1978 G30 van 5.7 liter 350 small block. The other day I had to punch the gas in order to cross a busy street. Immediately after and ever since it has a rough idle but runs smooth when u̶s̶i̶n̶g̶ ̶t̶h̶e̶ ̶g̶a̶s̶ accelerating.
Don’t understand this, what else would you be using?
Throttle body injection?
Nah, TBI didn’t show up until the mid-late 80’s in the truck/vans, probably has a quadrajet or Rochester 4 barrel
The diaphragm for the accelerator pump may have become inverted allowing gas to leak past it.
It’s the component at the upper left.
Tester
Not sure. Idk much about carburetors. It has an edelbrock.
Check the secondaries on the carburetor to see if they have closed, the throttle valves may need to be cleaned so that they open and close properly.
I know the regulars here have suffered my not being able to resist being a “smartazz”. Perhaps the gasoline was injured from being “punched”. Absolutely no offense to OP as “punching the gas” was a common slang term when I was a teenager 50 years ago. Nevada_545’s suggestion could result in an improper fuel/air ratio which could result in a rough idle.
Haven’t actually assaulted the gas but I have threatened the carburetor. I think tomorrow I’ll try spraying some carb cleaner (specifically seafoam) in it. If that dosn’t work I’ll have to learn how to rebuild it. Is there a way to check if it’s the carb or should I check anything else before I get in over my head and budget?
I would check the throttle linkage including the anti diesel solenoid to insure the throttle plates are opening and closing properly. It could be a weak return spring or a simple adjustment.
I would check the various externals very closely before committing to an overhaul.
Tester’s suggestion about the accelerator pump sounds worthy. If the seal is not sealing, that could be letting excess gas into the carb. On today’s cars that might be compensated for by the electronics and the idle air control motor. But not so on a carb.
The accelerator pump on my 1979 Toyota 4X4 had a leather seal that moved up and down in a cylinder - like a bike pump. I had to soak that seal in oil from time to time. It also had an AAP auxiliary accelerator pump which had a diaphragm with gas on one side and engine vacuum on the other. When that diaphragm failed, excess gas poured into the carb and eventually the oil.
So it is not the factory carb since Edelbrock bought the tooling to make Carter AFB carbs years ago. Sounds like you need a re-build.
Looks like my accelerate pump is kinda deep in the carburetor. This is the carb I have. Does the arrow point to the accelerator pump? Is there a way to check it without taking the top of the carb off?
Looks like yes and no. It’s quite possible you can get the top off without wrecking the gasket.
I don’t think the carb intake cleaner did much. Cleaned all the linkages, none seemed sticky. When I push in the accelerator linkage I noticed some squeaking noise which sounds like it’s coming from the accel pump. One more tid bit that I noticed was that it doesn’t run rough until a few seconds after letting off the gas pedal.
I think I’ll buy the gasket but if I can keep the old one intact I’ll take the new one back. You reckon it’s just one and what do I call that particular gasket?
I’d call it the gasket between the main body and the top. Best wishes, keep us informed.
Will do. Might be a couple days though.
Took the top off the carburetor. The accel pump doesn’t look damaged but it is dirty and old looking, may have shrunk. About to ride my bike to the parts store and get a new one. Should I soak the top while it’s off?
Did the accelerator pump squirt fuel into the primary bores when you operated the throttle? If it was not working it would cause a hesitation when the throttle is first opened, not a rough idle.
Wondering if it could be a sticky vacuum or mechanical advance in the distributor.
“Punching the gas” may have take engine RPM’s higher than they’ve been in a long time, causing the advance to go farther than it’s been in a long time and becoming stuck.