Hesitation corresponding to odd noise near air filter (S83P) (SCREW IT, I GIVE UP)

I searched the forum with no obvious results (I may not be a great searcher, so if you know of a thread related to this, please let me know).

Very recently I've been getting a random hesitation--almost always at low RPM's--with a simultaneous and odd "flappy" noise come from the engine. It's even died a few times while doing this.

So today I removed the engine cover, got it up to temp, and was able to reproduce the noise (and apparent hesitation, though this experiment is not under load, it's while parked) just by blipping the throttle. What happens is that I hear--just for a second--this odd "braap" noise that sounds like it's coming from the unit attached to the air cleaner housing (pics below). It happens once--additional throttle blips are normal. If I wait 10-15 seconds, I can reproduce it, but only once each time.

BTW, these are all new vacuum lines to this doodad (yes, I would love to know what it is, what it's called, and what it does).

Anyone been-there done-that? Any risks in opening this unit up (like is it a non-serviceable part)?

Thanks in advance for any advice-I need to get rid of this hesitation!

 

In stock trim, the engine can draw air from either frame rail. In warm weather it pulls it from the right rail, And in cooler weather, it pulls air from the left rail, through a heater box attached to the cat converter. Thus warming the air.

I think the valve pictured, is the hot air valve, which determines whether you are getting cool or heated air. The s80, and S81, had a manual lever, which you flipped one way for cold weather, and the other way for warm weather. Since then the switching is controlled by the air valve, which determines which way to move, based on a thermostatic vacuum valve. As the engine heats up the valve draws more air from the unheated Right frame rail.

 

Jigs,
Where would this manual lever be located?
Limestone

 

It was on the valve. Instead of the vacuum chamber shown, there was just a lever, and you switched back and forth seasonally.

 

THANKS, I don't remember seeing it! I'll check for it tomarow.
Limestone

 

I think it corresponded to the change from a manual choke to the thermostatic choke. That would make the most sense. But, I’m not sure of that.

 

Jigs,
That would make sense!
Limestone

 

Yeah, strike that. The flapping & hesitation are back. &*#%^@#^%!@#^&&*@!!!!!!!

Well, I think I have this solved. But all i really did was remove the air box, check all the vacuum lines, and clean everything up.

BUT...there was one thing. My airbox takes air from outside, obviously, but there's a a secondary air line that apparently pulls air from the tubular steel crossmember that goes under the truck and wraps around the motor (u-shaped, open end toward the rear). There's a little open 'box' welded to the center of this tube, facing down, with a hole into the tube. The little 'box' was packed full of dried mud. I wonder if the mud meant was starved for air, making the warm air diverter 'flap' when I applied the gas. Either way, the airbox has certainly pulled plenty of dusty dirty crap up through this tube, so I may do something about this.

Dunno, weird setup.​

 

JMM,
Absolutely, Air in air out! Good job!
Limestone

 

So after a few weeks of headaches, I had a mechanic buddy really dig in. We removed the carb, cleaned it up, but the truck continued to hesitate very badly.

Turns out the accelerator pump is bad. Just ordered a carb kit from Jimmy at Mactown--hopefully back to normal soon (don't jinx me, carburetor gods).

The "brrap" noise I was hearing was from backfiring--the accelerator pump is so weak that fuel isn't squirting into the throat, it's just pooling near the bottom of the accelerator jet. That fuel randomly ignites and the pressure waves are hitting the cold weather flap in the airbox, making it noisily flap around.

One thing...the truck never hesitated at all until I did a proper Seafoam treatment through the brake booster vac. line. I sincerely doubt that Seafoam would attack the accel. pump diaphragm--and maybe not everything is cause and effect--but this raised an eyebrow with my mechanic friend.

 

Sounds like your gaining on it!

 

The quarantine has given me plenty of time to rebuild the carb, and now that I'm done...I'm ready to set fire to this thing. Vac lines good, fuel good, filters, valves, timing...I give up. Going to buy a new carb or get this one professionally rebuilt. Hesitation is worse than ever, now the revs hang, low on power.

THIS IS WHY THE WORLD SWITCHED TO FUEL INJECTION.

 

If your gonna, might as well convert to electronic ignition! That's what my next move will be!

 

Pertronix makes a conversion for the Denso 3-cylinder distributors with points. Depends on whether the spin is clockwise or counterclockwise.