I'm having issue with really rough idle on my 67 olds, 425 4bbl. When it's cold it runs perfect, and no problem driving and cruising along after it's warmed up but when i come to light or idle for any reason it idles very rought as if it were bout to stall. The car wasn't driven much before i bought it and i have a theory that the valves are sticking after it's warmed up from alot of buildup...any thoughts?? anybody

 

My first suspicion would be a choke issue with the carburetor.

 

Methinks there may be a relation between these two threads, no?

- Eric

 

You think it's idling rough because my brake pedal is hard??

 

Bad PCV valve can act like a giant vacuum leak. If bad enough it could cause poor power brake performance.

Another thought, my sister had a problem that sounded a lot like what you have and it turned out to be the vacuum line to the brake booster was resting against the firewall and had rubbed a hole in it. Hard to see, hard to find. When idling with the hood up the hose was against the firewall and the hole was plugged. Couldn't even hear the leak. Hers was on a '97 Expedition so may not be something yours could do.

 

I think it's idling roughly because the vacuum hose to your brake booster is improperly connected, or your brake booster diaphragm has a hole in it.

- Eric

 

X2 Kenneth.


Remove aircleaner and observe the choke plate as the engine warms up. Immediately after a COLD START, the choke plate should open about 1/8". If it doesn't, suspect a bad choke vacuum pulloff (located right front of carb and a common failure).


If it opens as it should after cold start, watch it as the car warms up and make sure it's opening all the way. Time you've driven a couple miles or it's warmed enough to kick off all the way to curb idle speed, it should be 100% open unless in really cold weather. If it doesn't open fully, your choke thermostat spring is weak or your choke linkage is binding.


But do check all vacuum hoses and connections. Even a broken 1/8" hose can cause enough of a vacuum leak to cause rough idle and hard brake pedal. And after 49 years...

 

After considering this for a bit, I'm thinking that someone couldn't figure out the choke and adjusted the fuel mixture so as to start and run smoothly from a cold start without the choke. Seen that more than once on cars bought from people who didn't understand the importance of the choke in a carbureted engine. This is what my money's on, since an engine that starts and idles fine cold and idles like crap but drives fine hot seems to me to be a too-rich idle mixture condition. An excessively rich mixture would simulate a choke when the engine's cold, whereas I'd think that a choke that's stuck closed would result in the engine running poorly both at idle and while driving once warmed up.

I still do second what everyone says about checking choke function, as checking whether or not the choke is working is quick and easy insurance. With a cold engine push the gas pedal all the way to the floor once, let go, and then remove the air cleaner lid and see if the choke butterfly is closed. If the choke closed like it was supposed to, then warm the engine up and see if the choke pulls off or stays shut, like rocketraider was saying. On the other hand, if it stayed wide open when you pushed the pedal, then you could try warming the engine up to operating temperature and re-tuning the fuel/air mixture using a tach and/or vacuum gauge. A '67 Olds Chassis Service Manual would be useful for this, but I could provide the tune-up specs if you need them (feel free to PM me). Of course, if this turns out to be the problem then we'll also need to figure out why the choke is stuck open so your engine will still start well when cold once tuned correctly. One thing at a time, though.

Good luck, I'll stay tuned to hear what you find out!

PS: I think Eric's on point, that the two problems described in your two threads are interrelated. Given your mention in the other thread of a lack of vacuum when you disconnect the brake booster hose, I think the lack of brake assist could be a result of the rough idle, rather than the other way around. Peak manifold vacuum is one indicator of a proper idle mixture, so I'd think an over-rich idle mixture could cause the low manifold vacuum that you mentioned, which in turn would mean no brake assist. A blown-out brake booster would cause a lean condition, but that doesn't sound like the problem to me given all the other clues.

 

OR -- You could have a big vacuum leak that is mitigated when starting cold, because of the choke richening the mixture, and then is unmasked as the choke opens, causing a too-lean condition due to the extra air indroduced through the vacuum leak.

We shall see in the fullness of time...

- Eric

 

Hmm, true. Thinking back, when I had a vacuum hose come loose from the vac reservoir, it was hard to tell there was anything wrong until I was at a standstill (though I don't really remember how it behaved while cold with choke applied). Anything is certainly possible.

 

All points taken. I did test the choke before i posted and it seems to be functioning although i can't say if its perfect. I do know that the guy that I had working on it adjusted the fuel/air mix on the carb but didn't seem to make a difference one way or another.

I think i'll post a video and let ya'll figure it out

 

I'll just throw this out there, it's worth a quick check, a buddy of mine bought a really nice 78 cutlass with a 350, car started ok, ran decent cold and performance fell off when warm, it smelled rich, previous owner just had a tune up done and gave up on it and sold it cheap, after looking at the choke it was working, adjusting mixture screws made no difference, after some guessing I decided to check ignition, 2 plug wires were switched. I flip flopped the wires and it runs strong, my guess is choked at high idle the miss wasn't that noticeable, warm and off high idle it miss fire was more pronounced. Past experience tells me don't rule out a possible ignition issue,

 

I can't understand why Erics' advise is being ignored, it seems to be the most obvious reason for the OPs problem.......

 

I hope nobody thinks I'm trying to discount anyone's advise, I was just passing along an issue I ran across