Engine: Rough idle- any guesses

[JimC]

Okay, I haven't even had a chance to pop the hood yet and start poking around, but I thought I'd ask you experts to chime in on the best places for me to start looking tomorrow when I dive in.

Yesterday on a drive, my '63 suddenly started idling rough. There wasn't any particular event before during or after, it just started running like the engine was on the verge of stalling. It was still driving, and there are no unusual noises (aside from a noisy valve that's sounded the same since I bought it), but I did notice that there's a hesitation as I begin to depress the accelerator. Also, I noticed when revving it up a few times that the exhaust has a really rich smell of gas to it (though the car has always run a little rich since purchase)

Today I had to run a short errand, so I took it out. At the start of the trip it was running really awful, just like yesterday. By the end of the drive, it still wasn't the smooth ride I've been used to, but it was a whole lot better.

So, what the heck sort of problem can come up that varies in intensity like this?!? I'm totally perplexed. I was planning on doing a minor tune up this weekend anyway, so I hope I'll figure it out then, but in the mean time, if there's any little thing (vacuum line, etc) that you guys suspect it could be, I'll take a peek in the morning.

Thanks!

[Flashback]

Did you put gas in? What kind of carb? Could be trash in the needle and seat. Sounds like a rich fuel mixture problem, maybe due to leak by on needle and seat. Or, for goodness sake, don't let me say it is the fuel. You already mentioned a vacuum leak, and that's easy to check.

[JimC]

I didn't add gas, and I have a locking gas cap, so I doubt anyone else did either. I'm on a hair less than a half tank, and my last fill was premium 91 gas, which is what I've almost always used. I thought about a carb issue, and it may be, but can a carb issue "get better"??? I had a bad carb on an old plymouth, and it only seemed to get worse until I fixed it. That getting better issue is the perplexing part.

[bezhawk]

I would also say a bit of dirt on the float needle and seat.

[JimC]

Well, maybe it was just a little gunk in the carb. I took it on a short two mile drive and opened the throttle all the way a couple times. On the way back, it's running like normal. Go figure.

[Mike Van Veghten]

(aside from a noisy valve that's sounded the same since I bought it

Ok...so when "was" the last time the valve lash was checked ? These things need to be adjusted every now and again. Maybe it was telling you something back when you "bought it"..."hey...please adjust me.." !

Despite the fact that the engine type isn't mentioned, both the 6 and the 8's need care and maintance.
And yes, the exhaust valves will get tight, AND cause the engine to run rough at low rpm's when they need attention.

Just a thought.

Mike

[Dan Timberlake]

A few times In decades past I've found small amounts or even droplets of free water rolling around in the float bowls of 2 stroke motorcycle carbs. One of them would shut off under certain throttle conditions.

The ability of today's E10 gas to pull moisture out of the air is sometime startling. just last week we were playing with a buddy's 76 Corvette, and several ounces of gasoline ended up in a pyrex measuring cup. Granted, the large exposed surface to volume ratio was kind of extreme compared to a carb or gas tank, but within an hour or 2 the gas was very cloudy, which I always consider a sign of a problem needing correction. My devices' Gasoline must be crystal clear.
A few glugs of isopropyl dry gas caused a visual tempest of subsurface activity, then clarity returned.

At some point the water will fall out of suspension as a separate phase. (This EPA Memorandum asserts it would take 200 days for a gallon of probable normal gas to becom saturated with water - http://epa.gov/oms/regs/fuels/rfg/waterphs.pdf)

I think preventative additions of drygas or other fuel system cleaners may actually have something to offer, but would still take a sample of the fuel delivered to the carb for visual inspection.

[JimC]

Ok...so when "was" the last time the valve lash was checked ? These things need to be adjusted every now and again. Maybe it was telling you something back when you "bought it"..."hey...please adjust me.." !

Despite the fact that the engine type isn't mentioned, both the 6 and the 8's need care and maintance.
And yes, the exhaust valves will get tight, AND cause the engine to run rough at low rpm's when they need attention.

Just a thought.

Mike

Hi Mike,
Valves are on my checklist for the tune-up this weekend. I've only had her for about three months, and I work in a place where I don't usually get days off so much as I get 8 hour chunks of time off twice a week. But after about noon today, I'm off till Sunday night, so I'm going to get as much done as I can with the little time I have. (I also have to dismantle a Lark, so busy weekend!!)

[JimC]

Oh right, it's a 259. Forgot to add that.

Also, the nice thing about a state without emissions testing/requirements, is that we can still buy non-ethanol gas. Both myself and the prior owner (who had it several years) are anti E-10, so I doubt an ethanol problem would "suddenly arise". But you never know.

[irish]

I have seen a dirty distributor do something very similar to what you are describing. The distributor was dirty enough that sometimes the advance would stick. Also, don't rule out a failing condensor or coil they can cause erratic voltage which can cause the car to run temporarily rich or lean.

Joe

[RadioRoy]

It could be as simple as the mounting stud on the carb coming loose, a dirty air filter, loose distributor cap, bad rotor, bad spark plug wire (you can look at it in the dark and see if there is any corona) loose or cracked vacuum hoses, bad PCV valve, loose distributor clamp to the block, bad gas, idle mixture screws missing their springs, vacuum leaks anywhere, a low voltage wire rubbing on the engine and rubbed through the insulation.

Since it was running OK recently and suddenly is not, it's likely to be something simple. Don't fall into the trap of taking everything apart right off the bat. that usually causes new problems in addition to not finding the existing problem.