Step one: Pull the plugs for a look-see.
In trying to recollect dealing with driveability problems such as the OP’s back in the GOOD OLE DAYS I recall tipping the choke to significantly close it and listening for an improvement in the idle and also lifting the coil wire up slightly from the distributor cap. And to think how outrageous the electronic ignitions of the early 70s looked to mechanics at the time seems silly compared to today’s computerized COP ignitions.
Does it seem like it is running rich or lean? If running lean carb cleaning, rich replace needle and seat in the float bowl, a cheap easy thing to do.
I’ll take the stance that the ethanol fuel has crapped out the carb. I think the carb will need to come off for a rebuild or you can buy a new carb. Most rebuilds will use rubber parts that will handle the ethanol fuel in the future if we stay with 10% ethanol or lower.
Unfortunately that doesn’t update the rest of the fuel system, fuel pump, rubber fuel lines, etc. I’d highly recommend treating all the gas with a marine grade ethanol treatment.
Nevada_545: The “choke gadget” on top of the carb also looks like an accelerator pump to me.
@sgtrock21 … no, that triangle gadget on top of the carb – as shown in the photo-link I posted above – is not the accelerator pump. The accelerator pump is on the front of the carb. I’ve had to change the diaphragm in the accel pump a number of times over the years. But it’s likely not the current problem. When the accel pump diaphragm splits it leaks gasoline, which is easy to see. And it isn’t leaking.
That triangle gadget, not entirely sure everything what it does but part of its function is involved in the choke system. It seems to be what holds the choke plate slightly open once the engine starts. I did have to replace that gadget before due to a split diaphragm, but the current one seems to be functioning correctly now. At least it is cocking the choke plate ajar one the engine starts as expected. Easy enough to remove and eyeball the diaphragm though, and know for sure.
@ok4450 … Now I think about it, you are likely correct about partially clogged idle bleed screws. I’m placing that theory as of now as the highest priority. The symptom is very consistent, running horribly at idle, then runs fine with the throttle plate even slightly off idle. I did a little research on how carbs work the other day, both Rochester and Holley designs, and what the books said was the main idle port is below the throttle plate at full idle, so that’s the main path for gasoline to squirt into and run the engine with the throttle plate closed to its nominal idle position. But when the throttle plate is even slightly cracked open, that allows a secondary idle port just above the throttle plate nominal position to supply fuel. That’s the off-idle port. And by the engine performance I’m experiencing, that one must be open and working fine. The other thing consistent with your idea is I in fact did let the air cleaner go a little too long. It was noticeably dirty. I’ve installed a new one, but I’ll have to remove those idle air mixture screws and clean those holes out by squirting some carb cleaner in there.
@Nevada_545 and others, thanks, good comments. With a 40+ year old truck, it could be anything. If none of the quick fixes work, I’ll measure compression, fuel pressure, rebuild the carb, replace the fuel filter, spark plugs, points, condenser, dist cap, and wires and report back.
The triangle thing sounds like the high idle cam actuating lever. Is it connected to a drum shaped thing, or a thin rod that goes into a little housing? Is there a multistepped “stairway” stamped into it?
A drum shaped thing would contain a spiral bimetallic spring that senses temperature and adjusts the throttle plate to higher speeds as the temperature gets colder. If one apex is connected to a thin rod going into a housing, the bimetallic spring is under the little housing.
The triangular thing on the top of the carb is the choke pull off but that particular variety of pull off dates back to the early 70s I believe. How clean is the carburetor?
Manual chokes go back much farther than the '70s. By 1970 every vehicle I can remember had an automatic choke. Heck, my '64 Fairlane had an automatic choke.
Unless you’re thinkin’ it’s part of the assemblage I described operated by the bimetallic spring. Which it probably is.
Here’s a photo where you can pretty clearly see that it is connected to the choke plate. I think @“Rod Knox” is correct, it is just the choke pull off. Its purpose is to open the choke plate a little once the engine starts. Which does in fact happen.
@Nevada_545 mentioned a power valve as a potential suspect too. I rebuilt this carb years ago, and I seem to remember there was a power valve in it I replaced as part of the rebuild kit, but it must be lower down, not visible unless the carb is taken apart more.
The carb looks pretty clean on the inside anyway.
You can remove the idle air screws and spray carburetor cleaner into the ports. If the idle ports are plugged with residue as someone mentioned that could improve the situation. When you replace the idle screws turn them fully in LIGHTLY, then back them out 1 1/2 turns.
I’ll also measure how far they’re turned out now, by turning them all the way in to the stop first, counting the turns, before backing them out. If only I remember to write it down … lol …
How old are the spark plug wires?
@texases … The existing HV wires have seen 6 years, maybe 15,000 miles. I gave them the twist-and-look treatment and see no signs of insulation deterioration. They’re of the silicone rubber type, which seem to hold up pretty good. As suggested above, I idled the engine in the dark and see no signs of sparks jumping around under the hood. That and the fact that the symptom is very dependent on throttle position makes me think the most likely explanation is a fuel/air mixture problem at idle.
OK. Did you try squirting the base area with, say, wd40?
No, haven’t had the time for trying that yet. But it’s next on the list. No worries, my second car is a Corolla.
Update. Cleaned out idle-screw adjustment ports with carb spray with carb installed. Replaced fuel filter. Took a sample of the gas in the tank by re-routing the fuel line from the carb intake to a container and running the starter motor 15 seconds. No signs of fuel contamination or separation. Checked capacitance of ignition condenser, measured 0.22 uF , same as a new replacement, so left it in place.
Overall, definite improvement at idle rpm, but some idle missing and idle rpm surging still present. Off idle performance remains the same, like new. Next up, carb rebuild and egr replacement.
Measuring a capacitor’s value in µF tells you nothing about the cap’s performance at high voltages. It’s cheap enough, replace it.
Does that cap experience high voltages? How high?