Strange Engine Problem

[jonward786]

Nothing i like more than having to call AAA to get a tow home from the gym in town.  thought i might describe whats going on and see if someone on here has had it happen to them before.

drove to gym, its like 100 degrees here.  came out and hour later and started it up, sounds like its running on 5 cylinders, really rough, wont idle unless i keep my foot in it.  so i ran over to napa and grabbed new spark plug wires and new spark plugs, no change, still running rough as hell.

it doesnt do this very often but when its done it before i kinda just sit in it and keep tapping the gas and it eventually snaps itself back into a smooth idle.  

i am completely stumped.  any advice would be wonderful. thanks.

-Jon

[johnpro]

Get a phenolic spacer if you don't have one, and make sure your choke is opening correctly.  On a 100 degree day, if the choke is closing at all, it's probably running PIG rich until it opens.

[camachinist]

Easy stuff:
Choke fully open

A bit harder:
Power valve not leaking

If it's heat soaking, usually the fuel line will boil and push fuel over into the main wells and out the venturis and flood the engine with fuel, making starting exceedingly difficult, with attendant black smoke.

Vapor lock starves the engine for fuel and it'll just quit running, usually at a stop light.

A slight possibility is debris in the carb metering passages, swirling around and occasionally blocking orifices. I've had that problem. Sounds like it's running on five and then mysteriously clears up.

What brand and type of carburetor?

BTW, my first check would be to pop the air cleaner after a hot stop and an hour of sitting and see what the choke blade looks like, position-wise.

[jonward786]

Well it was towed home and i swapped coil and still no change.  it already has a 1 inch phenolic spacer and i assume if it was running rich i would see some black smoke, which there was none.  i cant see how its not ignition related, yet ive already ruled out 3 major ignition components.  *sigh* its sounds like a damn diesel volvo

its a holley 570 street avenger with an automatic choke.  when i went out after letting it sit and bake in the 100 degree heat for an hour the choke plate was completely open, as it should be.   

[camachinist]

You wouldn't by any chance have a Pertronix in there, would you? LOL...

I swear those things barf as much as the old Mallory Unilites used to...

Another possibility, if it's points, is a bad condensor.

[jonward786]

haha yes i do have a pertronix actually.  ive had one die on me leaving me stranded on the highway in san jose once, but that was my doing.  i made the mistake of using a flamethrower 2 coil with a ignitor 1 and the coil ate it for lunch.  its got the proper ignitor 2 in there now.  i suppose i could replace that too and see what happens.  its gonna be a brand new car before im done with it

[LSTCUP]

how about the distributer cap, any moisture, small crack?

[camachinist]

If you put a timing light on it while it's acting up and/or throw the points back in there, you'll have your answer. I'll bet it's the Pertronix.

[blue65coupe]

Jon, this could potentially drive you insane.  Ask me how I know.  I had the exact same problem and went through every possible scenario.  EVERY possible option.  It ended up being a blockage in the fuel line back at the tank.  The blockage wouldn't pull through the bend in the line and I changed everything I possibly could before finding that.  Good chance it's not your problem but who knows.  Any chance with the heat and it sitting after a run you may have popped a head gasket? 

[jonward786]

Oh trust me it is already driving me crazy.  normally things that go wrong with this car are pretty simple to diagnose, thats why i enjoy owning it so much.  i dont think it would be a bad idea to redo the whole fuel system, to my knowledge the tank never been replaced and i imagine the fuel lines from the tank to the pump are 40+ years old too.  back when i had one of those glass fuel filters i had A LOT of these tiny little black rice kernel shaped particles in the filter.  not sure what those were 

[Gigantopithecus]

If you were seeing chunks in the filter, I'd say its time to invest in a new tank.  New lines wouldn't hurt either.  All that crap in the filter was probably scaling and who knows what coming from the tank.

[blue65coupe]

If you replace the tank you need to replace EVERY fuel line.  Do not skip the hard line running from the tank just because it's gonna be a pain.

[jonward786]

This morning just for the hell of it i decided to run a fuel line directly from the fuel pump into a container of gas to see if perhaps there was some blockage in the hardline or at the tank.  It didnt fix the problem, still ran crappy.  I guess at least im eliminating things one by one.

[Fast68back]

Didnt see a fuel filter swap in your list of replaced parts

[jonward786]

Yeah i put a new filter on this morning as well.  A mechanic friend seems to think something could be wrong with the timing chain...

[crustycurmudgeon]

Well, if that were the case it wouldn't fix itself and suddenly run ok.

Although a slipped timing chain would cause those symptoms.

Frank

[jonward786]

i guess i cant rule that out either, i do hear this faint clicking when its idling so horribly

[blue65coupe]

Do you see ANY white smoke coming out of one of the exhaust pipes?

[daveSanborn]

to my knowledge the tank never been replaced and i imagine the fuel lines from the tank to the pump are 40+ years old too.  back when i had one of those glass fuel filters i had A LOT of these tiny little black rice kernel shaped particles in the filter.  not sure what those were

Um, those little particles are the rotting metal on the inside of your gas tank.

This morning just for the hell of it i decided to run a fuel line directly from the fuel pump into a container of gas to see if perhaps there was some blockage in the hardline or at the tank.  It didnt fix the problem, still ran crappy.  I guess at least im eliminating things one by one.

It's still running crappy because your carbuerator is fouled with "these tiny little black rice kernel shaped particles" that have gotten through the filter.


Until you replace the gas tank, the entire fuel line, the fuel filter and have the carb disassembled and cleaned..... IMO, you're wasting your time.

[apollard]

I'm with Dave - my '68 had a rusted tank. Had pretty much the same symptoms. Changed filters twice and rebult the carb once before I changed everything in the fuel system. After that, no issues.

[blue65coupe]

I'm with Dave - my '68 had a rusted tank. Had pretty much the same symptoms. Changed filters twice and rebult the carb once before I changed everything in the fuel system. After that, no issues.

Wow, I'm not alone.

[silverblueBP]

Wow, I'm not alone.

Yeah, but he said "once" not a dozen times.

[blue65coupe]

For your information Francis, I only took your advice once.  You just relived it a dozen times.

[silverblueBP]

 :lol

[jonward786]

Well i think i found the problem.  This morning as i started to disassemble the carb and give it a brakleen bath i noticed the rubber cap covering the large vacuum port was severely cracked and undoubtedly letting air pour in.  ill replace it and report back with the results.
its always the little thing!