an in line pressure gauge on my gas line and the needle is jumping up and down at idle. also the car wont start unless i spray carb cleaner down the carb, then it runs o.k. but stumbles at high excelleration. could my fuel pump need peplacing?

11-27-10, 06:35 AM
It could be your fuel pump or a filter, if that doesn't fix it I would drop the gas tank and inspect the pickup filter. A 40 year old car should be inspected for cracked and/or restricted lines (not just fuel) very closely as a safety precaution. This is a mechanical fuel pump if I remember correctly, not sure if it is driven directly by a cam mounted in front of the timing gear or there is a rod involved but there could be a problem in that area as well. I worked on a Chevy a while back that had a worn fuel pump pushrod that caused the same problems you are having.

11-27-10, 09:07 AM
You don't have a pushrod in your engine, the pump is driven off a concentric on the front of the cam. The fluctuation suggests a cracked diaphram. Check your dipstick. Sniff the oil. If in smells like gas, your pump is dumping into the sump. Change oil and pump.
Your hard start suggests the fuel is siphoning out the line. Your carb reservoir should hold enough fuel to start your engine. One of the inconveniences of the Quadrajet is the area known as the wells. The wells are located at the lower section of the carb main body. They are capped at the bottom and over time and several temperature changes, the caps can allow fuel to leak past and out of the reservoir. This is easily repaired using epoxy.

11-27-10, 09:26 AM
thanks!! can that issue with the carb and pump cause the car to stumble at high excceleration? the other day the pressure was steady at 5 lbs at idle and the car stumbled at high excceleration. could the carb only cause the erratic fuel pressure if the gas is draining out of the line?

11-27-10, 04:10 PM
Have you checked the filter that mounts in the front of the carb? Stumble at high engine speed could be caused by fuel supply feed not keeping up with demand. It could also be caused by a worn timing chain / cam gear. At higher engine speeds your fuel pump pressure fluctuations will not be as apparent as at lower engine speeds. Are you running a point distributor? Point bounce at higher engine speeds can also cause stumble. Because the Cadillac engines don't rely on higher rpm ranges to develop power, you probably have a set of standard grade points in the distributor. Distributors should be checked from time to time to check for wear. If your bushings are worn so that the shaft has .002 wobble, your point dwell increases and if the shaft has more wear the higher it climbs, it can be more. This doesn't mean you get stumble, but it contributes to your distributor's efficiency.

11-28-10, 08:11 PM
I would check for pin holes from rust in the fuel line. Most often at the highest point just over the rear wheels...This could be causing your problem..

11-29-10, 09:47 PM
i recently had the tank dropped and flushed, all new gas lines and filters (both at carb and pump) installed. i had a petronix ignitor and new vacuum advance put in. the carb was recently rebuilt (i think it was a BAD REBUILD) the car wont start up on a cold start unless i spray carb cleaner in the carb, and its getting fuel spray evenly out of both jets into the body. there are no vacuum leaks (checked that). before the numerous repairs the car had a rough start and rough idle but exccelerated fine, now after carb was rebuilt i get the stupid stumble upon excceleration. could all my issues be within the carb? or does the erratic pressure deal strictly with the pump?